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828 lines
33 KiB
Plaintext
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Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated: Sat Oct 24 00:12:23 EDT 1998
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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.shtml.
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Irix-specific questions are answered in
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http://postgreSQL.org/docs/faq-irix.shtml.
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HPUX-specific questions are answered in
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http://postgreSQL.org/docs/faq-hpux.shtml.
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_________________________________________________________________
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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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1.8) What documentation is available for 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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PostgreSQL?
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1.11) Are there ODBC drivers for PostgreSQL?
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1.12) What tools are available for hooking PostgreSQL to Web pages?
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1.13) Does PostgreSQL have a graphical user interface? A report
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generator? A embedded query language interface?
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1.14) How can I learn SQL?
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1.15) What languages are available to communicate with PostgreSQL?
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Installation/Configuration 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) When I run postmaster, I get a Bad System Call core dumped
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message.
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2.6) When I try to start the postmaster, I get IpcMemoryCreate errors.
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2.7) I have changed a source file, but a recompile does not see the
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change?
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2.8) How do I prevent other hosts from accessing my PostgreSQL
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database?
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2.9) I can't access the database as the root user.
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2.10) All my servers crash under concurrent table access. Why?
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2.11) How do I tune the database engine for better performance?
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2.12) What debugging features are available in PostgreSQL?
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2.13) How do I enable more than 64 concurrent backends?
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2.14) What non-unix ports are available?
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Operational questions
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3.1) Does PostgreSQL support nested subqueries?
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3.2) How can I write client applications for PostgreSQL?
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3.3) How do I set up a pg_group?
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3.4) What is the exact difference between binary cursors and normal
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cursors?
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3.5) What is an R-tree index and what is it used for?
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3.6) What is the maximum size for a tuple?
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3.7) I defined indices but my queries don't seem to make use of them.
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Why?
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3.8) How do I do regular expression searches? case-insensitive regexp
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searching?
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3.9) I experienced a server crash during a vacuum. How do I remove the
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lock file?
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3.10) What is the difference between the various character types?
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3.11) In a query, how do I detect if a field is NULL?
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3.12) How do I see how the query optimizer is evaluating my query?
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3.13) How do I create a serial/auto-incrementing field?
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3.14) What are the pg_psort.XXX files in my database directory?
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3.15) Why can't I connect to my database from another machine?
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3.16) How do I find out what indices or operations are defined in the
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database?
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3.17) What is an oid? What is a tid?
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3.18) What is the meaning of some of the terms used in PostgreSQL?
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3.19) What is Genetic Query Optimization?
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3.20) How do you remove a column from a table?
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3.21) How do I select only the first few rows of a query?
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3.22) How much database disk space is required to store data from a
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typical flat file?
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3.23) How do I get a list of tables, or other things I can see in
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psql?
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3.24) Why do I get the error "FATAL: palloc failure: memory
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exhausted?"
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3.25) How do I tell what PostgreSQL version I am running?
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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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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 now
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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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* aix - IBM on AIX 3.2.5 or 4.x
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* alpha - DEC Alpha AXP on Digital Unix 2.0, 3.2, 4.0
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* BSD44_derived - OSs derived from 4.4-lite BSD (NetBSD, FreeBSD)
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* bsdi - BSD/OS 2.x, 3.x, 4.x
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* dgux - DG/UX 5.4R4.11
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* hpux - HP PA-RISC on HP-UX 9.0, 10
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* i386_solaris - i386 Solaris
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* irix5 - SGI MIPS on IRIX 5.3
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* linux - Intel x86 on Linux 2.0 and Linux ELF SPARC on Linux ELF
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PPC on Linux Elf (For non-ELF Linux, see LINUX_ELF below).
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* sco - SCO 3.2v5
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* sparc_solaris - SUN SPARC on Solaris 2.4, 2.5, 2.5.1
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* sunos4 - SUN SPARC on SunOS 4.1.3
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* svr4 - Intel x86 on Intel SVR4 and MIPS
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* ultrix4 - DEC MIPS on Ultrix 4.4
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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 University of
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California, Berkeley. It is maintained through volunteer effort.
