forked from postgres/postgres
-
Notifications
You must be signed in to change notification settings - Fork 6
/
Copy pathFAQ
1168 lines (905 loc) · 51 KB
/
FAQ
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Mon Mar 29 00:07:11 EST 2004
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
The most recent version of this document can be viewed at
http://www.PostgreSQL.org/docs/faqs/FAQ.html.
Platform-specific questions are answered at
http://www.PostgreSQL.org/docs/index.html.
_________________________________________________________________
General Questions
1.1) What is PostgreSQL? How is it pronounced?
1.2) What is the copyright on PostgreSQL?
1.3) What Unix platforms does PostgreSQL run on?
1.4) What non-Unix ports are available?
1.5) Where can I get PostgreSQL?
1.6) Where can I get support?
1.7) What is the latest release?
1.8) What documentation is available?
1.9) How do I find out about known bugs or missing features?
1.10) How can I learn SQL?
1.11) Is PostgreSQL Y2K compliant?
1.12) How do I join the development team?
1.13) How do I submit a bug report?
1.14) How does PostgreSQL compare to other DBMSs?
1.15) How can I financially assist PostgreSQL?
User Client Questions
2.1) Are there ODBC drivers for PostgreSQL?
2.2) What tools are available for using PostgreSQL with Web pages?
2.3) Does PostgreSQL have a graphical user interface?
2.4) What languages are available to communicate with PostgreSQL?
Administrative Questions
3.1) How do I install PostgreSQL somewhere other than
/usr/local/pgsql?
3.2) When I start postmaster, I get a Bad System Call or core dumped
message. Why?
3.3) When I try to start postmaster, I get IpcMemoryCreate errors.
Why?
3.4) When I try to start postmaster, I get IpcSemaphoreCreate errors.
Why?
3.5) How do I control connections from other hosts?
3.6) How do I tune the database engine for better performance?
3.7) What debugging features are available?
3.8) Why do I get "Sorry, too many clients" when trying to connect?
3.9) What is in the pgsql_tmp directory?
3.10) Why do I need to do a dump and restore to upgrade PostgreSQL
releases?
Operational Questions
4.1) What is the difference between binary cursors and normal cursors?
4.2) How do I SELECT only the first few rows of a query? A random row?
4.3) How do I get a list of tables or other things I can see in psql?
4.4) How do you remove a column from a table, or change it's data
type?
4.5) What is the maximum size for a row, a table, and a database?
4.6) How much database disk space is required to store data from a
typical text file?
4.7) How do I find out what tables, indexes, databases, and users are
defined?
4.8) My queries are slow or don't make use of the indexes. Why?
4.9) How do I see how the query optimizer is evaluating my query?
4.10) What is an R-tree index?
4.11) What is the Genetic Query Optimizer?
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) In a query, how do I detect if a field is NULL?
4.14) What is the difference between the various character types?
4.15.1) How do I create a serial/auto-incrementing field?
4.15.2) How do I get the value of a SERIAL insert?
4.15.3) Don't currval() and nextval() lead to a race condition with
other 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?
4.16) What is an OID? What is a TID?
4.17) What is the meaning of some of the terms used in PostgreSQL?
4.18) Why do I get the error "ERROR: Memory exhausted in
AllocSetAlloc()"?
4.19) How do I tell what PostgreSQL version I am running?
4.20) Why does my large-object operations get "invalid large obj
descriptor"?
4.21) How do I create a column that will default to the current time?
4.22) Why are my subqueries using IN so slow?
4.23) How do I perform an outer join?
4.24) How do I perform queries using multiple databases?
4.25) How do I return multiple rows or columns from a function?
4.26) Why can't I reliably create/drop temporary tables in PL/PgSQL
functions?
4.27) What replication options are available?
4.28) What encryption options are available?
Extending PostgreSQL
5.1) I wrote a user-defined function. When I run it in psql, why does
it dump core?
5.2) How can I contribute some nifty new types and functions to
PostgreSQL?
5.3) How do I write a C function to return a tuple?
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. An audio file is available at
http://www.postgresql.org/postgresql.mp3 for those would like to hear
the pronunciation.
PostgreSQL is an enhancement of the POSTGRES database management
system (and is still sometimes reffered to as simply "Postgres"), 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 on how to join). This team is now responsible for all development
of PostgreSQL. It is a community project and is not controlled by any
company. To get involved, see the developer's FAQ at
http://www.PostgreSQL.org/docs/faqs/FAQ_DEV.html
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) 1996-2004, 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, any 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 and
http://momjian.postgresql.org/main/writings/pgsql/win32.html.
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 Freenode and EFNet, channel
#PostgreSQL. You can use the Unix command irc -c '#PostgreSQL' "$USER"
irc.phoenix.net. or irc -c '#PostgreSQL' "$USER" irc.freenode.net.
A list of commercial support companies is available at
http://techdocs.postgresql.org/companies.php.
1.7) What is the latest release?
The latest release of PostgreSQL is version 7.4.2.
We plan to have major releases every six to eight 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://techdocs.PostgreSQL.org/techdocs/bookreviews.php. 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 faster for multiple users, complex queries, and a
read/write query load. MySQL is faster for simple SELECT
queries done by a few users. Of course, MySQL does not have
most of the features mentioned in the Features section above.
We are built for reliability and features, and 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 Also, MySQL is
is a company that distributes its products via open source, and
requires a commercial license for close-source software, not an
open source development community like PostgreSQL.
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.)
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.
_________________________________________________________________
Also, if you have a success story about PostgreSQL, please submit it
to our advocacy site at http://advocacy.postgresql.org.
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 III
(http://www.pgadmin.org, 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 turn on tcpip_sockets in the postgresql.conf 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 ANALYZE 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? A random row?
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.
To SELECT a random row, use:
SELECT col
FROM tab
ORDER BY random()
LIMIT 1;
4.3) How do I get a list of tables or other things I can see in psql?
Use the \dt command to see tables in psql. For a complete list of
commands inside psql you can use \?. Alternatively 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. PostgreSQL
also provides an SQLi compliant INFORMATION SCHEMA interface you can
query to get information about the database.
4.4) How do you remove a column from a table, or change its data type?
DROP COLUMN 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;
To change the data type of a column, do this:
BEGIN;
ALTER TABLE tab ADD COLUMN new_col new_data_type;
UPDATE tab SET new_col = CAST(old_col AS new_data_type);
ALTER TABLE tab DROP COLUMN old_col;
COMMIT;
You might then want to do VACUUM FULL tab to reclaim the disk space
used by the expired rows.
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 (32 TB databases exist)
Maximum size for a table? 32 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 32 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 quadrupled
by increasing the default block size 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 as 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.
+ LIKE patterns must not start with %.
+ ~ (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.
* 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.
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
);
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, 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
<table>_<serialcolumn>_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 returned from the INSERT statement to
look up the default value, though this is probably the least portable
approach, and the oid value will wrap around when it reaches 4
billion. 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()"?