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1141 lines
38 KiB
Plaintext
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html>
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FREQUENTLY ASKED QUESTIONS (FAQ) FOR POSTGRESQL
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Last updated: Thu Dec 12 21:30:37 EST 1996
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Version: 6.0
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Current maintainer: Bruce Momjian (maillist@candle.pha.pa.us)
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The most recent version of this document can be viewed at the
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postgreSQL Web site, http://postgreSQL.org.
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Linux-specific questions are answered in
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http://postgreSQL.org/docs/FAQ-Linux.phtml.
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Irix-specific questions are answered in
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http://postgreSQL.org/docs/FAQ-Irix.phtml.
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Changes in this version (* = modified, + = new):
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*1.1) What is PostgreSQL?
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*1.5) Support for PostgreSQL
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*1.6) Latest release of PostgreSQL
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*1.10 Does PostgreSQL work with databases from earlier versions of
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postgres?
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*3.1) How do I specify a KEY or other constraints on a column?
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*3.3) How do I define a unique indices?
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*3.11) Why doesn't the != operator work?
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*3.26) Why are my table files not getting any smaller after a delete?
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*3.28) I get the error 'default index class unsupported' when creating
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an index. How do I do it?
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*3.37) What is the time-warp feature and how does it relate to vacuum?
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*5.1) How do I make a bug report?
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_________________________________________________________________
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Questions answered:
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1) General questions
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1.1) What is PostgreSQL?
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1.2) What does PostgreSQL run on?
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1.3) Where can I get PostgreSQL?
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1.4) What's the copyright on PostgreSQL?
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1.5) Support for PostgreSQL
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1.6) Latest release of PostgreSQL
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1.7) Is there a commercial version of PostgreSQL?
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1.9) What version of SQL does PostgreSQL use?
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1.10) Does PostgreSQL work with databases from earlier versions of
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postgres?
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1.11) How many people use PostgreSQL?
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2) Installation questions
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2.1) initdb doesn't run
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2.2) when I start up the postmaster, I get "FindBackend: could not
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find a backend to execute..." "postmaster: could not find backend to
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execute..."
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2.3) The system seems to be confused about commas, decimal points, and
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date formats.
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2.4) How do I install PostgreSQL somewhere other than
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/usr/local/pgsql?
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2.5) The backend compiled successfully, but compiling libpq resulted
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in a complaint: "libpq/pqcomm.h" not found when compiling fe-auth.c.
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2.6) When I run postmaster, I get a Bad System Call core dumped
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message.
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2.7) When I try to start the postmaster, I get IpcMemoryCreate errors.
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2.8) I have changed a source file, but a recompile does not see the
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change.
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3) PostgreSQL Features questions
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3.1) How do I specify a KEY or other constraints on a column?
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3.2) Does PostgreSQL support nested subqueries?
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3.3) How do I define a unique indices?
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3.4) I've having a lot of problems using rules.
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3.5) I can't seem to write into the middle of large objects reliably.
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3.6) Does PostgreSQL have a graphical user interface? A report
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generator? A embedded query language interface?
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3.7) How can I write client applications to PostgreSQL?
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3.8) How do I prevent other hosts from accessing my PostgreSQL
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3.9) How do I set up a pg_group?
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3.10) What is the exact difference between binary cursors and normal
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cursors?
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3.11) Why doesn't the != operator work?
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3.12) What is a R-tree index and what is it used for?
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3.13) What is the maximum size for a tuple?
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3.14) I defined indices but my queries don't seem to make use of them.
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Why?
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3.15) Are there ODBC drivers for PostgreSQL?
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3.16) How do I use postgres for multi-dimensional indexing (> 2
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dimensions)?
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3.17) How do I do regular expression searches? case-insensitive regexp
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searching?
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3.18) I can't access the database as the 'root' user.
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3.19) I experienced a server crash during a vacuum. How do I remove
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the lock file?
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3.20) What is the difference between the various character types?
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3.21) In a query, how do I detect if a field is NULL?
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3.22) How do I see how the query optimizer is evaluating my query?
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3.23) How do I create a serial field?
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3.24) How do I create a multi-column index?
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3.25) What are the temp_XXX files in my database directory?
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3.26) Why are my table files not getting any smaller after a delete?
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3.27) Why can't I connect to my database from another machine?
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3.28) I get the error 'default index class unsupported' when creating
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an index. How do I do it?
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3.29) Why does creating an index crash the backend server?
