FAQ 43 KB
Newer Older
Bruce Momjian's avatar
Bruce Momjian committed
1 2 3

                Frequently Asked Questions (FAQ) for PostgreSQL
                                       
Bruce Momjian's avatar
Bruce Momjian committed
4
   Last updated: Tue Oct 17 00:21:20 EDT 2000
Bruce Momjian's avatar
Bruce Momjian committed
5
   
Bruce Momjian's avatar
Bruce Momjian committed
6
   Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
Bruce Momjian's avatar
Bruce Momjian committed
7
   
Bruce Momjian's avatar
Bruce Momjian committed
8 9
   The most recent version of this document can be viewed at
   http://www.PostgreSQL.org/docs/faq-english.html.
Bruce Momjian's avatar
Bruce Momjian committed
10
   
Bruce Momjian's avatar
Bruce Momjian committed
11
   Platform-specific questions are answered at
Bruce Momjian's avatar
Bruce Momjian committed
12
   http://www.PostgreSQL.org/users-lounge/docs/faq.html.
Bruce Momjian's avatar
Bruce Momjian committed
13 14
     _________________________________________________________________
   
15
                             General Questions
Bruce Momjian's avatar
Bruce Momjian committed
16
                                      
Bruce Momjian's avatar
Bruce Momjian committed
17
   1.1) What is PostgreSQL?
18 19 20 21
   1.2) What's 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?
Bruce Momjian's avatar
Bruce Momjian committed
22 23 24
   1.6) Where can I get support?
   1.7) What is the latest release?
   1.8) What documentation is available?
25 26 27 28 29 30 31 32
   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 DBMS's?
   
                           User Client Questions
Bruce Momjian's avatar
Bruce Momjian committed
33
                                      
34
   2.1) Are there ODBC drivers for PostgreSQL?
Bruce Momjian's avatar
Bruce Momjian committed
35
   2.2) What tools are available for use PostgreSQL with Web pages?
36 37 38 39 40 41
   2.3) Does PostgreSQL have a graphical user interface? A report
   generator? An embedded query language interface?
   2.4) What languages are available to communicate with PostgreSQL?
   
                          Administrative Questions
                                      
Bruce Momjian's avatar
Bruce Momjian committed
42
   3.1) How do I install PostgreSQL somewhere other than
Bruce Momjian's avatar
Bruce Momjian committed
43
   /usr/local/pgsql?
Bruce Momjian's avatar
Bruce Momjian committed
44
   3.2) When I start the postmaster, I get a Bad System Call or core
Bruce Momjian's avatar
Bruce Momjian committed
45
   dumped message. Why?
Bruce Momjian's avatar
Bruce Momjian committed
46
   3.3) When I try to start the postmaster, I get IpcMemoryCreate errors.
Bruce Momjian's avatar
Bruce Momjian committed
47
   Why?
Bruce Momjian's avatar
Bruce Momjian committed
48
   3.4) When I try to start the postmaster, I get IpcSemaphoreCreate
Bruce Momjian's avatar
Bruce Momjian committed
49
   errors. Why?
Bruce Momjian's avatar
Bruce Momjian committed
50
   3.5) How do I prevent other hosts from accessing my PostgreSQL
51
   database?
Bruce Momjian's avatar
Bruce Momjian committed
52
   3.6) Why can't I connect to my database from another machine?
Bruce Momjian's avatar
Bruce Momjian committed
53 54 55 56
   3.7) How do I tune the database engine for better performance?
   3.8) What debugging features are available?
   3.9) I get "Sorry, too many clients" when trying to connect. Why?
   3.10) What are the pg_sorttempNNN.NN files in my database directory?
57 58
   
