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1028 lines
44 KiB
Plaintext
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Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated: Tue Mar 21 16:09:11 EST 2000
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Current maintainer: Bruce Momjian (pgman@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://www.PostgreSQL.org.
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Linux-specific questions are answered in
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http://www.PostgreSQL.org/docs/faq-linux.html.
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Irix-specific questions are answered in
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http://www.PostgreSQL.org/docs/faq-irix.html.
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HPUX-specific questions are answered in
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http://www.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's the copyright on PostgreSQL?
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1.3) What Unix platforms does PostgreSQL run on?
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1.4) What non-unix ports are available?
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1.5) Where can I get PostgreSQL?
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1.6) Where can I get support for PostgreSQL?
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1.7) What is the latest release of PostgreSQL?
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1.8) What documentation is available for PostgreSQL?
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1.9) How do I find out about known bugs or missing features?
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1.10) How can I learn SQL?
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1.11) Is PostgreSQL Y2K compliant?
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1.12) How do I join the development team?
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1.13) How do I submit a bug report?
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1.14) How does PostgreSQL compare to other DBMS's?
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User Client Questions
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2.1) Are there ODBC drivers for PostgreSQL?
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2.2) What tools are available for hooking PostgreSQL to Web pages?
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2.3) Does PostgreSQL have a graphical user interface? A report
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generator? An embedded query language interface?
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2.4) What languages are available to communicate with PostgreSQL?
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Administrative Questions
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3.1) Why does initdb fail?
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3.2) How do I install PostgreSQL somewhere other than
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/usr/local/pgsql?
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3.3) When I start the postmaster, I get a Bad System Call or core
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dumped message. Why?
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3.4) When I try to start the postmaster, I get IpcMemoryCreate
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errors3. Why?
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3.5) When I try to start the postmaster, I get IpcSemaphoreCreate
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errors. Why?
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3.6) How do I prevent other hosts from accessing my PostgreSQL
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database?
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3.7) Why can't I connect to my database from another machine?
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3.8) Why can't I access the database as the root user?
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3.9) All my servers crash under concurrent table access. Why?
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3.10) How do I tune the database engine for better performance?
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3.11) What debugging features are available in PostgreSQL?
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3.12) I get 'Sorry, too many clients' when trying to connect. Why?
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3.13) What are the pg_psort.XXX files in my database directory?
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3.14) How do I set up a pg_group?
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Operational Questions
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4.1) The system seems to be confused about commas, decimal points, and
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date formats.
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4.2) What is the exact difference between binary cursors and normal
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cursors?
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4.3) How do I select only the first few rows of a query?
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4.4) How do I get a list of tables, or other things I can see in psql?
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4.5) How do you remove a column from a table?
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4.6) What is the maximum size for a row, table, database?
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4.7) How much database disk space is required to store data from a
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typical flat file?
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4.8) How do I find out what indices or operations are defined in the
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database?
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4.9) My queries are slow or don't make use of the indexes. Why?
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4.10) How do I see how the query optimizer is evaluating my query?
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4.11) What is an R-tree index?
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4.12) What is Genetic Query Optimization?
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4.13) How do I do regular expression searches and case-insensitive
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regexp searching?
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4.14) In a query, how do I detect if a field is NULL?
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4.15) What is the difference between the various character types?
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4.16.1) How do I create a serial/auto-incrementing field?
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4.16.2) How do I get the value of a serial insert?
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4.16.3) Wouldn't use of currval() and nextval() lead to a race
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condition with other concurrent backend processes?
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4.17) What is an oid? What is a tid?
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4.18) What is the meaning of some of the terms used in PostgreSQL?
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4.19) Why do I get the error "FATAL: palloc failure: memory
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exhausted?"
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4.20) How do I tell what PostgreSQL version I am running?
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4.21) My large-object operations get invalid large obj descriptor.
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Why?
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4.22) How do I create a column that will default to the current time?
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4.23) Why are my subqueries using IN so slow?
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Extending PostgreSQL
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5.1) I wrote a user-defined function. When I run it in psql, why does
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it dumps core?
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5.2) What does the message: NOTICE:PortalHeapMemoryFree: 0x402251d0
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not in alloc set! mean?
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5.3) How can I contribute some nifty new types and functions for
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PostgreSQL?
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5.4) How do I write a C function to return a tuple?
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5.5) I have changed a source file. Why does the recompile does not see
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the change?