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3.30) How do I specify a decimal constant as a float8, or a string as
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a text? Why am I getting poor precision?
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3.31) How do I find out what indexes or operations are defined in the
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database?
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3.32) My database is corrupt. I can't do anything. What should I do?
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3.33) Createdb, destroydb, createuser, destroyuser don't run. Why?
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3.34) Why does 'createuser' return 'unexpected last match in input()'?
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3.35) All my servers crash under concurrent table access. Why?
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3.36) What tools are available for hooking postgres to Web pages?
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3.37) What is the time-warp feature and how does it relate to vacuum?
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3.38) How do I tune the database engine for better performance?
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3.39) What debugging features are available in PostgreSQL?
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3.40) What is an oid? What is a tid?
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3.41) What is the meaning of some of the terms used in Postgres?
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4) Questions about extending PostgreSQL
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4.1) I wrote a user-defined function and when I run it in psql, it
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dumps core.
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4.2) I get messages of the type NOTICE:PortalHeapMemoryFree:
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0x402251d0
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4.3) I've written some nifty new types and functions for PostgreSQL.
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4.4) How do I write a C function to return a tuple?
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5) Bugs
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5.1) How do I make a bug report?
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_________________________________________________________________
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Section 1: General Questions
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1.1) What is PostgreSQL?
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PostgreSQL is an enhancement of the POSTGRES database management
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system, a next-generation DBMS research prototype. While PostgreSQL
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retains the powerful data model and rich data types of POSTGRES, it
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replaces the PostQuel query language with an extended subset of SQL.
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PostgreSQL is free and the complete source is available.
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PostgreSQL development is being performed by a team of Internet
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developers who all subscribe to the PostgreSQL development mailing
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list. The current coordinator is Marc G. Fournier
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(scrappy@postgreSQL.org). (See below on how to join). This team is
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now responsible for all current and future development of PostgreSQL.
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The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen. Many
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others have contributed to the porting, testing, debugging and
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enhancement of the code. The original Postgres code, from which
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PostgreSQL is derived, was the effort of many graduate students,
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undergraduate students, and staff programmers working under the
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direction of Professor Michael Stonebraker at the University of
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California, Berkeley.
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The original name of the software at Berkeley was Postgres. When SQL
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functionality was added in 1995, its name was changed to Postgres95.
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The name was changed at the end of 1996 to PostgreSQL.
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1.2) What does PostgreSQL run on?
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The authors have compiled and tested PostgreSQL on the following
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platforms(some of these compiles require gcc 2.7.0):
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* DEC Alpha AXP on OSF/1 2.0
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* HP PA-RISC on HP-UX 9.0
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* i386 Solaris
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* SUN SPARC on Solaris 2.4
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* SUN SPARC on SunOS 4.1.3
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* DEC MIPS on Ultrix 4.4
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* Intel x86 on Linux 1.2 and Linux ELF
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* OSs derived from 4.4-lite BSD (NetBSD, FreeBSD)
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* IBM on AIX 3.2.5
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* BSD/OS 2.0, 2.01 & 2.1
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* SGI MIPS on IRIX 5.3
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The following ports are bundled with the PostgreSQL distribution. The
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authors do not have handy access to these platforms but the ports have
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been tested by the others.
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* Motorola MC68K or Intel x86 on NeXTSTEP 3.2
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* Intel x86 on Intel SVR4
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1.3) Where can I get PostgreSQL?
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The primary anonymous ftp site for PostgreSQL is:
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* ftp://ftp.postgreSQL.org/pub
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A mirror site exists at:
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* ftp://postgres95.vnet.net/pub/postgres95
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* ftp://ftp.luga.or.at/pub/postgres95
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* ftp://cal011111.student.utwente.nl/pub/postgres95
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* ftp://ftp.uni-trier.de/pub/database/rdbms/postgres/postgres95
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* ftp://rocker.sch.bme.hu
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1.4) What's the copyright on PostgreSQL?
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PostgreSQL is subject to the following COPYRIGHT.
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PostgreSQL Data Base Management System
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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.
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IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY PARTY
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FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES,
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INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND
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ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF CALIFORNIA HAS BEEN
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ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY WARRANTIES,
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INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
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MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE SOFTWARE
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PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE UNIVERSITY OF
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CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT,
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UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
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1.5) Support for PostgreSQL
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There is no official support for PostgreSQL from the original
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maintainers or from University of California, Berkeley. It is
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maintained through volunteer effort only.