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The main mailing list is: pgsql-general@postgreSQL.org. It is
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available for discussion of matters pertaining to PostgreSQL, For info
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on how to subscribe, send a mail with the lines in the body (not the
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subject line)
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subscribe
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end
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to pgsql-general-request@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: pgsql-general-digest-request@postgreSQL.org with a BODY of:
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subscribe
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end
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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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The bugs mailing list available. To subscribe to this list, send email
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to bugs-request@postgreSQL.org with a BODY of:
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There is also a developers discussion mailing list available. To
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subscribe to this list, send email to hackers-request@postgreSQL.org
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with a BODY of:
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subscribe
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end
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Additional mailing lists and information about PostgreSQL can be found
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via the PostgreSQL WWW home page at:
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http://postgreSQL.org
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There also an IRC channel on EFNet, channel #PostgreSQL. I use the
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unix command irc -c '#PostgreSQL' "$USER" irc.ais.net
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1.6) Latest release of PostgreSQL
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The latest release of PostgreSQL is version 6.4.
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We plan to have major releases every four months.
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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. For more information, contact
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sales@illustra.com
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1.8) What documentation is available for PostgreSQL?
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Several manuals, manual pages, and some small test examples are
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included in the distribution. See the /doc directory.
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psql has some nice \d commands to show information about types,
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operators, functions, aggregates, etc.
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The web page contains even more documentation.
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1.9) What version of SQL does PostgreSQL use?
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PostgreSQL supports an extended subset of SQL-92.
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It is Y2K compliant.
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1.10) Does PostgreSQL work with databases from earlier versions of
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PostgreSQL?
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Upgrading to 6.4 from release 6.3.* can be accomplished using the new
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pg_upgrade utility. Those upgrading from earlier releases require a
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dump and restore.
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Those ugrading from versions earlier than 1.09 must upgrade to 1.09
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first without a dump/reload, then dump the data from 1.09, and then
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load it into 6.4.
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1.11) Are there ODBC drivers for PostgreSQL?
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There are two ODBC drivers available, PostODBC and OpenLink ODBC.
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PostODBC is included in the distribution. More information about it
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can be gotten from: http://www.insightdist.com/psqlodbc
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OpenLink ODBC can be gotten from http://www.openlinksw.com. It works
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with their standard ODBC client software so you'll have PostgreSQL
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ODBC available on every client platform they support (Win, Mac, Unix,
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VMS).
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They will probably be selling this product to people who need
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commercial-quality support, but a freeware version will always be
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available. Questions to postgres95@openlink.co.uk.
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1.12) What tools are available for hooking PostgreSQL to Web pages?
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A nice introduction to Database-backed Web pages can be seen at:
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http://www.webtools.com
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For web integration, PHP is an excellent interface. The URL for that
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is http://www.php.net
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PHP is great for simple stuff, but for more complex stuff, some still
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use the perl interface and CGI.pm.
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An WWW gatway based on WDB using perl can be downloaded from
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http://www.eol.ists.ca/~dunlop/wdb-p95
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1.13) Does PostgreSQL have a graphical user interface? A report generator? A
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embedded query language interface?
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We have a nice graphical user interface called pgaccess, which is
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shipped as part of the distribtion. Pgaccess also has a report
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generator.
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The web page is http://www.flex.ro/pgaccess We also include ecpg,
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which is an embedded SQL query language interface for C.
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1.14) How can I learn SQL?
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There is a nice tutorial at http://w3.one.net/~jhoffman/sqltut.htm and
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at
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http://ourworld.compuserve.com/homepages/Graeme_Birchall/DB2_COOK.HTM.
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Many of our users like The Practical SQL Handbook, Bowman et al,
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Addison Wesley.
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1.15) What languages are available to communicate with PostgreSQL?
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We have:
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* C(interfaces/libpq)
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* C++(interfaces/libpq++)
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* Embedded C(interfaces/ecpg)
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* Java(interfaces/jdbc)
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* Perl(interfaces/perl5)
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* ODBC(interfaces/odbc)
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* Python(interfaces/python)
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* TCL(interfaces/libpgtcl)
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* A crude C/4GL(contrib/pginterface)
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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 that you don't have any of the previous version's binaries
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in your path (If you see the message WARN:heap_modifytuple: repl
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is \ 9, this is the problem.)