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_________________________________________________________________
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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'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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Portions copyright (c) 1996-2000, PostgreSQL, Inc Portions Copyright
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(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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1.3) What Unix platforms 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):
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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.*, 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 i86 Alpha SPARC PPC M68k
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* sco - SCO 3.2v5 Unixware
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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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1.4) What non-unix ports are available?
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It is possible to compile the libpq C library, psql, and other
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interfaces and binaries to run on MS Windows platforms. In this case,
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the client is running on MS Windows, and communicates via TCP/IP to a
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server running on one of our supported Unix platforms.
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A file win31.mak is included in the distribution for making a Win32
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libpq library and psql.
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The database server is now working on Windows NT using the Cygnus
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Unix/NT porting library. See pgsql/doc/README.NT in the distribution.
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There is also a web page at
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http://www.freebsd.org/~kevlo/postgres/portNT.html. There is another
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port using U/Win at http://surya.wipro.com/uwin/ported.html.
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1.5) 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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For mirror sites, see our main web site.
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1.6) Where can I get 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. To
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subscribe, send a mail with the lines in the body (not the subject
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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 is available. To subscribe to this list, send
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email to bugs-request@postgreSQL.org with a BODY of:
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subscribe
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end
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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 is also an IRC channel on EFNet, channel #PostgreSQL. I use the
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unix command irc -c '#PostgreSQL' "$USER" irc.phoenix.net
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Commercial support for PostgreSQL is available at
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http://www.pgsql.com/
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1.7) What is the latest release of PostgreSQL?
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The latest release of PostgreSQL is version 6.5.2.
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We plan to have major releases every four months.
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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 site contains even more documentation.
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1.9) How do I find out about known bugs or missing features?
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PostgreSQL supports an extended subset of SQL-92. See our TODO for a
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list of known bugs, missing features, and future plans.
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1.10) 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/HTM_COOK.HTM.
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Another one is "Teach Yourself SQL in 21 Days, Second Edition" at
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http://members.tripod.com/er4ebus/sql/index.htm
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Many of our users like The Practical SQL Handbook, Bowman et al.,
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Addison Wesley. Others like Lan Times Guide to SQL, Groff et al.,
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Osborne McGraw-Hill.
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1.11) Is PostgreSQL Y2K compliant?
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Yes, we easily handle dates past the year 2000AD, and before 2000BC.
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1.12) How do I join the development team?
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First, download the latest sources and read the PostgreSQL Developers
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documentation on our web site, or in the distribution. Second,
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subscribe to the pgsql-hackers and pgsql-patches mailing lists. Third,
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submit high-quality patches to pgsql-patches.
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There are about a dozen people who have COMMIT privileges to the
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PostgreSQL CVS archive. All of them have submitted so many
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high-quality patches that it was a pain for the existing committers to
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keep up, and we had confidence that patches they committed were likely
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to be of high quality.
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1.13) How do I submit a bug report?
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Fill out the "bug-template" file and send it to: bugs@postgreSQL.org
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Also check out our ftp site ftp://ftp.postgreSQL.org/pub to see if
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there is a more recent PostgreSQL version or patches.
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1.14) How does PostgreSQL compare to other DBMS's?
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There are several ways of measuring software: features, performance,
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reliability, support, and price.
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Features
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PostgreSQL has most features present in large commercial
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DBMS's, like transactions, subselects, triggers, views, and
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sophisticated locking. We have some features they don't have,
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like user-defined types, inheritance, rules, and multi-version
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concurrency control to reduce lock contention. We don't have
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foreign key referential integrity or outer joins, but are
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working on them for our next release.
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Performance
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PostgreSQL runs in two modes. Normal fsync mode flushes every
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completed transaction to disk, guaranteeing that if the OS
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crashes or loses power in the next few seconds, all your data
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is safely stored on disk. In this mode, we are slower than most
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commercial databases, partly because few of them do such
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conservative flushing to disk in their default modes. In
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no-fsync mode, we are usually faster than commercial databases,
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though in this mode, an OS crash could cause data corruption.
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We are working to provide an intermediate mode that suffers
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less performance overhead than full fsync mode, and will allow
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data integrity within 30 seconds of an OS crash. The mode is
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select-able by the database administrator.
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In comparison to MySQL or leaner database systems, we are
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slower on inserts/updates because we have transaction overhead.