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The main mailing list is: questions@postgreSQL.org. It is available
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for discussion o f matters pertaining to PostgreSQL, including but not
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limited to bug reports and fixes. For info on how to subscribe, send a
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mail with the lines in the body (not the subject line)
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help
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info questions
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to majordomo@postgreSQL.org.
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There is also a digest list available. To subscribe to this list, send
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email to: majordomo@postgreSQL.org with a BODY of:
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subscribe questions-digest
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Digests are sent out to members of this list whenever the main list
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has received around 30k of messages.
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There is also a developers mailing list available. To subscribe to
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this list, send email to hackers-request@postgreSQL.org with a BODY
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of:
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subscribe hackers
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Additional information about PostgreSQL can be found via the
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PostgreSQL WWW home page at:
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http://postgreSQL.org
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1.6) Latest release of PostgreSQL
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The latest release of PostgreSQL is version 1.09. The next release
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will be numbered 6.0 for historical reasons. This release is in beta
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and is available at our ftp site. We expect the beta period to be
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complete during the week of January 2, 1997. For information about
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what is new in 6.0, see our TODO list on our WWW page.
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We expect a 7.0 release in several months that will remove time-travel
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and reduce by 50% the size of on-disk system columns maintained for
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each row in a table. This release will also require a dump and
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restore.
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1.7) Is there a commercial version of PostgreSQL?
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Illustra Information Technology (a wholly owned subsidiary of Informix
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Software, Inc.) sells an object-relational DBMS called Illustra that
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was originally based on postgres. Illustra has cosmetic similarities
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to PostgreSQL but has more features, is more robust, performs better,
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and offers real documentation and support. On the flip side, it costs
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money. For more information, contact sales@illustra.com
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1.8) What documentation is available for PostgreSQL?
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A user manual, manual pages, and some small test examples are included
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in the distribution. The sql and built-in manual pages are
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particularly important.
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The www page contains pointers to an implementation guide and five
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papers written about postgres design concepts and features.
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1.9) What version of SQL does PostgreSQL use?
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PostgreSQL supports a subset of SQL-92. It has most of the important
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constructs but lacks some of the functionality. The most visible
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differences are:
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* no support for nested subqueries
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* no HAVING clause under a GROUP BY
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On the other hand, you get to create user-defined types, functions,
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inheritance etc. If you're willing to help with PostgreSQL coding,
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eventually we can also add the missing features listed above.
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1.10) Does PostgreSQL work with databases from earlier versions of postgres?
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PostgreSQL v1.09 is compatible with databases created with v1.01.
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Those upgrading from 1.0 should read the directions in the
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MIGRATION_1.0_TO_1.02 directory.
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Upgrading to 6.0 requires a dump and restore.
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1.11) How many people use PostgreSQL?
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Since we don't have any licensing or registration scheme, it's
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impossible to tell. We do know hundreds copies of PostgreSQL v1.* have
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been downloaded, and that there many hundreds of subscribers to the
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mailing lists.
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_________________________________________________________________
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Section 2: Installation Questions
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2.1) initdb doesn't run
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* check to see that you have the proper paths set
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* check that the 'postgres' user owns all the right files
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* ensure that there are files in $PGDATA/files, and that they are
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non-empty. If they aren't, then "gmake install" failed for some
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reason
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2.2) when I start up the postmaster, I get "FindBackend: could not find a
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backend to execute..." "postmaster: could not find backend to execute..."
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You probably do not have the right path set up. The 'postgres'
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executable needs to be in your path.
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2.3) The system seems to be confused about commas, decimal points, and date
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formats.
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Check your locale configuration. PostgreSQL uses the locale settings
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of the user that ran the postmaster process. Set those accordingly for
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your operating environment.
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2.4) How do I install PostgreSQL somewhere other than /usr/local/pgsql?
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You need to manually edit the paths in src/Makefile.global to your
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site configuration.
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2.5) The backend compiled successfully, but compiling libpq resulted in a
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complaint: "libpq/pqcomm.h" not found when compiling fe-auth.c.
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You've probably installed PostgreSQL somewhere other than
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/usr/local/pgsql, but didn't edit the src/Makefile.global accordingly.
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See question 2.6.
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2.6) When I run postmaster, I get a Bad System Call core dumped message.
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It could be a variety of problems, but first check to see that you
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have system V extensions installed on your kernel. PostgreSQL requires
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kernel support for shared memory.
