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710 lines
48 KiB
Plaintext
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PostgreSQL için Sıkça Sorulan Sorular (SSS) Son güncelleme : 19 Mayıs
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2003 Pazartesi - 03:05:21 Current maintainer: Bruce Momjian
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(pgman@candle.pha.pa.us ) Çeviren : Devrim GÜNDÜZ (devrim@gunduz.org )
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Nicolai Tufar (ntufar@yahoo.com ) Bu belgenin en güncel hali,
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http://www.PostgreSQL.org/docs/faqs/FAQ_turkish.html ve
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http://www.gunduz.org/seminer/pg/FAQ_turkish adreslerinde görülebilir.
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Platforma özel sorularınız,http://www.PostgreSQL.org/docs/index.html
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adresinde yanıtlanır..
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----------------------------------------------------------------------
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-- Genel Sorular 1.1 <#1.1>) PostgreSQL nedir? Nasıl okunur? 1.2
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<#1.2>) PostgreSQL' in hakları nedir? 1.3 <#1.3>) PostgreSQL, hangi
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Unix platformlarında çalısır? 1.4 <#1.4>) Hangi Unix olmayan
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uyarlamaları bulunmaktadır? 1.5 <#1.5>) PostgreSQL'i nereden
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indirebilirim? 1.6 <#1.6>) Desteği nereden alabilirim? 1.7 <#1.7>) En
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son sürümü nedir? 1.8 <#1.8>) Hangi belgelere ulasabilirim? 1.9
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<#1.9>) Bilinen hatalar ya da eksik özelliklere nereden ulasabilirim?
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1.10 <#1.10>) Nasıl SQL öğrenebilirim? 1.11 <#1.11>) PostgreSQL 2000
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yılına uyumlu mudur? 1.12 <#1.12>) Geliştirme takımına nasıl
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katılabilirim?? 1.13 <#1.13>) Bir hata raporunu nasıl gönderebilirim?
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1.14 <#1.14>) PostgreSQL, diger VTYS(DBMS lerle nasıl
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karşılaştırılabilir? 1.15 <#1.15>) PostgreSQL'e maddi açıdan nasıl
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destek olabilirim? Kullanıcı/istemci Soruları 2.1 <#2.1>) Are there
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ODBC drivers for PostgreSQL? 2.2 <#2.2>) What tools are available for
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using PostgreSQL with Web pages? 2.3 <#2.3>) Does PostgreSQL have a
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graphical user interface? 2.4 <#2.4>) What languages are available to
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communicate with PostgreSQL? Administrative Questions 3.1 <#3.1>) How
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do I install PostgreSQL somewhere other than //usr/local/pgsql/? 3.2
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<#3.2>) When I start /postmaster/, I get a /Bad System Call/ or core
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dumped message. Why? 3.3 <#3.3>) When I try to start /postmaster/, I
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get /IpcMemoryCreate/ errors. Why? 3.4 <#3.4>) When I try to start
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/postmaster/, I get /IpcSemaphoreCreate/ errors. Why? 3.5 <#3.5>) How
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do I control connections from other hosts? 3.6 <#3.6>) How do I tune
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the database engine for better performance? 3.7 <#3.7>) What debugging
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features are available? 3.8 <#3.8>) Why do I get /"Sorry, too many
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clients"/ when trying to connect? 3.9 <#3.9>) What is in the
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/pgsql_tmp/ directory? 3.10 <#3.10>) Why do I need to do a dump and
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restore to upgrade PostgreSQL releases? Operational Questions 4.1
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<#4.1>) What is the difference between binary cursors and normal
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cursors? 4.2 <#4.2>) How do I SELECT only the first few rows of a
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query? 4.3 <#4.3>) How do I get a list of tables or other things I can
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see in /psql/? 4.4 <#4.4>) How do you remove a column from a table?
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4.5 <#4.5>) What is the maximum size for a row, a table, and a
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database? 4.6 <#4.6>) How much database disk space is required to
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store data from a typical text file? 4.7 <#4.7>) How do I find out
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what tables, indexes, databases, and users are defined? 4.8 <#4.8>) My
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queries are slow or don't make use of the indexes. Why? 4.9 <#4.9>)
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How do I see how the query optimizer is evaluating my query? 4.10
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<#4.10>) What is an R-tree index? 4.11 <#4.11>) What is the Genetic
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Query Optimizer? 4.12 <#4.12>) How do I perform regular expression
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searches and case-insensitive regular expression searches? How do I
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use an index for case-insensitive searches? 4.13 <#4.13>) In a query,
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how do I detect if a field is NULL? 4.14 <#4.14>) What is the
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difference between the various character types? 4.15.1 <#4.15.1>) How
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do I create a serial/auto-incrementing field? 4.15.2 <#4.15.2>) How do
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I get the value of a SERIAL insert? 4.15.3 <#4.15.3>) Don't
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/currval()/ and /nextval()/ lead to a race condition with other users?
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4.15.4 <#4.15.4>) Why aren't my sequence numbers reused on transaction
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abort? Why are there gaps in the numbering of my sequence/SERIAL
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column? 4.16 <#4.16>) What is an OID? What is a TID? 4.17 <#4.17>)
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What is the meaning of some of the terms used in PostgreSQL? 4.18
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<#4.18>) Why do I get the error /"ERROR: Memory exhausted in
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AllocSetAlloc()"/? 4.19 <#4.19>) How do I tell what PostgreSQL version
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I am running? 4.20 <#4.20>) Why does my large-object operations get
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/"invalid large obj descriptor"/? 4.21 <#4.21>) How do I create a
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column that will default to the current time? 4.22 <#4.22>) Why are my
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subqueries using |IN| so slow? 4.23 <#4.23>) How do I perform an outer
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join? 4.24 <#4.24>) How do I perform queries using multiple databases?
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4.25 <#4.25>) How do I return multiple rows or columns from a
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function? 4.26 <#4.26>) Why can't I reliably create/drop temporary
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tables in PL/PgSQL functions? 4.27 <#4.27>) What replication options
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are available? 4.28 <#4.28>) What encryption options are available?