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Of course, MySQL doesn't have any of the features mentioned in
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the Features section above. We are built for flexibility and
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features, though we continue to improve performance through
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profiling and source code analysis.
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We handle each user connection by creating a Unix process.
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Backend processes share data buffers and locking information.
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With multiple CPU's, multiple backends can easily run on
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different CPU's.
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Reliability
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We realize that a DBMS must be reliable, or it is worthless. We
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strive to release well-tested, stable code that has a minimum
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of bugs. Each release has at least one month of beta testing,
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and our release history shows that we can provide stable, solid
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releases that are ready for production use. We believe we
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compare favorably to other database software in this area.
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Support
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Our mailing list provides a large group of developers and users
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to help resolve any problems encountered. While we can not
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guarantee a fix, commercial DBMS's don't always supply a fix
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either. Direct access to developers, the user community,
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manuals, and the source code often make PostgreSQL support
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superior to other DBMS's. There is commercial per-incident
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support available for those who need it. (See support FAQ
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item.)
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Price
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We are free for all use, both commercial and non-commercial.
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You can add our code to your product with no limitations,
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except those outlined in our BSD-style license stated above.
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_________________________________________________________________
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User Client Questions
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2.1) 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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2.2) 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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There is also one at http://www.phone.net/home/mwm/hotlist/.
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For web integration, PHP is an excellent interface. It is at:
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http://www.php.net
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PHP is great for simple stuff, but for more complex cases, many use
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the perl interface and CGI.pm.
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A WWW gateway 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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2.3) Does PostgreSQL have a graphical user interface? A report generator?
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An 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 distribution. Pgaccess also has a report
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generator. The web page is http://www.flex.ro/pgaccess
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We also include ecpg, which is an embedded SQL query language
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interface for C.
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2.4) What languages are available to communicate with PostgreSQL?
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We have:
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* C(libpq)
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* C++(libpq++)
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* Embedded C(ecpg)
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* Java(jdbc)
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* Perl(perl5)
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* ODBC(odbc)
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* Python(PyGreSQL)
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* TCL(libpgtcl)
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* A crude C/4GL(contrib/pginterface)
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* Embedded HTML(PHP from http://www.php.net)
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_________________________________________________________________
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Administrative Questions
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3.1) Why does initdb fail?
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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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3.2) How do I install PostgreSQL somewhere other than /usr/local/pgsql?
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The simplest way is to specify the --prefix option when running
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configure. If you forgot to do that, you can edit Makefile.global and
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change POSTGRESDIR accordingly, or create a Makefile.custom and define
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POSTGRESDIR there.
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3.3) When I start the postmaster, I get a Bad System Call or core dumped
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message. Why?
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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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3.4) When I try to start the postmaster, I get IpcMemoryCreate errors. Why?
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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 and backend processes you configure postmaster to run with.
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|
For most systems, with default numbers of buffers and processes, you
|
|
need a minimum of ~1MB.
|
|
|
|
3.5) When I try to start the postmaster, I get IpcSemaphoreCreate errors.
|
|
Why?
|
|
|
|
If the error message is IpcSemaphoreCreate: semget failed (No space
|
|
left on device) then your kernel is not configured with enough
|
|
semaphores. Postgres needs one semaphore per potential backend
|
|
process. A temporary solution is to start the postmaster with a
|
|
smaller limit on the number of backend processes. Use -N with a
|
|
parameter less than the default of 32. A more permanent solution is to
|
|
increase your kernel's SEMMNS and SEMMNI parameters.
|
|
|
|
If the error message is something else, you might not have semaphore
|
|
support configured in your kernel at all.
|
|
|
|
3.6) How do I prevent other hosts from accessing my PostgreSQL database?
|
|
|
|
By default, PostgreSQL only allows connections from the local machine
|
|
using unix domain sockets. Other machines will not be able to connect
|
|
unless you add the -i flag to the postmaster, and enable host-based
|
|
authentication by modifying the file $PGDATA/pg_hba.conf accordingly.
|
|
This will allow TCP/IP connections.
|
|
|
|
3.7) 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, make sure the
|
|
postmaster has been started with the -i option, and add an appropriate
|
|
host entry to the file pgsql/data/pg_hba.conf. See the pg_hba.conf
|
|
manual page.
|
|
|
|
3.8) Why can't I access the database as the root user?