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2.7) I get the error message "obj/fmgr.h: No such file or directory"
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This indicates that you did not generate the file fmgr.h properly.
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Something failed in the running of the
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src/backend/utils/Gen_fmgrtab.sh script. Check to see the paths used
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in that script is appropriate to your system.
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2.8) When I try to start the postmaster, I get IpcMemoryCreate errors.
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You either do not have shared memory configured properly in kernel or
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you need to enlarge the shared memory available in the kernel. The
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exact amount you need depends on your architecture and how many
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buffers you configure postmaster to run with. For most systems, with
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default buffer sizes, you need a minimum of ~760K.
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2.10) I have changed a source file, but a recompile does not see the change?
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The Makefiles do not have the proper dependencies for include files.
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You have to do a 'make clean' and then another 'make'.
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_________________________________________________________________
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Section 3: PostgreSQL Features
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3.1) How do I specify a KEY or other constraints on a column?
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Column constraints are not supported in PostgreSQL. As a consequence,
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the system does not check for duplicates.
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Under 6.0, create a unique index on the column. Attempts to create
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duplicate of that column will report an error.
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3.2) Does PostgreSQL support nested subqueries?
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Subqueries are not implemented, but they can be simulated using sql
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functions.
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3.3) How do I define a unique indices?
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PostgreSQL 6.0 supports unique indices.
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3.4) I've having a lot of problems using rules.
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Currently, the rule system in PostgreSQL is mostly broken. It works
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enough to support the view mechanism, but that's about it. Use
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PostgreSQL rules at your own peril.
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3.5) I can't seem to write into the middle of large objects reliably.
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The Inversion large object system in PostgreSQL is also mostly broken.
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It works well enough for storing large wads of data and reading them
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back out, but the implementation has some underlying problems. Use
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PostgreSQL large objects at your own peril.
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3.6) Does PostgreSQL have a graphical user interface? A report generator? A
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embedded query language interface?
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No. No. No. Not in the official distribution at least. Some users have
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reported some success at using 'pgbrowse' and 'onyx' as frontends to
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PostgreSQL. Several contributions are working on tk based frontend
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tools. Ask on the mailing list.
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3.7) How can I write client applications to PostgreSQL?
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PostgreSQL supports a C-callable library interface called libpq as
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well as a Tcl-based library interface called libtcl.
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Others have contributed a perl interface and a WWW gateway to
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PostgreSQL. See the PostgreSQL home pages for more details.
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3.8) How do I prevent other hosts from accessing my PostgreSQL backend?
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Use host-based authentication by modifying the file $PGDATA/pg_hba
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accordingly.
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3.9) How do I set up a pg_group?
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|
|
Currently, there is no easy interface to set up user groups. You have
|
|
to explicitly insert/update the pg_group table. For example:
|
|
|
|
|
|
jolly=> insert into pg_group (groname, grosysid, grolist)
|
|
jolly=> values ('posthackers', '1234', '{5443, 8261}');
|
|
INSERT 548224
|
|
jolly=> grant insert on foo to group posthackers;
|
|
CHANGE
|
|
jolly=>
|
|
|
|
|
|
|
|
The fields in pg_group are:
|
|
* groname: the group name. This a char16 and should be purely
|
|
alphanumeric. Do not include underscores or other punctuation.
|
|
* grosysid: the group id. This is an int4. This should be unique for
|
|
each group.
|
|
* grolist: the list of pg_user id's that belong in the group. This
|
|
is an int4[].
|
|
|
|
|
|
|
|
3.10) What is the exact difference between binary cursors and normal cursors?
|
|
|
|
|
|
|
|
Normal cursors return data back in ASCII format. Since data is stored
|
|
natively in binary format, the system must do a conversion to produce
|
|
the ASCII format. In addition, ASCII formats are often large in size
|
|
than binary format. Once the attributes come back in ASCII, often the
|
|
client application then has to convert it to a binary format to
|
|
manipulate it anyway.
|
|
|
|
Binary cursors give you back the data in the native binary
|
|
representation. Thus, binary cursors will tend to be a little faster
|
|
since there's less overhead of conversion.
|
|
|
|
However, ASCII is architectural neutral whereas binary representation
|
|
can differ between different machine architecture. Thus, if your
|
|
client machine uses a different representation than you server
|
|
machine, getting back attributes in binary format is probably not what
|
|
you want. Also, if your main purpose is displaying the data in ASCII,
|
|
then getting it back in ASCII will save you some effort on the client
|
|
side.
