From: Bruce Momjian Date: Mon, 2 Jun 2003 18:16:56 +0000 (+0000) Subject: Add Turkish FAQ, from Devrim GUNDUZ. X-Git-Tag: REL7_4_BETA1~488 X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=7facd5d10c3f5913224f598af57f4a78819a827d;p=postgresql Add Turkish FAQ, from Devrim GUNDUZ. --- diff --git a/doc/FAQ_turkish b/doc/FAQ_turkish new file mode 100644 index 0000000000..ce15326b6f --- /dev/null +++ b/doc/FAQ_turkish @@ -0,0 +1,709 @@ + + PostgreSQL için Sýkça Sorulan Sorular (SSS) Son güncelleme : 19 Mayýs + 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 sorularýnýz,http://www.PostgreSQL.org/docs/index.html + adresinde yanýtlanýr.. + ---------------------------------------------------------------------- + -- Genel Sorular 1.1 <#1.1>) PostgreSQL nedir? Nasýl okunur? 1.2 + <#1.2>) PostgreSQL' in haklarý nedir? 1.3 <#1.3>) PostgreSQL, hangi + Unix platformlarýnda çalýsýr? 1.4 <#1.4>) Hangi Unix olmayan + uyarlamalarý bulunmaktadýr? 1.5 <#1.5>) PostgreSQL'i nereden + indirebilirim? 1.6 <#1.6>) Desteði 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>) Nasýl SQL öðrenebilirim? 1.11 <#1.11>) PostgreSQL 2000 + yýlýna uyumlu mudur? 1.12 <#1.12>) Geliþtirme takýmýna nasýl + katýlabilirim?? 1.13 <#1.13>) Bir hata raporunu nasýl gönderebilirim? + 1.14 <#1.14>) PostgreSQL, diger VTYS(DBMS lerle nasýl + karþýlaþtýrýlabilir? 1.15 <#1.15>) PostgreSQL'e maddi açýdan nasýl + destek olabilirim? Kullanýcý/istemci Sorularý 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. diff --git a/doc/src/FAQ/FAQ_turkish.html b/doc/src/FAQ/FAQ_turkish.html new file mode 100644 index 0000000000..38944666bb --- /dev/null +++ b/doc/src/FAQ/FAQ_turkish.html @@ -0,0 +1,1255 @@ + + PostgreSQL için Sýkça Sorulan Sorular (SSS) + +Son güncelleme : 19 Mayýs 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 sorularýnýz,http://www.PostgreSQL.org/docs/index.html +adresinde yanýtlanýr.. + +------------------------------------------------------------------------ + + + Genel Sorular + +1.1 <#1.1>) PostgreSQL nedir? Nasýl okunur? +1.2 <#1.2>) PostgreSQL' in haklarý nedir? +1.3 <#1.3>) PostgreSQL, hangi Unix platformlarýnda çalýsýr? +1.4 <#1.4>) Hangi Unix olmayan uyarlamalarý bulunmaktadýr? +1.5 <#1.5>) PostgreSQL'i nereden indirebilirim? +1.6 <#1.6>) Desteði 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>) Nasýl SQL öðrenebilirim? +1.11 <#1.11>) PostgreSQL 2000 yýlýna uyumlu mudur? +1.12 <#1.12>) Geliþtirme takýmýna nasýl katýlabilirim?? +1.13 <#1.13>) Bir hata raporunu nasýl gönderebilirim? +1.14 <#1.14>) PostgreSQL, diger VTYS(DBMS lerle nasýl karþýlaþtýrýlabilir? +1.15 <#1.15>) PostgreSQL'e maddi açýdan nasýl destek olabilirim? + + + Kullanýcý/istemci Sorularý + +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 +. + +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 +. + +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 + 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. + There is a nice tutorial at +http://www.intermedia.net/support/sql/sqltut.shtm, + at +http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM, + +and at http://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 + + +*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 +. + +OpenLink ODBC can be gotten from http://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 +), PgAdmin II (http://www.pgadmin.org, +Win32-only), RHDB Admin (http://sources.redhat.com/rhdb/ ) and Rekall ( +http://www.thekompany.com/products/rekall/ +, proprietary). There is +also PHPPgAdmin ( http://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 + +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 +. + + + 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 + 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. +