|
|
|
|
You should not create database users with user id 0 (root). They will
|
|
be unable to access the database. This is a security precaution
|
|
because of the ability of any user to dynamically link object modules
|
|
into the database engine.
|
|
|
|
3.9) All my servers crash under concurrent table access. Why?
|
|
|
|
This problem can be caused by a kernel that is not configured to
|
|
support semaphores.
|
|
|
|
3.10) How do I tune the database engine for better performance?
|
|
|
|
Certainly, indices can speed up queries. The EXPLAIN command allows
|
|
you to see how PostgreSQL is interpreting your query, and which
|
|
indices are being used.
|
|
|
|
If you are doing a lot of INSERTs, consider doing them in a large
|
|
batch using the COPY command. This is much faster than single
|
|
individual INSERTS. Second, statements not in a BEGIN WORK/COMMIT
|
|
transaction block are considered to be in their own transaction.
|
|
Consider performing several statements in a single transaction block.
|
|
This reduces the transaction overhead. Also consider dropping and
|
|
recreating indices when making large data changes.
|
|
|
|
There are several tuning things that can be done. You can disable
|
|
fsync() by starting the postmaster with a -o -F option. This will
|
|
prevent fsync()'s from flushing to disk after every transaction.
|
|
|
|
You can also use the postmaster -B option to increase the number of
|
|
shared memory buffers used by the backend processes. If you make this
|
|
parameter too high, the postmaster may not start up because you've
|
|
exceeded your kernel's limit on shared memory space. Each buffer is 8K
|
|
and the default is 64 buffers.
|
|
|
|
You can also use the backend -S option to increase the maximum amount
|
|
of memory used by each backend process for temporary sorts. The -S
|
|
value is measured in kilobytes, and the default is 512 (ie, 512K). It
|
|
is unwise to make this value too large, or you may run out of memory
|
|
when a query invokes several concurrent sorts.
|
|
|
|
You can also use the CLUSTER command to group data in base tables to
|
|
match an index. See the cluster(l) manual page for more details.
|
|
|
|
3.11) What debugging features are available in PostgreSQL?
|
|
|
|
PostgreSQL has several features that report status information that
|
|
can be valuable for debugging purposes.
|
|
|
|
First, by running configure with the --enable-cassert option, many
|
|
assert()'s monitor the progress of the backend and halt the program
|
|
when something unexpected occurs.
|
|
|
|
Both postmaster and postgres have several debug options available.
|
|
First, whenever you start the postmaster, make sure you send the
|
|
standard output and error to a log file, like:
|
|
cd /usr/local/pgsql
|
|
./bin/postmaster >server.log 2>&1 &
|
|
|
|
This will put a server.log file in the top-level PostgreSQL directory.
|
|
This file contains useful information about problems or errors
|
|
encountered by the server. Postmaster has a -d option that allows even
|
|
more detailed information to be reported. The -d option takes a number
|
|
that specifies the debug level. Be warned that high debug level values
|
|
generate large log files.
|
|
|
|
You can actually run the postgres backend from the command line, and
|
|
type your SQL statement directly. This is recommended only for
|
|
debugging purposes. Note that a newline terminates the query, not a
|
|
semicolon. If you have compiled with debugging symbols, you can use a
|
|
debugger to see what is happening. Because the backend was not started
|
|
from the postmaster, it is not running in an identical environment and
|
|
locking/backend interaction problems may not be duplicated. Some
|
|
operating system can attach to a running backend directly to diagnose
|
|
problems.
|
|
|
|
The postgres program has -s, -A, and -t options that can be very
|
|
useful for debugging and performance measurements.
|
|
|
|
You can also compile with profiling to see what functions are taking
|
|
execution time. The backend profile files will be deposited in the
|
|
pgsql/data/base/dbname directory. The client profile file will be put
|
|
in the current directory.
|
|
|
|
3.12) I get 'Sorry, too many clients' when trying to connect. Why?
|
|
|
|
You need to increase the postmaster's limit on how many concurrent
|
|
backend processes it can start.
|
|
|
|
In Postgres 6.5.*, the default limit is 32 processes. You can increase
|
|
it by restarting the postmaster with a suitable -N value. With the
|
|
default configuration you can set -N as large as 1024; if you need
|
|
more, increase MAXBACKENDS in include/config.h and rebuild. You can
|
|
set the default value of -N at configuration time, if you like, using
|
|
configure's --with-maxbackends switch.