                           Operational Questions
Bruce Momjian's avatar
Bruce Momjian committed
59
                                      
Bruce Momjian's avatar
Bruce Momjian committed
60 61
   4.1) Why is the system confused about commas, decimal points, and date
   formats.
62
   4.2) What is the exact difference between binary cursors and normal
Bruce Momjian's avatar
Bruce Momjian committed
63
   cursors?
Bruce Momjian's avatar
Bruce Momjian committed
64 65
   4.3) How do I SELECT only the first few rows of a query?
   4.4) How do I get a list of tables or other things I can see in psql?
66 67 68
   4.5) How do you remove a column from a table?
   4.6) What is the maximum size for a row, table, database?
   4.7) How much database disk space is required to store data from a
Bruce Momjian's avatar
Bruce Momjian committed
69
   typical text file?
Bruce Momjian's avatar
Bruce Momjian committed
70
   4.8) How do I find out what tables or indexes are defined in the
71 72 73 74
   database?
   4.9) My queries are slow or don't make use of the indexes. Why?
   4.10) How do I see how the query optimizer is evaluating my query?
   4.11) What is an R-tree index?
Bruce Momjian's avatar
Bruce Momjian committed
75 76 77
   4.12) What is the Genetic Query Optimizer?
   4.13) How do I perform regular expression searches and
   case-insensitive regular expression searches?
78 79
   4.14) In a query, how do I detect if a field is NULL?
   4.15) What is the difference between the various character types?
Bruce Momjian's avatar
Bruce Momjian committed
80
   4.16.1) How do I create a serial/auto-incrementing field?
Bruce Momjian's avatar
Bruce Momjian committed
81
   4.16.2) How do I get the value of a SERIAL insert?
Bruce Momjian's avatar
Bruce Momjian committed
82
   4.16.3) Don't currval() and nextval() lead to a race condition with
Bruce Momjian's avatar
Bruce Momjian committed
83 84
   other users?
   4.17) What is an OID? What is a TID?
85 86
   4.18) What is the meaning of some of the terms used in PostgreSQL?
   4.19) Why do I get the error "FATAL: palloc failure: memory
Bruce Momjian's avatar
Bruce Momjian committed
87
   exhausted?"
88
   4.20) How do I tell what PostgreSQL version I am running?
Bruce Momjian's avatar
Bruce Momjian committed
89 90 91
   4.21) My large-object operations get invalid large obj descriptor.
   Why?
   4.22) How do I create a column that will default to the current time?
Bruce Momjian's avatar
Bruce Momjian committed
92
   4.23) Why are my subqueries using IN so slow?
Bruce Momjian's avatar
Bruce Momjian committed
93
   4.24) How do I perform an outer join?
Bruce Momjian's avatar
Bruce Momjian committed
94
   
95
                            Extending PostgreSQL
Bruce Momjian's avatar
Bruce Momjian committed
96
                                      
97
   5.1) I wrote a user-defined function. When I run it in psql, why does
Bruce Momjian's avatar
Bruce Momjian committed
98
   it dump core?
Bruce Momjian's avatar
Bruce Momjian committed
99 100
   5.2) What does the message "NOTICE:PortalHeapMemoryFree: 0x402251d0
   not in alloc set!" mean?
Bruce Momjian's avatar
Bruce Momjian committed
101
   5.3) How can I contribute some nifty new types and functions to
102 103
   PostgreSQL?
   5.4) How do I write a C function to return a tuple?
Bruce Momjian's avatar
Bruce Momjian committed
104 105
   5.5) I have changed a source file. Why does the recompile not see the
   change?
Bruce Momjian's avatar
Bruce Momjian committed
106 107
     _________________________________________________________________
   
108 109 110 111
                             General Questions
                                      
    1.1) What is PostgreSQL?
    
Bruce Momjian's avatar
Bruce Momjian committed
112 113 114 115 116 117
   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.
   
Bruce Momjian's avatar
Bruce Momjian committed
118 119 120 121 122
   PostgreSQL development is performed by a team of Internet developers
   who all subscribe to the PostgreSQL development mailing list. The
   current coordinator is Marc G. Fournier (scrappy@PostgreSQL.org). (See
   below on how to join). This team is now responsible for all
   development of PostgreSQL.
Bruce Momjian's avatar
Bruce Momjian committed
123 124
   
   The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen. Many
Bruce Momjian's avatar
Bruce Momjian committed
125
   others have contributed to the porting, testing, debugging, and
Bruce Momjian's avatar
Bruce Momjian committed
126 127 128 129 130 131 132 133 134 135
   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.
   