|
|
|
|
3.11) Why doesn't the != operator work?
|
|
|
|
|
|
|
|
SQL specifies <> as the inequality operator, and that is what we have
|
|
defined for the built-in types.
|
|
|
|
In 6.0, != is equivalent to <>.
|
|
|
|
3.12) What is a R-tree index and what is it used for?
|
|
|
|
|
|
|
|
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-tree's can handle multi-dimensional data. For
|
|
example, if a R-tree index can be built on an attribute of type
|
|
'point', the system can more efficient answer queries like 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." Proc 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"
|
|
|
|
3.13) What is the maximum size for a tuple?
|
|
|
|
|
|
|
|
Tuples are limited to 8K bytes. Taking into account system attributes
|
|
and other overhead, one should stay well shy of 8,000 bytes to be on
|
|
the safe side. To use attributes larger than 8K, try using the large
|
|
objects interface.
|
|
|
|
Tuples do not cross 8k boundaries so a 5k tuple will require 8k of
|
|
storage.
|
|
|
|
3.14) I defined indices but my queries don't seem to make use of them. Why?
|
|
|
|
|
|
|
|
PostgreSQL does not automatically maintain statistics. One has to make
|
|
an explicit 'vacuum' call to update the statistics. After statistics
|
|
are updated, the optimizer has a better shot at using indices. Note
|
|
that the optimizer is limited and does not use indices in some
|
|
circumstances (such as OR clauses).
|
|
|
|
If the system still does not see the index, it is probably because you
|
|
have created an index on a field with the improper *_ops type. For
|
|
example, you have created a CHAR(4) field, but have specified a
|
|
char_ops index type_class.
|
|
|
|
See the create_index manual page for information on what type classes
|
|
are available. It must match the field type.
|
|
|
|
Postgres does not warn the user when the improper index is created.
|
|
|
|
Indexes not used for ORDER BY operations.
|
|
|
|
3.15) Are there ODBC drivers for PostgreSQL?
|
|
|
|
|
|
|
|
There are two ODBC drivers available, PostODBC and OpenLink ODBC.
|
|
|
|
For all people being interested in PostODBC, there are now two mailing
|
|
lists devoted to the discussion of PostODBC. The mailing lists are:
|
|
* postodbc-users@listserv.direct. net
|
|
* postodbc-developers@listse rv.direct.net
|
|
|
|
|
|
|
|
these lists are ordinary majordomo mailing lists. You can subscribe by
|
|
sending a mail to:
|
|
* majordomo@listserv.direct.net
|
|
|
|
|
|
|
|
OpenLink ODBC is currently in beta under Linux. You can get it from
|
|
http://www.openlinksw.com/postgres.html. It works with our standard
|
|
ODBC client software so you'll have Postgres ODBC available on every
|
|
client platform we support (Win, Mac, Unix, VMS).
|
|
|
|
We will probably be selling this product to people who need
|
|
commercial-quality support, but a freeware version will always be
|
|
available. Questions to postgres95@openlink.co.uk.
|
|
|
|
3.16) How do I use postgres for multi-dimensional indexing (> 2
|
|
dimensions">)?
|
|
|
|
|
|
|
|
Builtin 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 require a bit of work and we don't currently have
|
|
any documentation on how to do it.
|
|
|
|
3.17) How do I do regular expression searches? case-insensitive regexp
|
|
searching?
|
|
|
|
|
|
|
|
PostgreSQL supports the SQL LIKE syntax as well as more general
|
|
regular expression searching with the ~ operator. The !~ is the
|
|
negated regexp operator. ~* and !~* are the case-insensitive regular
|
|
expression operators.
|
|
|
|
3.18) I can't access the database as the 'root' user.
|
|
|
|
|
|
|
|
You should not create database users with user id 0(root). They will
|
|
be unable to access the database. This is a security precaution
|
|
because of the ability of any user to dynamically link object modules
|
|
into the database engine.
|
|
|
|
3.19) I experienced a server crash during a vacuum. How do I remove the lock
|
|
file?
|
|
|
|
|
|
|
|
If the server crashes during a vacuum command, chances are it will
|
|
leave a lock file hanging around. Attempts to re-run the vacuum
|
|
command result in
|
|
|
|
|
|
WARN:can't create lock file -- another vacuum cleaner running?
|
|
|
|
|
|
|
|
If you are sure that no vacuum is actually running, you can remove the
|
|
file called "pg_vlock" in your database directory (which is
|
|
$PGDATA/base/)
|
|
|
|
3.20) What is the difference between the various character types?