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Extending PostgreSQL 5.1 <#5.1>) I wrote a user-defined function. When
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I run it in /psql/, why does it dump core? 5.2 <#5.2>) How can I
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contribute some nifty new types and functions to PostgreSQL? 5.3
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<#5.3>) How do I write a C function to return a tuple? 5.4 <#5.4>) I
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have changed a source file. Why does the recompile not see the change?
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----------------------------------------------------------------------
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-- General Questions 1.1) What is PostgreSQL? How is it pronounced?
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PostgreSQL is pronounced /Post-Gres-Q-L/. PostgreSQL is an enhancement
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of the POSTGRES database management system, a next-generation DBMS
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research prototype. While PostgreSQL retains the powerful data model
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and rich data types of POSTGRES, it replaces the PostQuel query
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language with an extended subset of SQL. PostgreSQL is free and the
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complete source is available. PostgreSQL development is performed by a
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team of developers who all subscribe to the PostgreSQL development
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mailing list. The current coordinator is Marc G. Fournier
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(scrappy@PostgreSQL.org ). (See section 1.6 <#1.6> on how to join).
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This team is now responsible for all development of PostgreSQL. The
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authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen. Many others
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have contributed to the porting, testing, debugging, and enhancement
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of the code. The original Postgres code, from which PostgreSQL is
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derived, was the effort of many graduate students, undergraduate
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students, and staff programmers working under the direction of
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Professor Michael Stonebraker at the University of California,
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Berkeley. The original name of the software at Berkeley was Postgres.
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When SQL functionality was added in 1995, its name was changed to
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Postgres95. The name was changed at the end of 1996 to PostgreSQL.
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1.2) What is the copyright on PostgreSQL? PostgreSQL is subject to the
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following COPYRIGHT: PostgreSQL Data Base Management System Portions
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copyright (c) 1996-2002, PostgreSQL Global Development Group Portions
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Copyright (c) 1994-6 Regents of the University of California
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Permission to use, copy, modify, and distribute this software and its
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documentation for any purpose, without fee, and without a written
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agreement is hereby granted, provided that the above copyright notice
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and this paragraph and the following two paragraphs appear in all
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copies. IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO
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ANY PARTY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL
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DAMAGES, INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS
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SOFTWARE AND ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF CALIFORNIA
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HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. THE UNIVERSITY OF
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CALIFORNIA SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING, BUT NOT
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LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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A PARTICULAR PURPOSE. THE SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS"
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BASIS, AND THE UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE
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MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS. The
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above is the BSD license, the classic open-source license. It has no
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restrictions on how the source code may be used. We like it and have
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no intention of changing it. 1.3) What Unix platforms does PostgreSQL
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run on? In general, a modern Unix-compatible platform should be able
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to run PostgreSQL. The platforms that had received explicit testing at
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the time of release are listed in the installation instructions. 1.4)
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What non-Unix ports are available? *Client* It is possible to compile
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the /libpq/ C library, psql, and other interfaces and client
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applications to run on MS Windows platforms. In this case, the client
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is running on MS Windows, and communicates via TCP/IP to a server
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running on one of our supported Unix platforms. A file /win32.mak/ is
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included in the distribution for making a Win32 /libpq/ library and
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/psql/. PostgreSQL also communicates with ODBC clients. *Server* The
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database server can run on Windows NT and Win2k using Cygwin, the
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Cygnus Unix/NT porting library. See /pgsql/doc/FAQ_MSWIN/ in the
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distribution or the MS Windows FAQ at
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http://www.PostgreSQL.org/docs/faqs/text/FAQ_MSWIN
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www.PostgreSQL.org/docs/faqs/text/FAQ_MSWIN>. A native port to MS Win
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NT/2000/XP is currently being worked on. For more details on the
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current status of PostgreSQL on Windows see
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http://techdocs.postgresql.org/guides/Windows
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techdocs.postgresql.org/guides/Windows>. There is also a Novell
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Netware 6 port at http://forge.novell.com. 1.5) Where can I get
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PostgreSQL? The primary anonymous ftp site for PostgreSQL is
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ftp://ftp.PostgreSQL.org/pub. For mirror sites, see our main web site.
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1.6) Where can I get support? The main mailing list is:
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pgsql-general@PostgreSQL.org . It is available for discussion of
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matters pertaining to PostgreSQL. To subscribe, send mail with the
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following lines in the body (not the subject line): subscribe end to
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pgsql-general-request@PostgreSQL.org . There is also a digest list
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available. To subscribe to this list, send email to:
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pgsql-general-digest-request@PostgreSQL.org with a body of: subscribe
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end Digests are sent out to members of this list whenever the main
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list has received around 30k of messages. The bugs mailing list is
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available. To subscribe to this list, send email to
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pgsql-bugs-request@PostgreSQL.org with a body of: subscribe end There
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is also a developers discussion mailing list available. To subscribe
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to this list, send email to pgsql-hackers-request@PostgreSQL.org with
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a body of: subscribe end Additional mailing lists and information
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about PostgreSQL can be found via the PostgreSQL WWW home page at:
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http://www.PostgreSQL.org There is also an IRC channel on EFNet and
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OpenProjects, channel /#PostgreSQL/. I use the Unix command |irc -c
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'#PostgreSQL' "$USER" irc.phoenix.net.| A list of commercial support
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companies is available at
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http://www.ca.PostgreSQL.org/users-lounge/commercial-support.html.
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1.7) What is the latest release? The latest release of PostgreSQL is
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version 7.3.3. We plan to have major releases every four months. 1.8)
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What documentation is available? Several manuals, manual pages, and
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some small test examples are included in the distribution. See the
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//doc/ directory. You can also browse the manuals online at
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http://www.PostgreSQL.org/docs. There are two PostgreSQL books
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available online at http://www.PostgreSQL.org/docs/awbook.html and
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http://www.commandprompt.com/ppbook/. There is a list of PostgreSQL
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books available for purchase at http://www.ca.PostgreSQL.org/books/.