Bruce Momjian's avatar
Bruce Momjian committed
136 137
   It is pronounced Post-Gres-Q-L.
   
138 139
    1.2) What's the copyright on PostgreSQL?
    
Bruce Momjian's avatar
Bruce Momjian committed
140
   PostgreSQL is subject to the following COPYRIGHT:
Bruce Momjian's avatar
Bruce Momjian committed
141 142 143
   
   PostgreSQL Data Base Management System
   
Bruce Momjian's avatar
Bruce Momjian committed
144 145
   Portions copyright (c) 1996-2001, PostgreSQL Global Development Group
   Portions Copyright (c) 1994-6 Regents of the University of California
Bruce Momjian's avatar
Bruce Momjian committed
146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165
   
   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.
   
166 167
    1.3) What Unix platforms does PostgreSQL run on?
    
Bruce Momjian's avatar
Bruce Momjian committed
168 169 170 171
   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.
   
172 173
    1.4) What non-unix ports are available?
    
Bruce Momjian's avatar
Bruce Momjian committed
174 175
        Client
        
176 177 178
   It is possible to compile the libpq C library, psql, and other
   interfaces and binaries to run on MS Windows platforms. In this case,
   the client is running on MS Windows, and communicates via TCP/IP to a
Bruce Momjian's avatar
Bruce Momjian committed
179 180
   server running on one of our supported Unix platforms. A file
   win31.mak is included in the distribution for making a Win32 libpq
Bruce Momjian's avatar
Bruce Momjian committed
181
   library and psql. PostgreSQL also communicates with ODBC clients.
182
   
Bruce Momjian's avatar
Bruce Momjian committed
183 184
        Server
        
Bruce Momjian's avatar
Bruce Momjian committed
185 186
   The database server can run on Windows NT and later using Cygwin, the
   Cygnus Unix/NT porting library. See pgsql/doc/FAQ_MSWIN in the
Bruce Momjian's avatar
Bruce Momjian committed
187 188 189 190
   distribution or the MS Windows FAQ on our web site. The database
   server does not run on MS Windows 9X because Cygwin does not support
   the required features on those platforms. We have no plan to do a
   native port to any Microsoft platform.
191 192 193 194
   
    1.5) Where can I get PostgreSQL?
    
   The primary anonymous ftp site for PostgreSQL is
Bruce Momjian's avatar
Bruce Momjian committed
195
   ftp://ftp.PostgreSQL.org/pub. For mirror sites, see our main Web site.
196
   
Bruce Momjian's avatar
Bruce Momjian committed
197
    1.6) Where can I get support?
198
    
Bruce Momjian's avatar
Bruce Momjian committed
199
   The main mailing list is: pgsql-general@PostgreSQL.org. It is
Bruce Momjian's avatar
Bruce Momjian committed
200
   available for discussion of matters pertaining to PostgreSQL. To
Bruce Momjian's avatar
Bruce Momjian committed
201 202
   subscribe, send mail with the following lines in the body (not the
   subject line)
Bruce Momjian's avatar
Bruce Momjian committed
203 204
    subscribe
    end
Bruce Momjian's avatar
Bruce Momjian committed
205

Bruce Momjian's avatar
Bruce Momjian committed
206
   to pgsql-general-request@PostgreSQL.org.
Bruce Momjian's avatar
Bruce Momjian committed
207 208
   
   There is also a digest list available. To subscribe to this list, send
Bruce Momjian's avatar
Bruce Momjian committed
209
   email to: pgsql-general-digest-request@PostgreSQL.org with a body of:
Bruce Momjian's avatar
Bruce Momjian committed
210 211
    subscribe
    end
Bruce Momjian's avatar
Bruce Momjian committed
212 213 214 215

   Digests are sent out to members of this list whenever the main list
   has received around 30k of messages.
   