|
|
|
|
Type Internal Name Notes
|
|
--------------------------------------------------
|
|
CHAR char 1 character }
|
|
CHAR2 char2 2 characters }
|
|
CHAR4 char4 4 characters } optimized for a fixed length
|
|
CHAR8 char8 8 characters }
|
|
CHAR16 char16 16 characters }
|
|
CHAR(#) bpchar blank padded to the specified fixed length
|
|
VARCHAR(#) varchar size specifies maximum length, no padding
|
|
TEXT text length limited only by maximum tuple length
|
|
BYTEA bytea variable-length array of bytes
|
|
|
|
|
|
|
|
Remember, you need to use the internal name when creating indexes on
|
|
these fields or when doing other internal operations.
|
|
|
|
The last four types above are "varlena" types (i.e. the first four
|
|
bytes is the length, followed by the data). CHAR(#) and VARCHAR(#)
|
|
allocate the maximum number of bytes no matter how much data is stored
|
|
in the field. TEXT and BYTEA are the only character types that have
|
|
variable length on the disk.
|
|
|
|
3.21) In a query, how do I detect if a field is NULL?
|
|
|
|
|
|
|
|
PostgreSQL has two builtin keywords, "isnull" and "notnull" (note no
|
|
spaces). Version 1.05 and later and 6.* understand IS NULL and IS NOT
|
|
NULL.
|
|
|
|
3.22) How do I see how the query optimizer is evaluating my query?
|
|
|
|
|
|
|
|
Place the word 'EXPLAIN' at the beginning of the query, for example:
|
|
|
|
|
|
EXPLAIN SELECT * FROM table1 WHERE age = 23;
|
|
|
|
|
|
|
|
3.23) How do I create a serial field?
|
|
|
|
|
|
|
|
Postgres does not allow the user to specifiy a user column as type
|
|
SERIAL. Instead, you can use each row's oid field as a unique value.
|
|
However, if you need to dump and reload the database, you need to be
|
|
using postgres version 1.07 or later or 6.* with pgdump's -o option or
|
|
COPY's WITH OIDS option to preserver the oids.
|
|
|
|
Another valid way of doing this is to create a function:
|
|
|
|
|
|
create table my_oids (f1 int4);
|
|
insert into my_oids values (1);
|
|
create function new_oid () returns int4 as
|
|
'update my_oids set f1 = f1 + 1; select f1 from my_oids; '
|
|
language 'sql';
|
|
|
|
|
|
|
|
then:
|
|
|
|
|
|
create table my_stuff (my_key int4, value text);
|
|
insert into my_stuff values (new_oid(), 'hello');
|
|
|
|
|
|
|
|
However, keep in mind there is a race condition here where one server
|
|
could do the update, then another one do an update, and they both
|
|
could select the same new id. This statement should be performed
|
|
within a transaction.
|
|
|
|
3.24) How do I create a multi-column index?
|
|
|
|
|
|
|
|
You can not directly create a multi-column index using create index.
|
|
You need to define a function which acts on the multiple columns, then
|
|
use create index with that function.
|
|
|
|
3.25) What are the temp_XXX files in my database directory?
|
|
|
|
|
|
|
|
They are temp_ files generated by the query executor. For example, if
|
|
a sort needs to be done to satisfy an ORDER BY, some temp files are
|
|
generated as a result of the sort.
|
|
|
|
If you have no transactions or sorts running at the time, it is safe
|
|
to delete the temp_ files.
|
|
|
|
3.26) Why are my table files not getting any smaller after a delete?
|
|
|
|
|
|
|
|
If you run vacuum in pre-6.0, unused rows will be marked for reuse,
|
|
but the file blocks are not released.
|
|
|
|
In 6.0, vacuum properly shrinks tables.
|
|
|
|
3.27) Why can't I connect to my database from another machine?
|
|
|
|
|
|
|
|
The default configuration allows only connections from tcp/ip host
|
|
localhost. You need to add a host entry to the file pgsql/data/pg_hba.
|
|
|
|
|
|
3.28) I get the error 'default index class unsupported' when creating an
|
|
index. How do I do it?
|
|
|
|
|
|
|
|
You probably used:
|
|
|
|
|
|
create index idx1 on person using btree (name);
|
|
|
|
|
|
|
|
PostgreSQL indexes are extensible, and therefore in pre-6.0, you must
|
|
specify a class_type when creating an index. Read the manual page for
|
|
create index (called create_index).