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There is also a collection of PostgreSQL technical articles at
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http://techdocs.PostgreSQL.org/. /psql/ has some nice \d commands to
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show information about types, operators, functions, aggregates, etc.
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Our web site contains even more documentation. 1.9) How do I find out
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about known bugs or missing features? PostgreSQL supports an extended
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subset of SQL-92. See our TODO developer.PostgreSQL.org/todo.php> list
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for known bugs, missing features, and future plans. 1.10) How can I
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learn SQL? The PostgreSQL book at
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http://www.PostgreSQL.org/docs/awbook.html teaches SQL. There is
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another PostgreSQL book at http://www.commandprompt.com/ppbook.
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www.commandprompt.com/ppbook/> There is a nice tutorial at
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http://www.intermedia.net/support/sql/sqltut.shtm,
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www.intermedia.net/support/sql/sqltut.shtm> at
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http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,
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ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM> and at
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http://sqlcourse.com. sqlcourse.com/> Another one is "Teach Yourself
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SQL in 21 Days, Second Edition" at
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http://members.tripod.com/er4ebus/sql/index.htm Many of our users like
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/The Practical SQL Handbook/, Bowman, Judith S., et al.,
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Addison-Wesley. Others like /The Complete Reference SQL/, Groff et
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al., McGraw-Hill. 1.11) Is PostgreSQL Y2K compliant? Yes, we easily
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handle dates past the year 2000 AD, and before 2000 BC. 1.12) How do I
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join the development team? First, download the latest source and read
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the PostgreSQL Developers documentation on our web site, or in the
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distribution. Second, subscribe to the /pgsql-hackers/ and
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/pgsql-patches/ mailing lists. Third, submit high quality patches to
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pgsql-patches. There are about a dozen people who have commit
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privileges to the PostgreSQL CVS archive. They each have submitted so
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many high-quality patches that it was impossible for the existing
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committers to keep up, and we had confidence that patches they
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committed were of high quality. 1.13) How do I submit a bug report?
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Please visit the PostgreSQL BugTool page at
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http://www.PostgreSQL.org/bugs/bugs.php, which gives guidelines and
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directions on how to submit a bug report. Also check out our ftp site
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ftp://ftp.PostgreSQL.org/pub to see if there is a more recent
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PostgreSQL version or patches. 1.14) How does PostgreSQL compare to
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other DBMSs? There are several ways of measuring software: features,
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performance, reliability, support, and price. *Features* PostgreSQL
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has most features present in large commercial DBMSs, like
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transactions, subselects, triggers, views, foreign key referential
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integrity, and sophisticated locking. We have some features they do
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not have, like user-defined types, inheritance, rules, and
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multi-version concurrency control to reduce lock contention.
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*Performance* PostgreSQL has performance similar to other commercial
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and open source databases. it is faster for some things, slower for
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others. In comparison to MySQL or leaner database systems, we are
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slower on inserts/updates because of transaction overhead. Of course,
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MySQL does not have any of the features mentioned in the /Features/
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section above. We are built for reliability and features, though we
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continue to improve performance in every release. There is an
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interesting Web page comparing PostgreSQL to MySQL at
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http://openacs.org/philosophy/why-not-mysql.html
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openacs.org/philosophy/why-not-mysql.html> *Reliability* We realize
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that a DBMS must be reliable, or it is worthless. We strive to release
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well-tested, stable code that has a minimum of bugs. Each release has
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at least one month of beta testing, and our release history shows that
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we can provide stable, solid releases that are ready for production
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use. We believe we compare favorably to other database software in
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this area. *Support* Our mailing lists provide contact with a large
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group of developers and users to help resolve any problems
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encountered. While we cannot guarantee a fix, commercial DBMSs do not
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always supply a fix either. Direct access to developers, the user
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community, manuals, and the source code often make PostgreSQL support
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superior to other DBMSs. There is commercial per-incident support
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available for those who need it. (See FAQ section 1.6 <#1.6>.) *Price*
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We are free for all use, both commercial and non-commercial. You can
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add our code to your product with no limitations, except those
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outlined in our BSD-style license stated above. 1.15) How can I
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financially assist PostgreSQL? PostgreSQL has had a first-class
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infrastructure since we started in 1996. This is all thanks to Marc
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Fournier, who has created and managed this infrastructure over the
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years. Quality infrastructure is very important to an open-source
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project. It prevents disruptions that can greatly delay forward
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movement of the project. Of course, this infrastructure is not cheap.
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There are a variety of monthly and one-time expenses that are required
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to keep it going. If you or your company has money it can donate to
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help fund this effort, please go to http://store.pgsql.com/shopping/
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and make a donation. Although the web page mentions PostgreSQL, Inc,
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the "contributions" item is solely to support the PostgreSQL project
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and does not fund any specific company. If you prefer, you can also
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send a check to the contact address.
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----------------------------------------------------------------------
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-- User Client Questions 2.1) Are there ODBC drivers for PostgreSQL?
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There are two ODBC drivers available, PsqlODBC and OpenLink ODBC. You
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can download PsqlODBC from
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http://gborg.postgresql.org/project/psqlodbc/projdisplay.php
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gborg.postgresql.org/project/psqlodbc/projdisplay.php>. OpenLink ODBC
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can be gotten from http://www.openlinksw.com www.openlinksw.com/>. It
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works with their standard ODBC client software so you'll have
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PostgreSQL ODBC available on every client platform they support (Win,
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Mac, Unix, VMS). They will probably be selling this product to people
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who need commercial-quality support, but a freeware version will
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always be available. Please send questions to
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postgres95@openlink.co.uk . 2.2) What tools are available for using
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PostgreSQL with Web pages? A nice introduction to Database-backed Web
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pages can be seen at: http://www.webreview.com For Web integration,
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PHP is an excellent interface. It is at http://www.php.net. For
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complex cases, many use the Perl interface and CGI.pm or mod_perl.