Bruce Momjian's avatar
Bruce Momjian committed
216
   The bugs mailing list is available. To subscribe to this list, send
Bruce Momjian's avatar
Bruce Momjian committed
217
   email to pgsql-bugs-request@PostgreSQL.org with a body of:
Bruce Momjian's avatar
Bruce Momjian committed
218 219
    subscribe
    end
Bruce Momjian's avatar
Bruce Momjian committed
220

Bruce Momjian's avatar
Bruce Momjian committed
221
   There is also a developers discussion mailing list available. To
Bruce Momjian's avatar
Bruce Momjian committed
222 223
   subscribe to this list, send email to
   pgsql-hackers-request@PostgreSQL.org with a body of:
Bruce Momjian's avatar
Bruce Momjian committed
224 225
    subscribe
    end
Bruce Momjian's avatar
Bruce Momjian committed
226

Bruce Momjian's avatar
Bruce Momjian committed
227 228
   Additional mailing lists and information about PostgreSQL can be found
   via the PostgreSQL WWW home page at:
Bruce Momjian's avatar
Bruce Momjian committed
229
   
Bruce Momjian's avatar
Bruce Momjian committed
230
     http://www.PostgreSQL.org
Bruce Momjian's avatar
Bruce Momjian committed
231
     
Bruce Momjian's avatar
Bruce Momjian committed
232
   There is also an IRC channel on EFNet, channel #PostgreSQL. I use the
Bruce Momjian's avatar
Bruce Momjian committed
233
   unix command irc -c '#PostgreSQL' "$USER" irc.phoenix.net.
Bruce Momjian's avatar
Bruce Momjian committed
234
   
Bruce Momjian's avatar
Bruce Momjian committed
235
   A list of commercial support companies is available at
Bruce Momjian's avatar
Bruce Momjian committed
236
   http://www.postgresql.org/users-lounge/commercial-support.html.
237
   
Bruce Momjian's avatar
Bruce Momjian committed
238
    1.7) What is the latest release?
239
    
Bruce Momjian's avatar
Bruce Momjian committed
240
   The latest release of PostgreSQL is version 7.0.3.
Bruce Momjian's avatar
Bruce Momjian committed
241
   
Bruce Momjian's avatar
Bruce Momjian committed
242
   We plan to have major releases every four months.
Bruce Momjian's avatar
Bruce Momjian committed
243
   
Bruce Momjian's avatar
Bruce Momjian committed
244
    1.8) What documentation is available?
245
    
Bruce Momjian's avatar
Bruce Momjian committed
246
   Several manuals, manual pages, and some small test examples are
Bruce Momjian's avatar
Bruce Momjian committed
247
   included in the distribution. See the /doc directory. You can also
Bruce Momjian's avatar
Bruce Momjian committed
248
   browse the manual online at
Bruce Momjian's avatar
Bruce Momjian committed
249
   http://www.PostgreSQL.org/users-lounge/docs/.
Bruce Momjian's avatar
Bruce Momjian committed
250
   
Bruce Momjian's avatar
Bruce Momjian committed
251 252
   There is a PostgreSQL book available at
   http://www.PostgreSQL.org/docs/awbook.html.
Bruce Momjian's avatar
Bruce Momjian committed
253 254 255
   
   psql has some nice \d commands to show information about types,
   operators, functions, aggregates, etc.
Bruce Momjian's avatar
Bruce Momjian committed
256
   
Bruce Momjian's avatar
Bruce Momjian committed
257
   Our Web site contains even more documentation.
258 259 260
   
    1.9) How do I find out about known bugs or missing features?
    
Bruce Momjian's avatar
Bruce Momjian committed
261 262
   PostgreSQL supports an extended subset of SQL-92. See our TODO list
   for known bugs, missing features, and future plans.
263 264 265
   
    1.10) How can I learn SQL?
    
