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803 lines
32 KiB
Plaintext
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Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated: Sat Sep 13 22:00:02 EDT 1997
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Version: 6.2beta
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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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Changes in this version (* = modified, + = new):
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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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1.8) What version of SQL does PostgreSQL use?
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1.9) What documentation is available for 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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1.11) Are there ODBC drivers for PostgreSQL?
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1.12) What tools are available for hooking postgres 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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2) 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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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 32 concurrent backends?
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3) Operational questions
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3.1) Does PostgreSQL support nested subqueries?
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3.2) I've having a lot of problems using rules.
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3.3) I can't seem to write into the middle of large objects reliably.
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3.4) How can I write client applications to PostgreSQL?
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3.5) How do I set up a pg_group?
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3.6) What is the exact difference between binary cursors and normal
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cursors?
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3.7) What is a R-tree index and what is it used for?
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3.8) What is the maximum size for a tuple?
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3.9) I defined indices but my queries don't seem to make use of them.
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Why?
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3.10) How do I do regular expression searches? case-insensitive regexp
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searching?
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3.11) I experienced a server crash during a vacuum. How do I remove
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the lock file?
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3.12) What is the difference between the various character types?
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3.13) In a query, how do I detect if a field is NULL?
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3.14) How do I see how the query optimizer is evaluating my query?
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3.15) How do I create a serial field?
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3.16) What are the pg_psort.XXX files in my database directory?
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3.17) Why can't I connect to my database from another machine?
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3.18) How do I find out what indexes or operations are defined in the
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database?
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3.19) What is the time-warp feature and how does it relate to vacuum?
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3.20) What is an oid? What is a tid?
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3.21) What is the meaning of some of the terms used in Postgres?
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3.22) What is Genetic Query Optimization?
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3.23) How do you remove a column from a table?
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3.24) How do SELECT only the first few rows of a query?
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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 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 2.0, 2.01, 2.1, 3.0
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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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The following platforms have known problems/bugs:
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* nextstep - Motorola MC68K or Intel x86 on NeXTSTEP 3.2
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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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subscribe
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end
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to questions-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: questions-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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There is a bugs mailing list available. To subscribe to this list,
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send email 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 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 6.1.1, which was released
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on July 22, 1997. For information about what is new in 6.1.1, see our
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TODO list on our WWW page.
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We expect a 6.2 release at the end of September.
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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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Upgrading to 6.1 requires a dump and restore from previous releases.
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Upgrading from 6.1 to 6.1.1 requires running configure, compile of the
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new release, recompile of all your custom applications to use the new
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libpq, and then an install while the postmaster is temporarily
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stopped.
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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 6.1.1.
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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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For all people being interested in PostODBC, there are now two mailing
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lists devoted to the discussion of PostODBC. The mailing lists are:
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* postodbc-users@listserv.direct. net
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* postodbc-developers@listse rv.direct.net
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these lists are ordinary majordomo mailing lists. You can subscribe by
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sending a mail to:
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* majordomo@listserv.direct.net
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OpenLink ODBC is very popular. You can get it from
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http://www.openlinksw.com/postgres.html. It works with our standard
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ODBC client software so you'll have Postgres ODBC available on every
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client platform we support (Win, Mac, Unix, VMS).
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We 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 postgres to Web pages?
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For web integration, PHP/FI is an excellent interface. The URL for
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that is http://www.vex.net/php/
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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 example of using WWW with C to talk to Postgres is can be tried at:
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* http://postgreSQL.org/~mlc
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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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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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_________________________________________________________________
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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 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.
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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 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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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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There are two things that can be done. You can use Openlink's option
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to disable fsync() by starting the postmaster with a '-o -F' option.
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This will prevent fsync()'s from flushing to disk after every
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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 shared among the backend processes. If you make
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this parameter too high, the process will not start or crash
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unexpectedly. Each buffer is 8K and the defualt is 64 buffers.
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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 can contain 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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1-3 that specifies the debug level. The query plans in a verbose debug
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file can be formatted using the 'indent' program. Be warned that a
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debug level of three generates large log files.