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2.3) Does PostgreSQL have a graphical user interface? Yes, there are
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several graphical interfaces to PostgreSQL available. These include
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PgAccess http://www.pgaccess.org www.pgaccess.org>), PgAdmin II
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(http://www.pgadmin.org, Win32-only), RHDB Admin
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(http://sources.redhat.com/rhdb/ ) and Rekall (
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http://www.thekompany.com/products/rekall/
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www.thekompany.com/products/rekall/>, proprietary). There is also
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PHPPgAdmin ( http://phppgadmin.sourceforge.net/
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phppgadmin.sourceforge.net/>), a web-based interface to PostgreSQL.
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See http://techdocs.postgresql.org/guides/GUITools for a more detailed
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list. 2.4) What languages are able to communicate with PostgreSQL?
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Most popular programming languages contain an interface to PostgreSQL.
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Check your programming language's list of extension modules. The
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following interfaces are included in the PostgreSQL distribution: * C
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(libpq) * Embedded C (ecpg) * Java (jdbc) * Python (PyGreSQL) * TCL
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(libpgtcl) Additional interfaces are available at
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http://gborg.postgresql.org in the /Drivers/Interfaces/ section.
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----------------------------------------------------------------------
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-- Administrative Questions 3.1) How do I install PostgreSQL somewhere
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other than //usr/local/pgsql/? Specify the /--prefix/ option when
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running /configure/. 3.2) When I start /postmaster/, I get a /Bad
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System Call/ or core dumped message. Why? It could be a variety of
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problems, but first check to see that you have System V extensions
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installed in your kernel. PostgreSQL requires kernel support for
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shared memory and semaphores. 3.3) When I try to start /postmaster/, I
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get /IpcMemoryCreate/ errors. Why? You either do not have shared
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memory configured properly in your kernel or you need to enlarge the
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shared memory available in the kernel. The exact amount you need
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depends on your architecture and how many buffers and backend
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processes you configure for /postmaster/. For most systems, with
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default numbers of buffers and processes, you need a minimum of ~1 MB.
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See the PostgreSQL Administrator's Guide
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www.PostgreSQL.org/docs/view.php?version=current&idoc=1&file=kernel-re
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sources.html> for more detailed information about shared memory and
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semaphores. 3.4) When I try to start /postmaster/, I get
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/IpcSemaphoreCreate/ errors. Why? If the error message is
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/IpcSemaphoreCreate: semget failed (No space left on device)/ then
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your kernel is not configured with enough semaphores. Postgres needs
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one semaphore per potential backend process. A temporary solution is
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to start /postmaster/ with a smaller limit on the number of backend
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processes. Use /-N/ with a parameter less than the default of 32. A
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more permanent solution is to increase your kernel's SEMMNS and SEMMNI
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parameters. Inoperative semaphores can also cause crashes during heavy
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database access. If the error message is something else, you might not
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have semaphore support configured in your kernel at all. See the
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PostgreSQL Administrator's Guide for more detailed information about
|
||
shared memory and semaphores. 3.5) How do I control connections from
|
||
other hosts? By default, PostgreSQL only allows connections from the
|
||
local machine using Unix domain sockets. Other machines will not be
|
||
able to connect unless you add the /-i/ flag to /postmaster/, *and*
|
||
enable host-based authentication by modifying the file
|
||
/$PGDATA/pg_hba.conf/ accordingly. This will allow TCP/IP connections.
|
||
3.6) How do I tune the database engine for better performance?
|
||
Certainly, indexes can speed up queries. The EXPLAIN command allows
|
||
you to see how PostgreSQL is interpreting your query, and which
|
||
indexes are being used. If you are doing many INSERTs, consider doing
|
||
them in a large batch using the COPY command. This is much faster than
|
||
individual INSERTS. Second, statements not in a BEGIN WORK/COMMIT
|
||
transaction block are considered to be in their own transaction.
|
||
Consider performing several statements in a single transaction block.
|
||
This reduces the transaction overhead. Also, consider dropping and
|
||
recreating indexes when making large data changes. There are several
|
||
tuning options. You can disable /fsync()/ by starting /postmaster/
|
||
with a /-o -F/ option. This will prevent /fsync()/s from flushing to
|
||
disk after every transaction. You can also use the /postmaster/ /-B/
|
||
option to increase the number of shared memory buffers used by the
|
||
backend processes. If you make this parameter too high, the
|
||
/postmaster/ may not start because you have exceeded your kernel's
|
||
limit on shared memory space. Each buffer is 8K and the default is 64
|
||
buffers. You can also use the backend /-S/ option to increase the
|
||
maximum amount of memory used by the backend process for temporary
|
||
sorts. The /-S/ value is measured in kilobytes, and the default is 512
|
||
(i.e. 512K). You can also use the CLUSTER command to group data in
|
||
tables to match an index. See the CLUSTER manual page for more
|
||
details. 3.7) What debugging features are available? PostgreSQL has
|
||
several features that report status information that can be valuable
|
||
for debugging purposes. First, by running /configure/ with the
|
||
--enable-cassert option, many /assert()/s monitor the progress of the
|
||
backend and halt the program when something unexpected occurs. Both
|
||
/postmaster/ and /postgres/ have several debug options available.
|
||
First, whenever you start /postmaster/, make sure you send the
|
||
standard output and error to a log file, like: cd /usr/local/pgsql
|
||
./bin/postmaster >server.log 2>&1 & This will put a server.log file in
|
||
the top-level PostgreSQL directory. This file contains useful
|
||
information about problems or errors encountered by the server.
|
||
/Postmaster/ has a /-d/ option that allows even more detailed
|
||
information to be reported. The /-d/ option takes a number that
|
||
specifies the debug level. Be warned that high debug level values
|
||
generate large log files. If /postmaster/ is not running, you can
|
||
actually run the /postgres/ backend from the command line, and type
|
||
your SQL statement directly. This is recommended *only* for debugging
|
||
purposes. Note that a newline terminates the query, not a semicolon.