Bruce Momjian's avatar
Bruce Momjian committed
266
   The PostgreSQL book at http://www.PostgreSQL.org/docs/awbook.html
Bruce Momjian's avatar
Bruce Momjian committed
267
   teaches SQL. There is a nice tutorial at
Bruce Momjian's avatar
Bruce Momjian committed
268
   http://w3.one.net/~jhoffman/sqltut.htm and at
Bruce Momjian's avatar
Bruce Momjian committed
269
   http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM.
Bruce Momjian's avatar
Bruce Momjian committed
270 271
   
   Another one is "Teach Yourself SQL in 21 Days, Second Edition" at
Bruce Momjian's avatar
Bruce Momjian committed
272
   http://members.tripod.com/er4ebus/sql/index.htm
273
   
Bruce Momjian's avatar
Bruce Momjian committed
274
   Many of our users like The Practical SQL Handbook, Bowman, Judith S.,
Bruce Momjian's avatar
Bruce Momjian committed
275
   et al., Addison-Wesley. Others like The Complete Reference SQL, Groff
Bruce Momjian's avatar
Bruce Momjian committed
276
   et al., McGraw-Hill.
277 278 279 280
   
    1.11) Is PostgreSQL Y2K compliant?
    
   Yes, we easily handle dates past the year 2000AD, and before 2000BC.
Bruce Momjian's avatar
Bruce Momjian committed
281
   
282 283
    1.12) How do I join the development team?
    
Bruce Momjian's avatar
Bruce Momjian committed
284
   First, download the latest source and read the PostgreSQL Developers
Bruce Momjian's avatar
Bruce Momjian committed
285
   documentation on our Web site, or in the distribution. Second,
286 287
   subscribe to the pgsql-hackers and pgsql-patches mailing lists. Third,
   submit high-quality patches to pgsql-patches.
Bruce Momjian's avatar
Bruce Momjian committed
288
   
Bruce Momjian's avatar
Bruce Momjian committed
289
   There are about a dozen people who have commit privileges to the
Bruce Momjian's avatar
Bruce Momjian committed
290
   PostgreSQL CVS archive. They each have submitted so many high-quality
Bruce Momjian's avatar
Bruce Momjian committed
291 292 293
   patches that it was impossible for the existing committers to keep up,
   and we had confidence that patches they committed were of high
   quality.
Bruce Momjian's avatar
Bruce Momjian committed
294
   
295 296
    1.13) How do I submit a bug report?
    
Bruce Momjian's avatar
Bruce Momjian committed
297 298
   Fill out the "bug-template" file and send it to:
   pgsql-bugs@PostgreSQL.org
Bruce Momjian's avatar
Bruce Momjian committed
299
   
Bruce Momjian's avatar
Bruce Momjian committed
300
   Also check out our ftp site ftp://ftp.PostgreSQL.org/pub to see if
301 302 303 304 305 306 307 308 309
   there is a more recent PostgreSQL version or patches.
   
    1.14) How does PostgreSQL compare to other DBMS's?
    
   There are several ways of measuring software: features, performance,
   reliability, support, and price.
   
   Features
          PostgreSQL has most features present in large commercial
Bruce Momjian's avatar
Bruce Momjian committed
310 311 312 313
          DBMS's, like transactions, subselects, triggers, views, foreign
          key referential integrity, and sophisticated locking. We have
          some features they don't have, like user-defined types,
          inheritance, rules, and multi-version concurrency control to
Bruce Momjian's avatar
Bruce Momjian committed
314
          reduce lock contention.
315 316 317 318
          