|
|
|
|
Version 6.0, if you do not specify a class_type, it defaults to the
|
|
proper type for the column.
|
|
|
|
3.29) Why does creating an index crash the backend server?
|
|
|
|
|
|
|
|
You have probably defined an incorrect *_ops type class for the field
|
|
you are indexing.
|
|
|
|
3.30) How do I specify a decimal constant as a float8, or a string as a text?
|
|
Why am I getting poor precision?
|
|
|
|
|
|
|
|
Use the :: operator. It is needed only when the default promotion
|
|
rules fail. i.e.:
|
|
|
|
|
|
insert into tab1 values (4.23::float8, '2343'::text)
|
|
|
|
|
|
|
|
The default floating-point constant is a float4 in releases prior to
|
|
1.05. Later releases default to float8.
|
|
|
|
3.31) How do I find out what indexes or operations are defined in the
|
|
database?
|
|
|
|
|
|
|
|
Run the file pgsql/src/tutorial/syscat.source. It illustrates many of
|
|
the 'select's needed to get information out of the database system
|
|
tables.
|
|
|
|
3.32) My database is corrupt. I can't do anything. What should I do?
|
|
|
|
|
|
|
|
The 1.02 release has a README file and utility that describes a
|
|
possible cause of the problem and a workaround.
|
|
|
|
This bug is fixed in 1.02.1.
|
|
|
|
3.33) Createdb, destroydb, createuser,destroyuser don't run. Why?
|
|
|
|
|
|
|
|
Release 1.02 does not have this problem.
|
|
|
|
The 1.01 release of PostgreSQL uses a variable called PAGER to filter
|
|
the output of SELECT statements. Unfortunately, this PAGER is used
|
|
even when the standard output is not a terminal.
|
|
|
|
3.34) Why does 'createuser' return 'unexpected last match in input(">)'?
|
|
|
|
|
|
|
|
You have compile postgres with flex version 2.5.3. There is bug in
|
|
this version of flex. Use flex version 2.5.2 or flex 2.5.4 instead.
|
|
There is a doc/README.flex file which will properly patch the flex
|
|
2.5.3 source code.
|
|
|
|
3.35) All my servers crash under concurrent table access. Why?
|
|
|
|
|
|
|
|
This problem can be caused by a kernel that is not configured to
|
|
support semaphores.
|
|
|
|
3.36) What tools are available for hooking postgres to Web pages?
|
|
|
|
|
|
|
|
For web integration, PHP/FI is an excellent interface. The URL for
|
|
that is http://www.vex.net/php/
|
|
|
|
PHP is great for simple stuff, but for more complex stuff, some still
|
|
use the perl interface and CGI.pm.
|
|
|
|
An example of using WWW with C to talk to Postgres is can be tried at:
|
|
* http://postgreSQL.org/~mlc
|
|
|
|
|
|
|
|
An WWW gatway based on WDB using perl can be downloaded from:
|
|
* http://www.eol.ists.ca/~dunlop/wdb -p95
|
|
|
|
3.37) What is the time-warp feature and how does it relate to vacuum?
|
|
|
|
|
|
|
|
PostgreSQL handles data changes differently than most database
|
|
systems. When a row is changed in a table, the original row is marked
|
|
with the time it was changed, and a new row is created with the
|
|
current data. By default, only current rows are used in a table. If
|
|
you specify a date/time after the table name in a FROM clause, you can
|
|
access the data that was current at that time, i.e.
|
|
|
|
|
|
SELECT *
|
|
FROM employees ['July 24, 1996 09:00:00']
|
|
|
|
|
|
|
|
displays employee rows in the table at the specified time. You can
|
|
specify intervals like [date,date], [date,], [,date], or [,]. This
|
|
last option accesses all rows that ever existed.
|
|
|
|
INSERTed rows get a timestamp too, so rows that were not in the table
|
|
at the desired time will not appear.
|
|
|
|
Vacuum removes rows that are no longer current. This time-warp feature
|
|
is used by the engine for rollback and crash recovery. Expiration
|
|
times can be set with purge.
|
|
|
|
In 6.0, once a table is vacuumed, the creation time of a row may be
|
|
incorrect, causing time-traval to fail.
|
|
|
|
The time-travel feature will be removed in 7.0.
|
|
|
|
3.38) How do I tune the database engine for better performance?