|
||
If you have compiled with debugging symbols, you can use a debugger to
|
||
see what is happening. Because the backend was not started from
|
||
/postmaster/, it is not running in an identical environment and
|
||
locking/backend interaction problems may not be duplicated. If
|
||
/postmaster/ is running, start /psql/ in one window, then find the PID
|
||
of the /postgres/ process used by /psql/. Use a debugger to attach to
|
||
the /postgres/ PID. You can set breakpoints in the debugger and issue
|
||
queries from /psql/. If you are debugging /postgres/ startup, you can
|
||
set PGOPTIONS="-W n", then start /psql/. This will cause startup to
|
||
delay for /n/ seconds so you can attach to the process with the
|
||
debugger, set any breakpoints, and continue through the startup
|
||
sequence. The /postgres/ program has /-s, -A/, and /-t/ options that
|
||
can be very useful for debugging and performance measurements. You can
|
||
also compile with profiling to see what functions are taking execution
|
||
time. The backend profile files will be deposited in the
|
||
/pgsql/data/base/dbname/ directory. The client profile file will be
|
||
put in the client's current directory. Linux requires a compile with
|
||
/-DLINUX_PROFILE/ for proper profiling. 3.8) Why do I get /"Sorry, too
|
||
many clients"/ when trying to connect? You need to increase
|
||
/postmaster/'s limit on how many concurrent backend processes it can
|
||
start. The default limit is 32 processes. You can increase it by
|
||
restarting /postmaster/ with a suitable /-N/ value or modifying
|
||
/postgresql.conf/. Note that if you make /-N/ larger than 32, you must
|
||
also increase /-B/ beyond its default of 64; /-B/ must be at least
|
||
twice /-N/, and probably should be more than that for best
|
||
performance. For large numbers of backend processes, you are also
|
||
likely to find that you need to increase various Unix kernel
|
||
configuration parameters. Things to check include the maximum size of
|
||
shared memory blocks, SHMMAX; the maximum number of semaphores, SEMMNS
|
||
and SEMMNI; the maximum number of processes, NPROC; the maximum number
|
||
of processes per user, MAXUPRC; and the maximum number of open files,
|
||
NFILE and NINODE. The reason that PostgreSQL has a limit on the number
|
||
of allowed backend processes is so your system won't run out of
|
||
resources. 3.9) What is in the /pgsql_tmp/ directory? This directory
|
||
contains temporary files generated by the query executor. For example,
|
||
if a sort needs to be done to satisfy an ORDER BY and the sort
|
||
requires more space than the backend's /-S/ parameter allows, then
|
||
temporary files are created here to hold the extra data. The temporary
|
||
files are usually deleted automatically, but might remain if a backend
|
||
crashes during a sort. A stop and restart of the /postmaster/ will
|
||
remove files from those directories. 3.10) Why do I need to do a dump
|
||
and restore to upgrade between major PostgreSQL releases? The
|
||
PostgreSQL team makes only small changes between minor releases, so
|
||
upgrading from 7.2 to 7.2.1 does not require a dump and restore.
|
||
However, major releases (e.g. from 7.2 to 7.3) often change the
|
||
internal format of system tables and data files. These changes are
|
||
often complex, so we don't maintain backward compatability for data
|
||
files. A dump outputs data in a generic format that can then be loaded
|
||
in using the new internal format. In releases where the on-disk format
|
||
does not change, the /pg_upgrade/ script can be used to upgrade
|
||
without a dump/restore. The release notes mention whether /pg_upgrade/
|
||
is available for the release.
|
||
----------------------------------------------------------------------
|
||
-- Operational Questions 4.1) What is the difference between binary
|
||
cursors and normal cursors? See the DECLARE manual page for a
|
||
description. 4.2) How do I SELECT only the first few rows of a query?
|
||
See the FETCH manual page, or use SELECT ... LIMIT.... The entire
|
||
query may have to be evaluated, even if you only want the first few
|
||
rows. Consider using a query that has an ORDER BY. If there is an
|
||
index that matches the ORDER BY, PostgreSQL may be able to evaluate
|
||
only the first few records requested, or the entire query may have to
|
||
be evaluated until the desired rows have been generated. 4.3) How do I
|
||
get a list of tables or other things I can see in /psql/? You can read
|
||
the source code for /psql/ in file /pgsql/src/bin/psql/describe.c/. It
|
||
contains SQL commands that generate the output for psql's backslash
|
||
commands. You can also start /psql/ with the /-E/ option so it will
|
||
print out the queries it uses to execute the commands you give. 4.4)
|
||
How do you remove a column from a table? This functionality was added
|
||
in release 7.3 with ALTER TABLE DROP COLUMN. In earlier versions, you
|
||
can do this: BEGIN; LOCK TABLE old_table; SELECT ... -- select all
|
||
columns but the one you want to remove INTO TABLE new_table FROM
|
||
old_table; DROP TABLE old_table; ALTER TABLE new_table RENAME TO
|
||
old_table; COMMIT; 4.5) What is the maximum size for a row, a table,
|
||
and a database? These are the limits: Maximum size for a database?