   Performance
          PostgreSQL runs in two modes. Normal fsync mode flushes every
          completed transaction to disk, guaranteeing that if the OS
Bruce Momjian's avatar
Bruce Momjian committed
319
          crashes or loses power in the next few seconds, all your data
320 321 322 323 324 325
          is safely stored on disk. In this mode, we are slower than most
          commercial databases, partly because few of them do such
          conservative flushing to disk in their default modes. In
          no-fsync mode, we are usually faster than commercial databases,
          though in this mode, an OS crash could cause data corruption.
          We are working to provide an intermediate mode that suffers
Bruce Momjian's avatar
Bruce Momjian committed
326
          less performance overhead than full fsync mode, and will allow
Bruce Momjian's avatar
Bruce Momjian committed
327
          data integrity within 30 seconds of an OS crash.
328
          In comparison to MySQL or leaner database systems, we are
Bruce Momjian's avatar
Bruce Momjian committed
329 330 331 332
          slower on inserts/updates because we have transaction overhead.
          Of course, MySQL doesn't have any of the features mentioned in
          the Features section above. We are built for flexibility and
          features, though we continue to improve performance through
Bruce Momjian's avatar
Bruce Momjian committed
333
          profiling and source code analysis. There is an interesting Web
Bruce Momjian's avatar
Bruce Momjian committed
334 335
          page comparing PostgreSQL to MySQL at
          http://openacs.org/why-not-mysql.html
Bruce Momjian's avatar
Bruce Momjian committed
336 337 338 339
          We handle each user connection by creating a Unix process.
          Backend processes share data buffers and locking information.
          With multiple CPU's, multiple backends can easily run on
          different CPU's.
340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363
          
   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 list provides a large group of developers and users
          to help resolve any problems encountered. While we can not
          guarantee a fix, commercial DBMS's don't always supply a fix
          either. Direct access to developers, the user community,
          manuals, and the source code often make PostgreSQL support
          superior to other DBMS's. There is commercial per-incident
          support available for those who need it. (See support FAQ
          item.)
          
   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.
     _________________________________________________________________
Bruce Momjian's avatar
Bruce Momjian committed
364
   
365 366 367 368
                           User Client Questions
                                      
    2.1) Are there ODBC drivers for PostgreSQL?
    
Bruce Momjian's avatar
Bruce Momjian committed
369
   There are two ODBC drivers available, PsqlODBC and OpenLink ODBC.
Bruce Momjian's avatar
Bruce Momjian committed
370
   
Bruce Momjian's avatar
Bruce Momjian committed
371
   PsqlODBC is included in the distribution. More information about it
Bruce Momjian's avatar
Bruce Momjian committed
372
   can be gotten from ftp://ftp.PostgreSQL.org/pub/odbc/.
Bruce Momjian's avatar
Bruce Momjian committed
373
   
Bruce Momjian's avatar
Bruce Momjian committed
374 375 376 377 378 379
   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
Bruce Momjian's avatar
Bruce Momjian committed
380 381 382
   commercial-quality support, but a freeware version will always be
   available. Questions to postgres95@openlink.co.uk.
   
Bruce Momjian's avatar
Bruce Momjian committed
383 384
   See also the ODBC chapter of the Programmer's Guide.
   
Bruce Momjian's avatar
Bruce Momjian committed
385
    2.2) What tools are available for using PostgreSQL with Web pages?
386
    
Bruce Momjian's avatar
Bruce Momjian committed
387 388 389
   A nice introduction to Database-backed Web pages can be seen at:
   http://www.webtools.com
   
390 391
   There is also one at http://www.phone.net/home/mwm/hotlist/.
   
Bruce Momjian's avatar
Bruce Momjian committed
392
   For Web integration, PHP is an excellent interface. It is at
393
   http://www.php.net
Bruce Momjian's avatar
Bruce Momjian committed
394
   
Bruce Momjian's avatar
Bruce Momjian committed
395
   For complex cases, many use the Perl interface and CGI.pm.
Bruce Momjian's avatar
Bruce Momjian committed
396
   
Bruce Momjian's avatar
Bruce Momjian committed
397
   A WWW gateway based on WDB using Perl can be downloaded from
Bruce Momjian's avatar
Bruce Momjian committed
398 399
   http://www.eol.ists.ca/~dunlop/wdb-p95
   
400 401 402
    2.3) Does PostgreSQL have a graphical user interface? A report generator?
    An embedded query language interface?
    
Bruce Momjian's avatar
Bruce Momjian committed
403
   We have a nice graphical user interface called pgaccess, which is
Bruce Momjian's avatar
Bruce Momjian committed
404
   shipped as part of the distribution. Pgaccess also has a report
Bruce Momjian's avatar
Bruce Momjian committed
405
   generator. The Web page is http://www.flex.ro/pgaccess
Bruce Momjian's avatar
Bruce Momjian committed
406
   
407 408
   We also include ecpg, which is an embedded SQL query language
   interface for C.
Bruce Momjian's avatar
Bruce Momjian committed
409
   
410 411
    2.4) What languages are available to communicate with PostgreSQL?
    