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* check to see that you have the proper paths set
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* check that the postgres user owns the proper files
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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 edit Makefile.global and change POSTGRESDIR accordingly,
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or create a Makefile.custom and define POSTGRESDIR there.
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2.5) 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 and semaphores.
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2.6) 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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2.7) 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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2.8) How do I prevent other hosts from accessing my PostgreSQL database?
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By default, PostgreSQL only allows connections from the local machine
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using unix domain sockets. You must add the -i flag to the postmaster,
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and enable host-based authentication by modifying the file
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$PGDATA/pg_hba accordingly.
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2.9) I can't access the database as the root user.
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You should not create database users with user id 0(root). They will
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be unable to access the database. This is a security precaution
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because of the ability of any user to dynamically link object modules
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into the database engine.
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2.10) All my servers crash under concurrent table access. Why?
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This problem can be caused by a kernel that is not configured to
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support semaphores.
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2.11) How do I tune the database engine for better performance?
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Certainly, indices can speed up queries. The explain command allows
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you to see how PostgreSQL is interpreting your query, and which
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indices are being used.
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If you are doing a lot of inserts, consider doing them in a large
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batch using the copy command. This is much faster than single
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individual inserts. Second, statements not in a begin work/commit
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transaction block are considered to be their in their own transaction.
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Consider performing several statements in a single transaction block.
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This reduces the transaction overhead. Also consider dropping and
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recreating indices when making large data changes.
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There are several tuning things that can be done. You can disable
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fsync() by starting the postmaster with a -o -F option. This will
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prevent fsync()'s from flushing to disk after every transaction.
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You can also use the postmaster -B option to increase the number of
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shared memory buffers used by the backend processes. If you make this
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parameter too high, the backends will not start or crash unexpectedly.
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Each buffer is 8K and the default is 64 buffers.
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You can also use the postgres -S option to increase the maximum amount
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of memory used by each backend process for temporary sorts. Each
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buffer is 1K and the defualt is 512 buffers.
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You can also use the cluster command to group data in base tables to
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match an index. See the cluster(l) manual page for more details.
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2.12) What debugging features are available in PostgreSQL?
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PostgreSQL has several features that report status information that
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can be valuable for debugging purposes.
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First, by running configure with the -enable-cassert option, many
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assert()'s monitor the progress of the backend and halt the program
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when something unexpected occurs.
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Both postmaster and postgres have several debug options available.
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First, whenever you start the postmaster, make sure you send the
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standard output and error to a log file, like:
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cd /usr/local/pgsql
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./bin/postmaster >server.log 2>&1 &
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This will put a server.log file in the top-level PostgreSQL directory.
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This file contains useful information about problems or errors
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encountered by the server. Postmaster has a -d option that allows even
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more detailed information to be reported. The -d option takes a number
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that specifies the debug level. Be warned that high debug level values
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generates large log files.
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You can actually run the postgres backend from the command line, and
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type your SQL statement directly. This is recommended only for
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debugging purposes. Note that a newline terminates the query, not a
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semicolon. If you have compiled with debugging symbols, you can use a
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debugger to see what is happening. Because the backend was not started
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from the postmaster, it is not running in an identical environment and
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locking/backend interaction problems may not be duplicated. Some
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operating system can attach to a running backend directly to diagnose
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problems.
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|
|
|
The postgres program has a -s, -A, -t options that can be very usefull
|
|
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 current directory.
|
|
|
|
2.13) How do I enable more than 64 concurrent backends?
|
|
|
|
Edit include/storage/sinvaladt.h, and change the value of
|
|
MaxBackendId. In the future, we plan to make this a configurable
|
|
prameter.
|
|
|
|
2.14) What non-unix ports are available?
|
|
|
|
It is possible to compile the libpq C library, psql, and other
|
|
interfaces and binaries to run on MS Windows platforms. In this case,
|
|
the client is running on MS Windows, and communicates via TCP/IP to a
|
|
server running on one of our supported Unix platforms.