|
||
unlimited (4 TB databases exist) Maximum size for a table? 16 TB
|
||
Maximum size for a row? 1.6TB Maximum size for a field? 1 GB Maximum
|
||
number of rows in a table? unlimited Maximum number of columns in a
|
||
table? 250-1600 depending on column types Maximum number of indexes on
|
||
a table? unlimited Of course, these are not actually unlimited, but
|
||
limited to available disk space and memory/swap space. Performance may
|
||
suffer when these values get unusually large. The maximum table size
|
||
of 16 TB does not require large file support from the operating
|
||
system. Large tables are stored as multiple 1 GB files so file system
|
||
size limits are not important. The maximum table size and maximum
|
||
number of columns can be increased if the default block size is
|
||
increased to 32k. 4.6) How much database disk space is required to
|
||
store data from a typical text file? A PostgreSQL database may require
|
||
up to five times the disk space to store data from a text file. As an
|
||
example, consider a file of 100,000 lines with an integer and text
|
||
description on each line. Suppose the text string avergages twenty
|
||
bytes in length. The flat file would be 2.8 MB. The size of the
|
||
PostgreSQL database file containing this data can be estimated as 6.4
|
||
MB: 36 bytes: each row header (approximate) 24 bytes: one int field
|
||
and one text field + 4 bytes: pointer on page to tuple
|
||
---------------------------------------- 64 bytes per row The data
|
||
page size in PostgreSQL is 8192 bytes (8 KB), so: 8192 bytes per page
|
||
------------------- = 128 rows per database page (rounded down) 64
|
||
bytes per row 100000 data rows -------------------- = 782 database
|
||
pages (rounded up) 128 rows per page 782 database pages * 8192 bytes
|
||
per page = 6,406,144 bytes (6.4 MB) Indexes do not require as much
|
||
overhead, but do contain the data that is being indexed, so they can
|
||
be large also. NULLs are stored in bitmaps, so they use very little
|
||
space. 4.7) How do I find out what tables, indexes, databases, and
|
||
users are defined? /psql/ has a variety of backslash commands to show
|
||
such information. Use \? to see them. There are also system tables
|
||
beginning with /pg_/ that describe these too. Also, /psql -l/ will
|
||
list all databases. Also try the file
|
||
/pgsql/src/tutorial/syscat.source/. It illustrates many of the SELECTs
|
||
needed to get information from the database system tables. 4.8) My
|
||
queries are slow or don't make use of the indexes. Why? Indexes are
|
||
not automatically used by every query. Indexes are only used if the
|
||
table is larger than a minimum size, and the query selects only a
|
||
small percentage of the rows in the table. This is because the random
|
||
disk access caused by an index scan can be slower than a straight read
|
||
through the table, or sequential scan. To determine if an index should
|
||
be used, PostgreSQL must have statistics about the table. These
|
||
statistics are collected using VACUUM ANALYZE, or simply ANALYZE.
|
||
Using statistics, the optimizer knows how many rows are in the table,
|
||
and can better determine if indexes should be used. Statistics are
|
||
also valuable in determining optimal join order and join methods.
|
||
Statistics collection should be performed periodically as the contents
|
||
of the table change. Indexes are normally not used for ORDER BY or to
|
||
perform joins. A sequential scan followed by an explicit sort is
|
||
usually faster than an index scan of a large table. However, LIMIT
|
||
combined with ORDER BY often will use an index because only a small
|
||
portion of the table is returned. In fact, though MAX() and MIN()
|
||
don't use indexes, it is possible to retrieve such values using an
|
||
index with ORDER BY and LIMIT: SELECT col FROM tab ORDER BY col [ DESC
|
||
] LIMIT 1; If you believe the optimizer is incorrect in choosing a
|
||
sequential scan, use |SET enable_seqscan TO 'off'| and run tests to
|
||
see if an index scan is indeed faster. When using wild-card operators
|
||
such as LIKE or /~/, indexes can only be used in certain
|
||
circumstances: * The beginning of the search string must be anchored
|
||
to the start of the string, i.e. o LIKE patterns must not start with
|
||
/%/. o /~/ (regular expression) patterns must start with /^/. * The
|
||
search string can not start with a character class, e.g. [a-e]. *
|
||
Case-insensitive searches such as ILIKE and /~*/ do not utilise
|
||
indexes. Instead, use functional indexes, which are described in
|
||
section 4.12 <#4.12>. * The default /C/ locale must be used during
|
||
/initdb/. 4.9) How do I see how the query optimizer is evaluating my
|
||
query? See the EXPLAIN manual page. 4.10) What is an R-tree index? An
|
||
R-tree index is used for indexing spatial data. A hash index can't
|
||
handle range searches. A B-tree index only handles range searches in a
|
||
single dimension. R-trees can handle multi-dimensional data. For
|
||
example, if an R-tree index can be built on an attribute of type
|
||
/point/, the system can more efficiently answer queries such as
|
||
"select all points within a bounding rectangle." The canonical paper
|
||
that describes the original R-tree design is: Guttman, A. "R-trees: A
|
||
Dynamic Index Structure for Spatial Searching." Proceedings of the
|
||
1984 ACM SIGMOD Int'l Conf on Mgmt of Data, 45-57. You can also find
|
||
this paper in Stonebraker's "Readings in Database Systems". Built-in
|
||
R-trees can handle polygons and boxes. In theory, R-trees can be
|
||
extended to handle higher number of dimensions. In practice, extending
|
||
R-trees requires a bit of work and we don't currently have any
|
||
documentation on how to do it. 4.11) What is the Genetic Query
|
||
Optimizer? The GEQO module speeds query optimization when joining many
|
||
tables by means of a Genetic Algorithm (GA). It allows the handling of
|
||
large join queries through nonexhaustive search. 4.12) How do I
|
||
perform regular expression searches and case-insensitive regular
|
||
expression searches? How do I use an index for case-insensitive
|
||
searches? The /~/ operator does regular expression matching, and /~*/
|
||
does case-insensitive regular expression matching. The
|
||
case-insensitive variant of LIKE is called ILIKE. Case-insensitive
|
||
equality comparisons are normally expressed as: SELECT * FROM tab
|
||
WHERE lower(col) = 'abc'; This will not use an standard index.
|
||
However, if you create a functional index, it will be used: CREATE
|
||
INDEX tabindex ON tab (lower(col)); 4.13) In a query, how do I detect
|
||
if a field is NULL? You test the column with IS NULL and IS NOT NULL.
|
||
4.14) What is the difference between the various character types? Type
|
||
Internal Name Notes --------------------------------------------------
|
||
VARCHAR(n) varchar size specifies maximum length, no padding CHAR(n)
|
||
bpchar blank padded to the specified fixed length TEXT text no
|
||
specific upper limit on length BYTEA bytea variable-length byte array
|
||
(null-byte safe) "char" char one character You will see the internal
|
||
name when examining system catalogs and in some error messages. The
|
||
first four types above are "varlena" types (i.e., the first four bytes
|
||
on disk are the length, followed by the data). Thus the actual space
|
||
used is slightly greater than the declared size. However, these data
|
||
types are also subject to compression or being stored out-of-line by
|
||
TOAST, so the space on disk might also be less than expected.
