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