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You can actuall 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
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perhaps use a debugger to see what is happening. Because the backend
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was not started from the postmaster, it is not running in an identical
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environment and locking/backend interaction problems may not be
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duplicated. Some operating system can attach to a running backend
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directly to diagnose problems.
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The postgres program has a -s, -A, -t options that can be very usefull
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for debugging and performance measurements.
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The EXPLAIN command (see this FAQ) allows you to see how PostgreSQL is
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iterpreting your query.
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You can also compile with profiling to see what functions are taking
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execution time.
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2.13) How do I enable more than 32 concurrent backends?
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Edit include/storage/sinvaladt.h, and change the value of
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MaxBackendId. In the future, we plan to make this a configurable
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prameter.
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_________________________________________________________________
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Section 3: PostgreSQL Features
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3.1) 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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3.2) I've having a lot of problems using rules.
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|
Currently, the rule system in PostgreSQL is mostly broken. It works
|
|
enough to support the view mechanism, but that's about it. Use
|
|
PostgreSQL rules at your own peril.
|
|
|
|
3.3) I can't seem to write into the middle of large objects reliably.
|
|
|
|
The Inversion large object system in PostgreSQL is also mostly broken.
|
|
It works well enough for storing large wads of data and reading them
|
|
back out, but the implementation has some underlying problems. Use
|
|
PostgreSQL large objects at your own peril.
|
|
|
|
3.4) How can I write client applications to PostgreSQL?
|
|
|
|
PostgreSQL supports a C-callable library interface called libpq as
|
|
well as many others. See the /src/interfaces directory.
|
|
|
|
Others have contributed a perl interface and a WWW gateway to
|
|
PostgreSQL. See the PostgreSQL home pages for more details.
|
|
|
|
3.5) 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 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[].
|
|
|
|
3.6) What is the exact difference between binary cursors and normal cursors?
|
|
|
|
See the declare manual page for a description.
|
|
|
|
3.7) 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"
|
|
|
|
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.
|
|
|
|
3.8) 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.
|
|
|
|
3.9) 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). For column-specific optimization
|
|
statistics, use 'vacuum analyze'.
|
|
|
|
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.
|
|
|
|
3.10) How do I do regular expression searches? case-insensitive regexp
|
|
searching?
|
|
|
|
See the pgbuiltin manual page. Search for regular expression.
|
|
|
|
3.11) I experienced a server crash during a vacuum. How do I remove the lock
|
|
file?
|
|
|
|
See the vacuum manual page.
|
|
|
|
3.12) 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.
|
|
|
|
3.13) In a query, how do I detect if a field is NULL?
|
|
|
|
You test the column with IS NULL and IS NOT NULL.
|
|
|
|
3.14) How do I see how the query optimizer is evaluating my query?
|
|
|
|
See the explain manual page.
|
|
|
|
3.15) 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 use
|
|
pgdump's -o option or COPY's WITH OIDS option to preserver the oids.
|
|
|
|
We also have a SEQUENCE function that is similar to SERIAL. See the
|
|
create_sequence manual page.
|
|
|
|
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.
|
|
|
|
3.16) 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.
|
|
|
|
3.17) 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.
|
|
See the hba_conf manual page.
|
|
|
|
3.18) 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.
|
|
|
|
3.19) 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.
|
|
|
|
3.20) 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.
|
|
|
|
3.21) 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.
|
|
|
|
3.22) 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>.
|
|
|
|
3.23) 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;
|
|
|
|
3.23) How do 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
|
|
first few rows. Consider a query that has and ORDER BY. There is no
|
|
way to return any rows until the entire query is evaluated and sorted.
|
|
_________________________________________________________________
|
|
|
|
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. The short answer is
|
|
... you can't. This capability is forthcoming in the future.
|
|
_________________________________________________________________
|
|
|
|
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.
|
|
|
|
You can also fill out the "bug-template" file and send it to:
|
|
* bugs@postgreSQL.org
|
|
|
|
This is the address of the developers mailing list.
|