|
||
VARCHAR(n) is best when storing variable-length strings and it limits
|
||
how long a string can be. TEXT is for strings of unlimited length,
|
||
with a maximum of one gigabyte. CHAR(n) is for storing strings that
|
||
are all the same length. CHAR(n) pads with blanks to the specified
|
||
length, while VARCHAR(n) only stores the characters supplied. BYTEA is
|
||
for storing binary data, particularly values that include NULL bytes.
|
||
All the types described here have similar performance characteristics.
|
||
4.15.1) How do I create a serial/auto-incrementing field? PostgreSQL
|
||
supports a SERIAL data type. It auto-creates a sequence and index on
|
||
the column. For example, this: CREATE TABLE person ( id SERIAL, name
|
||
TEXT ); is automatically translated into this: CREATE SEQUENCE
|
||
person_id_seq; CREATE TABLE person ( id INT4 NOT NULL DEFAULT
|
||
nextval('person_id_seq'), name TEXT ); CREATE UNIQUE INDEX
|
||
person_id_key ON person ( id ); See the /create_sequence/ manual page
|
||
for more information about sequences. You can also use each row's
|
||
/OID/ field as a unique value. However, if you need to dump and reload
|
||
the database, you need to use /pg_dump/'s /-o/ option or COPY WITH
|
||
OIDS option to preserve the OIDs. 4.15.2) How do I get the value of a
|
||
SERIAL insert? One approach is to retrieve the next SERIAL value from
|
||
the sequence object with the /nextval()/ function /before/ inserting
|
||
and then insert it explicitly. Using the example table in 4.15.1
|
||
<#4.15.1>, an example in a pseudo-language would look like this:
|
||
new_id = execute("SELECT nextval('person_id_seq')"); execute("INSERT
|
||
INTO person (id, name) VALUES (new_id, 'Blaise Pascal')"); You would
|
||
then also have the new value stored in |new_id| for use in other
|
||
queries (e.g., as a foreign key to the |person| table). Note that the
|
||
name of the automatically created SEQUENCE object will be named
|
||
__/seq/, where /table/ and /serialcolumn/ are the names of your table
|
||
and your SERIAL column, respectively. Alternatively, you could
|
||
retrieve the assigned SERIAL value with the /currval()/ function
|
||
/after/ it was inserted by default, e.g., execute("INSERT INTO person
|
||
(name) VALUES ('Blaise Pascal')"); new_id = execute("SELECT
|
||
currval('person_id_seq')"); Finally, you could use the OID <#4.16>
|
||
returned from the INSERT statement to look up the default value,
|
||
though this is probably the least portable approach. In Perl, using
|
||
DBI with Edmund Mergl's DBD::Pg module, the oid value is made
|
||
available via /$sth->{pg_oid_status}/ after /$sth->execute()/. 4.15.3)
|
||
Don't /currval()/ and /nextval()/ lead to a race condition with other
|
||
users? No. /currval()/ returns the current value assigned by your
|
||
backend, not by all users. 4.15.4) Why aren't my sequence numbers
|
||
reused on transaction abort? Why are there gaps in the numbering of my
|
||
sequence/SERIAL column? To improve concurrency, sequence values are
|
||
given out to running transactions as needed and are not locked until
|
||
the transaction completes. This causes gaps in numbering from aborted
|
||
transactions. 4.16) What is an OID? What is a TID? OIDs are
|
||
PostgreSQL's answer to unique row ids. Every row that is created in
|
||
PostgreSQL gets a unique OID. All OIDs generated during /initdb/ are
|
||
less than 16384 (from /include/access/transam.h/). All user-created
|
||
OIDs are equal to or greater than this. By default, all these OIDs are
|
||
unique not only within a table or database, but unique within the
|
||
entire PostgreSQL installation. PostgreSQL uses OIDs in its internal
|
||
system tables to link rows between tables. These OIDs can be used to
|
||
identify specific user rows and used in joins. It is recommended you
|
||
use column type OID to store OID values. You can create an index on
|
||
the OID field for faster access. OIDs are assigned to all new rows
|
||
from a central area that is used by all databases. If you want to
|
||
change the OID to something else, or if you want to make a copy of the
|
||
table, with the original OIDs, there is no reason you can't do it:
|
||
CREATE TABLE new_table(old_oid oid, mycol int); SELECT old_oid, mycol
|
||
INTO new FROM old; COPY new TO '/tmp/pgtable'; DELETE FROM new; COPY
|
||
new WITH OIDS FROM '/tmp/pgtable'; OIDs are stored as 4-byte integers,
|
||
and will overflow at 4 billion. No one has reported this ever
|
||
happening, and we plan to have the limit removed before anyone does.
|
||
TIDs are used to identify specific physical rows with block and offset
|
||
values. TIDs change after rows are modified or reloaded. They are used
|
||
by index entries to point to physical rows. 4.17) What is the meaning
|
||
of some of the terms used in PostgreSQL? Some of the source code and
|
||
older documentation use terms that have more common usage. Here are
|
||
some: * table, relation, class * row, record, tuple * column, field,
|
||
attribute * retrieve, select * replace, update * append, insert * OID,
|
||
serial value * portal, cursor * range variable, table name, table
|
||
alias A list of general database terms can be found at:
|
||
http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary
|
||
/glossary.html 4.18) Why do I get the error /"ERROR: Memory exhausted
|
||
in AllocSetAlloc()"/? You probably have run out of virtual memory on
|
||
your system, or your kernel has a low limit for certain resources. Try
|
||
this before starting /postmaster/: ulimit -d 262144 limit datasize
|
||
256m Depending on your shell, only one of these may succeed, but it
|
||
will set your process data segment limit much higher and perhaps allow
|
||
the query to complete. This command applies to the current process,
|
||
and all subprocesses created after the command is run. If you are
|
||
having a problem with the SQL client because the backend is returning
|
||
too much data, try it before starting the client. 4.19) How do I tell
|
||
what PostgreSQL version I am running? From /psql/, type |SELECT
|
||
version();| 4.20) Why does my large-object operations get /"invalid
|
||
large obj descriptor"/? You need to put |BEGIN WORK| and |COMMIT|
|
||
around any use of a large object handle, that is, surrounding
|
||
|lo_open| ... |lo_close.| Currently PostgreSQL enforces the rule by
|
||
closing large object handles at transaction commit. So the first
|
||
attempt to do anything with the handle will draw /invalid large obj
|
||
descriptor/. So code that used to work (at least most of the time)
|
||
will now generate that error message if you fail to use a transaction.