|
|
|
|
Note that if you make -N larger than 32, you should consider
|
|
increasing -B beyond its default of 64. For large numbers of backend
|
|
processes, you are also likely to find that you need to increase
|
|
various Unix kernel configuration parameters. Things to check include
|
|
the maximum size of shared memory blocks, SHMMAX, the maximum number
|
|
of semaphores, SEMMNS and SEMMNI, the maximum number of processes,
|
|
NPROC, the maximum number of processes per user, MAXUPRC, and the
|
|
maximum number of open files, NFILE and NINODE. The reason that
|
|
Postgres has a limit on the number of allowed backend processes is so
|
|
that you can ensure that your system won't run out of resources.
|
|
|
|
In Postgres versions prior to 6.5, the maximum number of backends was
|
|
64, and changing it required a rebuild after altering the MaxBackendId
|
|
constant in include/storage/sinvaladt.h.
|
|
|
|
3.13) What are the pg_tempNNN.NN files in my database directory?
|
|
|
|
They are temporary files generated by the query executor. For example,
|
|
if a sort needs to be done to satisfy an ORDER BY, and the sort
|
|
requires more space than the backend's -S parameter allows, then temp
|
|
files are created to hold the extra data.
|
|
|
|
The temp files should go away automatically, but might not if a
|
|
backend crashes during a sort. If you have no transactions running at
|
|
the time, it is safe to delete the pg_tempNNN.NN files.
|
|
|
|
3.14) 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[].
|
|
_________________________________________________________________
|
|
|
|
Operational Questions
|
|
|
|
4.1) The system seems to be confused about commas, decimal points, and date
|
|
formats.
|
|
|
|
Check your locale configuration. PostgreSQL uses the locale settings
|
|
of the user that ran the postmaster process. There are postgres and
|
|
psql SET commands to control the date format. Set those accordingly
|
|
for your operating environment.
|
|
|
|
4.2) What is the exact difference between binary cursors and normal
|
|
cursors?
|
|
|
|
See the DECLARE manual page for a description.
|
|
|
|
4.3) How do I SELECT only the first few rows of a query?
|
|
|
|
See the FETCH manual page, or use SELECT ... LIMIT....
|
|
|
|
The entire query may have to be evaluated, even if you only want the
|
|
first few rows. Consider a query that has an ORDER BY. If there is an
|
|
index that matches the ORDER BY, PostgreSQL may be able to evaluate
|
|
only the first few records requested, or the entire query may have to
|
|
be evaluated until the desired rows have been generated.
|
|
|
|
4.4) How do I get a list of tables, or other information I see in psql?
|
|
|
|
You can read the source code for psql, file pgsql/src/bin/psql/psql.c.
|
|
It contains SQL commands that generate the output for psql's backslash
|
|
commands. Beginning in Postgres 6.5, you can also start psql with the
|
|
-E option so that it will print out the queries it uses to execute the
|
|
commands you give.
|
|
|
|
4.5) 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;
|
|
|
|
4.6) What is the maximum size for a row, table, database?
|
|
|
|
Rows are limited to 8K bytes, but this can be changed by editing
|
|
include/config.h and changing BLCKSZ. To use attributes larger than
|
|
8K, you can also use the large object interface.
|
|
|
|
Rows do not cross 8k boundaries so a 5k row will require 8k of
|
|
storage.
|
|
|
|
Table and database sizes are unlimited. There are many databases that
|
|
are tens of gigabytes, and probably some that are hundreds.
|
|
|
|
4.7)How much database disk space is required to store data from a typical
|
|
flat file?
|
|
|
|
A Postgres database can require about six and a half times the disk
|
|
space required to store the data in a flat file.
|
|
|
|
Consider a file of 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 at 14MB:
|
|
36 bytes: each row header (approximate)
|
|
+ 8 bytes: two int fields @ 4 bytes each
|
|
+ 4 bytes: pointer on page to tuple
|
|
----------------------------------------
|
|
48 bytes per row
|
|
|
|
The data page size in PostgreSQL is 8192 bytes (8 KB), so:
|
|
|
|
8192 bytes per page
|
|
------------------- = 171 rows per database page (rounded up)
|
|
48 bytes per row
|
|
|
|
300000 data rows
|
|
-------------------- = 1755 database pages
|
|
171 rows per page
|
|
|
|
1755 database pages * 8192 bytes per page = 14,376,960 bytes (14MB)
|
|
|
|
Indexes do not contain as much overhead, but do contain the data that
|
|
is being indexed, so they can be large also.
