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