|
||
If you are using a client interface like ODBC you may need to set
|
||
|auto-commit off.| 4.21) How do I create a column that will default to
|
||
the current time? Use /CURRENT_TIMESTAMP/: |CREATE TABLE test (x int,
|
||
modtime timestamp DEFAULT CURRENT_TIMESTAMP ); | 4.22) Why are my
|
||
subqueries using |IN| so slow? Currently, we join subqueries to outer
|
||
queries by sequentially scanning the result of the subquery for each
|
||
row of the outer query. If the subquery returns only a few rows and
|
||
the outer query returns many rows, |IN| is fastest. To speed up other
|
||
queries, replace |IN| with |EXISTS|: SELECT * FROM tab WHERE col IN
|
||
(SELECT subcol FROM subtab); to: SELECT * FROM tab WHERE EXISTS
|
||
(SELECT subcol FROM subtab WHERE subcol = col); For this to be fast,
|
||
|subcol| should be an indexed column. This preformance problem will be
|
||
fixed in 7.4. 4.23) How do I perform an outer join? PostgreSQL
|
||
supports outer joins using the SQL standard syntax. Here are two
|
||
examples: SELECT * FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col); or
|
||
SELECT * FROM t1 LEFT OUTER JOIN t2 USING (col); These identical
|
||
queries join t1.col to t2.col, and also return any unjoined rows in t1
|
||
(those with no match in t2). A RIGHT join would add unjoined rows of
|
||
t2. A FULL join would return the matched rows plus all unjoined rows
|
||
from t1 and t2. The word OUTER is optional and is assumed in LEFT,
|
||
RIGHT, and FULL joins. Ordinary joins are called INNER joins. In
|
||
previous releases, outer joins can be simulated using UNION and NOT
|
||
IN. For example, when joining /tab1/ and /tab2/, the following query
|
||
does an /outer/ join of the two tables: SELECT tab1.col1, tab2.col2
|
||
FROM tab1, tab2 WHERE tab1.col1 = tab2.col1 UNION ALL SELECT
|
||
tab1.col1, NULL FROM tab1 WHERE tab1.col1 NOT IN (SELECT tab2.col1
|
||
FROM tab2) ORDER BY col1 4.24) How do I perform queries using multiple
|
||
databases? There is no way to query a database other than the current
|
||
one. Because PostgreSQL loads database-specific system catalogs, it is
|
||
uncertain how a cross-database query should even behave.
|
||
/contrib/dblink/ allows cross-database queries using function calls.
|
||
Of course, a client can make simultaneous connections to different
|
||
databases and merge the results on the client side. 4.25) How do I
|
||
return multiple rows or columns from a function? In 7.3, you can
|
||
easily return multiple rows or columns from a function,
|
||
http://techdocs.postgresql.org/guides/SetReturningFunctions
|
||
techdocs.postgresql.org/guides/SetReturningFunctions>. 4.26) Why can't
|
||
I reliably create/drop temporary tables in PL/PgSQL functions?
|
||
PL/PgSQL caches function contents, and an unfortunate side effect is
|
||
that if a PL/PgSQL function accesses a temporary table, and that table
|
||
is later dropped and recreated, and the function called again, the
|
||
function will fail because the cached function contents still point to
|
||
the old temporary table. The solution is to use EXECUTE for temporary
|
||
table access in PL/PgSQL. This will cause the query to be reparsed
|
||
every time. 4.27) What replication options are available? There are
|
||
several master/slave replication options available. These allow only
|
||
the master to make database changes and the slave can only do database
|
||
reads. The bottom of
|
||
http://gborg.PostgreSQL.org/genpage?replication_research
|
||
gborg.PostgreSQL.org/genpage?replication_research> lists them. A
|
||
multi-master replication solution is being worked on at
|
||
http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php.
|
||
4.28) What encryption options are available? * /contrib/pgcrypto/
|
||
contains many encryption functions for use in SQL queries. * The only
|
||
way to encrypt transmission from the client to the server is by using
|
||
/hostssl/ in /pg_hba.conf/. * Database user passwords are
|
||
automatically encrypted when stored in version 7.3. In previous
|
||
versions, you must enable the option /PASSWORD_ENCRYPTION/ in
|
||
/postgresql.conf/. * The server can run using an encrypted file
|
||
system.
|
||
----------------------------------------------------------------------
|
||
-- Extending PostgreSQL 5.1) I wrote a user-defined function. When I
|
||
run it in /psql/, why does it dump core? The problem could be a number
|
||
of things. Try testing your user-defined function in a stand-alone
|
||
test program first. 5.2) How can I contribute some nifty new types and
|
||
functions to PostgreSQL? Send your extensions to the /pgsql-hackers/
|
||
mailing list, and they will eventually end up in the /contrib//
|
||
subdirectory. 5.3) How do I write a C function to return a tuple? In
|
||
versions of PostgreSQL beginning with 7.3, table-returning functions
|
||
are fully supported in C, PL/PgSQL, and SQL. See the Programmer's
|
||
Guide for more information. An example of a table-returning function
|
||
defined in C can be found in /contrib/tablefunc/. 5.4) I have changed
|
||
a source file. Why does the recompile not see the change? The
|
||
/Makefiles/ do not have the proper dependencies for include files. You
|
||
have to do a /make clean/ and then another /make/. If you are using
|
||
GCC you can use the /--enable-depend/ option of /configure/ to have
|
||
the compiler compute the dependencies automatically.
|