|
|
|
|
4.8) 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 from the database system
|
|
tables.
|
|
|
|
4.9) My queries are slow or don't make use of the indexes. 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 sequential
|
|
scan would be faster.
|
|
|
|
For column-specific optimization statistics, use VACUUM ANALYZE.
|
|
VACUUM ANALYZE is important for complex multi-join queries, so the
|
|
optimizer can estimate the number of rows returned from each table,
|
|
and choose the proper join order. The backend does not keep track of
|
|
column statistics on its own, so VACUUM ANALYZE must be run to collect
|
|
them periodically.
|
|
|
|
Indexes are not used for ORDER BY operations.
|
|
|
|
When using wild-card operators such as 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 ^. If you have
|
|
locale enabled, indexes can't be used for wild-card searches.
|
|
|
|
4.10) How do I see how the query optimizer is evaluating my query?
|
|
|
|
See the EXPLAIN manual page.
|
|
|
|
4.11) What is an R-tree index?
|
|
|
|
An r-tree index is used for indexing spatial data. A hash index can't
|
|
handle range searches. A B-tree index only handles range searches in a
|
|
single dimension. R-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.
|
|
|
|
4.12) 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 the documentation.
|
|
|
|
4.13) How do I do regular expression searches and case-insensitive regexp
|
|
searching?
|
|
|
|
~ and ~* are probably what you want. See psql's \do command.
|
|
|
|
4.14) In a query, how do I detect if a field is NULL?
|
|
|
|
You test the column with IS NULL and IS NOT NULL.
|
|
|
|
4.15) 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 row 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.
|
|
|
|
4.16.1) How do I create a serial/auto-incrementing field?
|
|
|
|
PostgreSQL supports SERIAL data type. It auto-creates a sequence and
|
|
index on the column. For example, this...
|
|
CREATE TABLE person (
|
|
id SERIAL,
|
|
name TEXT
|
|
);
|
|
|
|
...is automatically translated into this...
|
|
CREATE SEQUENCE person_id_seq;
|
|
CREATE TABLE person (
|
|
id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
|
|
name TEXT
|
|
);
|
|
CREATE UNIQUE INDEX person_id_key ON person ( id );
|
|
|
|
See the create_sequence manual page for more information about
|
|
sequences. You can also use each row's oid field as a unique value.
|
|
However, if you need to dump and reload the database, you need to use
|
|
pg_dump's -o option or COPY WITH OIDS option to preserve the oids.
|
|
|
|
For more details, see Bruce Momjian's chapter on Numbering Rows.
|
|
|
|
4.16.2) How do I get the back the generated SERIAL value after an insert?
|
|
|
|
Probably the simplest approach is to to retrieve the next SERIAL value
|
|
from the sequence object with the nextval() function before inserting
|
|
and then insert it explicitly. Using the example table in 4.16.1, that
|
|
might look like this:
|
|
$newSerialID = nextval('person_id_seq');
|
|
INSERT INTO person (id, name) VALUES ($newSerialID, 'Blaise Pascal');
|
|
|
|
You would then also have the new value stored in $newSerialID for use
|
|
in other queries (e.g., as a foreign key to the person table). Note
|
|
that the name of the automatically-created SEQUENCE object will be
|
|
named <table>_<serialcolumn>_seq, where table and serialcolumn are the
|
|
names of your table and your SERIAL column, respectively.
|
|
|
|
Similarly, you could retrieve the just-assigned SERIAL value with the
|
|
currval() function after it was inserted by default, e.g.,
|
|
INSERT INTO person (name) VALUES ('Blaise Pascal');
|
|
$newID = currval('person_id_seq');
|
|
|
|
Finally, you could use the oid returned from the INSERT statement to
|
|
lookup the default value, though this is probably the least portable
|
|
approach. In perl, using DBI with Edmund Mergl's DBD::Pg module, the
|
|
oid value is made available via $sth->{pg_oid_status} after
|
|
$sth->execute().
|
|
|
|
4.16.3) Wouldn't use of currval() and nextval() lead to a race condition
|
|
with other concurrent backend processes?
|
|
|
|
No. That has been handled by the backends.