|
|
|
|
A file win32.mak is included in the distributiion for making a Win32
|
|
libpq library and psql.
|
|
|
|
Someone is attempting to port our PostgreSQL database server to
|
|
Windows NT using the Cygnus Unix/NT porting library. He has gotten it
|
|
compiled, but initdb is currently failing.
|
|
_________________________________________________________________
|
|
|
|
Section 3: PostgreSQL Features
|
|
|
|
3.1) Does PostgreSQL support nested subqueries?
|
|
|
|
Yes, fully supported, but only in the where clause, not in the target
|
|
list.
|
|
|
|
3.2) How can I write client applications for PostgreSQL?
|
|
|
|
PostgreSQL supports a C-callable library interface called libpq as
|
|
well as many others. See the above list of supported languages.
|
|
|
|
3.3) How do I set up a pg_group?
|
|
|
|
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 name 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[].
|
|
|
|
3.4) What is the exact difference between binary cursors and normal cursors?
|
|
|
|
See the declare manual page for a description.
|
|
|
|
3.5) What is an 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 an 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"
|
|
|
|
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.
|
|
|
|
3.6) 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.
|
|
|
|
3.7) 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 knows how many rows in the table, and can
|
|
better decide if it should use indices. Note that the optimizer does
|
|
not use indices in cases when the table is small because a sequentail
|
|
scan would be faster. For column-specific optimization statistics, use
|
|
vacuum analyze.
|
|
|
|
Indexes are not used for order by operations.
|
|
|
|
When using wildcard operators like LIKE or ~, indices can only be used
|
|
if the beginning of the search is anchored to the start of the string.
|
|
So, to use indices, LIKE searches can should not begin with %, and
|
|
~(regular expression searches) should start with ^.
|
|
|
|
3.8) How do I do regular expression searches? case-insensitive regexp
|
|
searching?
|
|
|
|
See psql's \do command.
|
|
|
|
3.9) I experienced a server crash during a vacuum. How do I remove the lock
|
|
file?
|
|
|
|
See the vacuum manual page.
|
|
|
|
3.10) What is the difference between the various character types?
|
|
|
|
Type Internal Name Notes
|
|
--------------------------------------------------
|
|
CHAR char 1 character
|
|
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
|
|
|
|
You need to use the internal name when doing internal operations.
|
|
|
|
The last four types above are "varlena" types (i.e. the first four
|
|
bytes are the length, followed by the data). char(#) allocates the
|
|
maximum number of bytes no matter how much data is stored in the
|
|
field. text, varchar(#), and bytea all have variable length on the
|
|
disk, and because of this, there is a small performance penalty for
|
|
using them. Specifically, the penalty is for access to all columns
|
|
after the first column of this type.
|
|
|
|
3.11) In a query, how do I detect if a field is NULL?
|
|
|
|
You test the column with IS NULL and IS NOT NULL.
|
|
|
|
3.12) How do I see how the query optimizer is evaluating my query?
|
|
|
|
See the explain manual page.
|
|
|
|
3.13) 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. 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 pgdump's -o option or copy with oids option to
|
|
preserve the oids.
|
|
|
|
3.14) What are the pg_psort.XXX files in my database directory?
|
|
|
|
They are temporary sort 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 pg_psort.XXX files.
|
|
|
|
3.15) Why can't I connect to my database from another machine?
|
|
|
|
The default configuration allows only unix domain socket connections
|
|
from the local machine. To enable TCP/IP connections, use the
|
|
postmaster -i option You need to add a host entry to the file
|
|
pgsql/data/pg_hba. See the pg_hba.conf manual page.
|
|
|
|
3.16) How do I find out what indices or operations are defined in the
|
|
database?
|
|
|
|
psql has a variety of backslash commands to show such information. Use
|
|
\? to see them.
|
|
|
|
Also try the file pgsql/src/tutorial/syscat.source. It illustrates
|
|
many of the selects needed to get information out of the database
|
|
system tables.