|
|
|
|
|
|
|
|
There are two things that can be done. You can use Openlink's option
|
|
to disable fsync() by starting the 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 shared among the backend processes. If you make
|
|
this parameter too high, the process will not start or crash
|
|
unexpectedly. Each buffer is 8K and the defualt is 64 buffers.
|
|
|
|
3.39) What debugging features are available in PostgreSQL?
|
|
|
|
|
|
|
|
PostgreSQL has several features that report status information that
|
|
can be valuable for debugging purposes.
|
|
|
|
First, by compiling with DEBUG defined, 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 the 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 can contain 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
|
|
1-3 that specifies the debug level. The query plans in a verbose debug
|
|
file can be formatted using the 'indent' program. (You may need to
|
|
remove the '====' lines in 1.* releases.) Be warned that a debug level
|
|
greater than one generates large log files in 1.* releases.
|
|
|
|
You can actuall 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
|
|
perhaps use a debugger to see what is happening. Because the backend
|
|
was not started from the postmaster, it is not running in an identical
|
|
environment and locking/backend interaction problems may not be
|
|
duplicated. Some operating system can attach to a running backend
|
|
directly to diagnose problems.
|
|
|
|
The postgres program has a -s, -A, -t options that can be very usefull
|
|
for debugging and performance measurements.
|
|
|
|
The EXPLAIN command (see this FAQ) allows you to see how PostgreSQL is
|
|
iterpreting your query.
|
|
|
|
3.40) What is an oid? What is a tid?
|
|
|
|
|
|
|
|
Oids are Postgres's answer to unique row ids or serial columns. Every
|
|
row that is created in Postgres gets a unique oid. All oids generated
|
|
by initdb are less than 16384 (from backend/access/transam.h). All
|
|
post-initdb (user-created) oids are equal or greater that this. All
|
|
these oids are unique not only within a table, or database, but unique
|
|
within the entire postgres installation.
|
|
|
|
Postgres uses oids in its internal system tables to link rows in
|
|
separate 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. See the sql(l) manual page to see the other internal
|
|
columns.
|
|
|
|
Tids are used to indentify 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. They can not be
|
|
accessed through sql.
|
|
|
|
3.41) What is the meaning of some of the terms used in Postgres?
|
|
|
|
|
|
|
|
Some of the source code and older documentation use terms that have
|
|
more common usage. Here are some:
|
|
* row, record, tuple
|
|
* attribute, field, column
|
|
* table, class
|
|
* retrieve, select
|
|
* replace, update
|
|
* append, insert
|
|
* oid, serial value
|
|
* portal, cursor
|
|
* range variable, table name, table alias
|
|
|
|
|
|
|
|
Please let me know if you think of any more.
|
|
|
|
|
|
_________________________________________________________________
|
|
|
|
Section 4: Extending PostgreSQL
|
|
|
|
|
|
|
|
4.1) I wrote a user-defined function and when I run it in psql, it dumps
|
|
core.
|
|
|
|
|
|
|
|
The problem could be a number of things. Try testing your user-defined
|
|
function in a stand alone test program first. Also, make sure you are
|
|
not sending elog NOTICES when the front-end is expecting data, such as
|
|
during a type_in() or type_out() functions
|
|
|
|
4.2) I get messages of the type NOTICE:PortalHeapMemoryFree: 0x402251d0 not
|
|
in alloc set!
|
|
|
|
|
|
|
|
You are pfree'ing something that was not palloc'ed. When writing
|
|
user-defined functions, do not include the file "libpq-fe.h". Doing so
|
|
will cause your palloc to be a malloc instead of a free. Then, when
|
|
the backend pfrees the storage, you get the notice message.
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4.3) I've written some nifty new types and functions for PostgreSQL.
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Please share them with other PostgreSQL users. Send your extensions to
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mailing list, and they will eventually end up in the contrib/
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subdirectory.
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4.4) How do I write a C function to return a tuple?
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This requires extreme wizardry, so extreme that the authors have not
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ever tried it, though in principle it can be done. The short answer is
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... you can't. This capability is forthcoming in the future.
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_________________________________________________________________
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Section 5: Bugs
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5.1) How do I make a bug report?
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Check the current FAQ at http://postgreSQL.org
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Also check out our ftp site ftp://ftp.postgreSQL.org/pub to see if
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there is a more recent PostgreSQL version.
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You can also fill out the "bug-template" file and send it to:
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* hackers@postgreSQL.org
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This is the address of the developers mailing list.
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