|
|
|
|
4.17) 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(old_oid oid, mycol int);
|
|
SELECT INTO new SELECT old_oid, mycol FROM old;
|
|
COPY new TO '/tmp/pgtable';
|
|
DELETE FROM new;
|
|
COPY new WITH OIDS FROM '/tmp/pgtable';
|
|
|
|
Tids are used to identify specific physical rows with block and offset
|
|
values. Tids change after rows are modified or reloaded. They are used
|
|
by index entries to point to physical rows.
|
|
|
|
4.18) 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:
|
|
* table, relation, class
|
|
* row, record, tuple
|
|
* column, field, attribute
|
|
* retrieve, select
|
|
* replace, update
|
|
* append, insert
|
|
* oid, serial value
|
|
* portal, cursor
|
|
* range variable, table name, table alias
|
|
|
|
4.19) 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 you are having a
|
|
problem with the SQL client because the backend is returning too much
|
|
data, try it before starting the client.
|
|
|
|
4.20) How do I tell what PostgreSQL version I am running?
|
|
|
|
From psql, type select version();
|
|
|
|
4.21) My large-object operations get invalid large obj descriptor. Why?
|
|
|
|
You need to put BEGIN WORK and COMMIT around any use of a large object
|
|
handle, that is, surrounding lo_open ... lo_close.
|
|
|
|
The documentation has always stated that lo_open must be wrapped in a
|
|
transaction, but PostgreSQL versions prior to 6.5 didn't enforce that
|
|
rule. Instead, they'd just fail occasionally if you broke it.
|
|
|
|
Current PostgreSQL enforces the rule by closing large object handles
|
|
at transaction commit, which will be instantly upon completion of the
|
|
lo_open command if you are not inside a transaction. So the first
|
|
attempt to do anything with the handle will draw invalid large obj
|
|
descriptor. So code that used to work (at least most of the time) will
|
|
now generate that error message if you fail to use a transaction.
|
|
|
|
If you are using a client interface like ODBC you may need to set
|
|
auto-commit off.
|
|
|
|
4.22) How do I create a column that will default to the current time?
|
|
|
|
The tempation is to do:
|
|
create table test (x int, modtime timestamp default 'now');
|
|
|
|
but this makes the column default to the time of table creation, not
|
|
the time of row insertion. Instead do:
|
|
CREATE TABLE test (x int, modtime timestamp default now() );
|
|
|
|
The calling of the function now() prevents the default value from
|
|
being computed at table creation time, and delays it until insertion
|
|
time. We believe this will not be a problem in post-6.5.* releases.
|
|
|
|
4.23) Why are my subqueries using IN so slow?
|
|
|
|
Currently, we join subqueries to outer queries by sequential scanning
|
|
the result of the subquery for each row of the outer query. A
|
|
workaround is to replace IN with EXISTS. For example, change:
|
|
SELECT *
|
|
FROM tab
|
|
WHERE col1 IN (SELECT col2 FROM TAB2)
|
|
|
|
to:
|
|
SELECT *
|
|
FROM tab
|
|
WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 = col2)
|
|
|
|
We hope to fix this limitation in a future release.
|
|
_________________________________________________________________
|
|
|
|
Extending PostgreSQL
|
|
|
|
5.1) I wrote a user-defined function. When I run it in psql, why does it
|
|
dump core?
|
|
|
|
The problem could be a number of things. Try testing your user-defined
|
|
function in a stand alone test program first. 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
|
|
|
|
5.2) What does the message: NOTICE:PortalHeapMemoryFree: 0x402251d0 not in
|
|
alloc set! mean?
|
|
|
|
You are pfree'ing something that was not palloc'ed. Beware of mixing
|
|
malloc/free and palloc/pfree.
|
|
|
|
5.3) How can I contribute some nifty new types and functions for
|
|
PostgreSQL?
|
|
|
|
Send your extensions to the pgsql-hackers mailing list, and they will
|
|
eventually end up in the contrib/ subdirectory.
|
|
|
|
5.4) How do I write a C function to return a tuple?
|
|
|
|
This requires wizardry so extreme that the authors have never tried
|
|
it, though in principle it can be done.
|
|
|
|
5.5) I have changed a source file. Why does the recompile does not see the
|
|
change?
|
|
|
|
The Makefiles do not have the proper dependencies for include files.
|
|
You have to do a make clean and then another make.
|