|
|
|
|
3.17) 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 backend/access/transam.h). All
|
|
user-created oids are equal or greater that 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. See the sql(l) manual page to see the other internal
|
|
columns. 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 oid's, there
|
|
is no reason you can't do it:
|
|
CREATE TABLE new_table (mycol int);
|
|
INSERT INTO new_table SELECT oid, mycol FROM old_table;
|
|
|
|
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.
|
|
|
|
3.18) 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:
|
|
* 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
|
|
|
|
3.19) What is Genetic Query Optimization?
|
|
|
|
The GEQO module in PostgreSQL is intended to solve the query
|
|
optimization problem of joining many tables by means of a Genetic
|
|
Algorithm (GA). It allows the handling of large join queries through
|
|
non-exhaustive search.
|
|
|
|
For further information see README.GEQO <utesch@aut.tu-freiberg.de>.
|
|
|
|
3.20) How do you remove a column from a table?
|
|
|
|
We do not support alter table drop column, but do this:
|
|
|
|
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;
|
|
|
|
3.21) How do I select only the first few rows of a query?
|
|
|
|
See the fetch manual page.
|
|
|
|
This only prevents all row results from being transfered to the
|
|
client. The entire query must be evaluated, even if you only want just
|
|
the first few rows. Consider a query that has an order by. There is no
|
|
way to return any rows until the entire query is evaluated and sorted.
|
|
|
|
3.22)How much database disk space is required to store data from a typical
|
|
flat file?
|
|
|
|
Consider a file with 300,000 lines with two integers on each line. The
|
|
flat file is 2.4MB. The size of the PostgreSQL database file
|
|
containing this data can be estimated:
|
|
40 bytes + each row header (approximate)
|
|
8 bytes + two int fields @ 4 bytes each
|
|
4 bytes + pointer on page to tuple
|
|
-------- =
|
|
52 bytes per row
|
|
|
|
The data page size in PostgreSQL is 8192(8k) bytes, so:
|
|
|
|
8192 bytes per page
|
|
------------------- = 157 rows per database page (rounded up)
|
|
52 bytes per row
|
|
|
|
300000 data rows
|
|
----------------- = 1911 database pages
|
|
157 rows per page
|
|
|
|
1911 database pages * 8192 bytes per page = 15,654,912 or 15.5MB
|
|
|
|
|
|
Indexes do not contain as much overhead, but do contain the data that is
|
|
being indexed, so they can be large also.
|
|
|
|
3.23) How do I get a list of tables, or other things I can see in psql?
|
|
|
|
See the file pgsql/src/bin/psql/psql.c. It contains SQL commands that
|
|
generate the output for psql's backslash commands.
|
|
|
|
3.24) Why do I get the error "FATAL: palloc failure: memory exhausted?"
|
|
|
|
It is possible you have run out of virtual memory on your system, or
|
|
your kernel has a low limit for certain resources. Try this before
|
|
starting the postmaster:
|
|
|
|
ulimit -d 65536
|
|
limit datasize 64m
|
|
|
|
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 are having a
|
|
problem with the SQL client because the backend is returning too much
|
|
data, try it before starting the client.
|
|
|
|
3.25) How do I tell what PostgreSQL version I am running?
|
|
|
|
From psql, type select version();
|
|
_________________________________________________________________
|
|
|
|
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.
|
|
|
|
4.3) I've written some nifty new types and functions for PostgreSQL.
|
|
|
|
Please share them with other PostgreSQL users. Send your extensions to
|
|
mailing list, and they will eventually end up in the contrib/
|
|
subdirectory.
|
|
|
|
4.4) How do I write a C function to return a tuple?
|
|
|
|
This requires extreme wizardry so extreme that the authors have not
|
|
ever tried it, though in principle it can be done.
|
|
_________________________________________________________________
|
|
|
|
Section 5: Bugs
|
|
|
|
5.1) How do I make a bug report?
|
|
|
|
Check the current FAQ at http://postgreSQL.org
|
|
|
|
Also check out our ftp site ftp://ftp.postgreSQL.org/pub to see if
|
|
there is a more recent PostgreSQL version or patches.
|
|
|
|
You can also fill out the "bug-template" file and send it to:
|
|
bugs@postgreSQL.org
|