Bruce Momjian's avatar
Bruce Momjian committed
412
   We have:
Bruce Momjian's avatar
Bruce Momjian committed
413 414 415 416 417 418 419 420 421
     * C (libpq)
     * C++ (libpq++)
     * Embedded C (ecpg)
     * Java (jdbc)
     * Perl (perl5)
     * ODBC (odbc)
     * Python (PyGreSQL)
     * TCL (libpgtcl)
     * C Easy API (libpgeasy)
Bruce Momjian's avatar
Bruce Momjian committed
422
     * Embedded HTML (PHP from http://www.php.net)
Bruce Momjian's avatar
Bruce Momjian committed
423 424
     _________________________________________________________________
   
425 426
                          Administrative Questions
                                      
Bruce Momjian's avatar
Bruce Momjian committed
427
    3.1) How do I install PostgreSQL somewhere other than /usr/local/pgsql?
428
    
Bruce Momjian's avatar
Bruce Momjian committed
429
   Specify the --prefix option when running configure.
Bruce Momjian's avatar
Bruce Momjian committed
430
   
Bruce Momjian's avatar
Bruce Momjian committed
431
    3.2) When I start the postmaster, I get a Bad System Call or core dumped
432 433
    message. Why?
    
Bruce Momjian's avatar
Bruce Momjian committed
434
   It could be a variety of problems, but first check to see that you
Bruce Momjian's avatar
Bruce Momjian committed
435
   have System V extensions installed in your kernel. PostgreSQL requires
Bruce Momjian's avatar
Bruce Momjian committed
436
   kernel support for shared memory and semaphores.
Bruce Momjian's avatar
Bruce Momjian committed
437
   
Bruce Momjian's avatar
Bruce Momjian committed
438
    3.3) When I try to start the postmaster, I get IpcMemoryCreate errors. Why?
439
    
Bruce Momjian's avatar
Bruce Momjian committed
440 441 442
   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
Bruce Momjian's avatar
Bruce Momjian committed
443 444 445
   many buffers and backend processes you configure for the postmaster.
   For most systems, with default numbers of buffers and processes, you
   need a minimum of ~1MB.
Bruce Momjian's avatar
Bruce Momjian committed
446
   
Bruce Momjian's avatar
Bruce Momjian committed
447
    3.4) When I try to start the postmaster, I get IpcSemaphoreCreate errors.
448 449 450 451 452 453 454 455 456 457 458 459
    Why?
    
   If the error message is IpcSemaphoreCreate: semget failed (No space
   left on device) then your kernel is not configured with enough
   semaphores. Postgres needs one semaphore per potential backend
   process. A temporary solution is to start the postmaster with a
   smaller limit on the number of backend processes. Use -N with a
   parameter less than the default of 32. A more permanent solution is to
   increase your kernel's SEMMNS and SEMMNI parameters.
   
   If the error message is something else, you might not have semaphore
   support configured in your kernel at all.
Bruce Momjian's avatar
Bruce Momjian committed
460
   
Bruce Momjian's avatar
Bruce Momjian committed
461
    3.5) How do I prevent other hosts from accessing my PostgreSQL database?
462
    
Bruce Momjian's avatar
Bruce Momjian committed
463
   By default, PostgreSQL only allows connections from the local machine
Bruce Momjian's avatar
Bruce Momjian committed
464
   using Unix domain sockets. Other machines will not be able to connect
Bruce Momjian's avatar
Bruce Momjian committed
465 466
   unless you add the -i flag to the postmaster, and enable host-based
   authentication by modifying the file $PGDATA/pg_hba.conf accordingly.
467
   This will allow TCP/IP connections.
Bruce Momjian's avatar
Bruce Momjian committed
468
   
Bruce Momjian's avatar
Bruce Momjian committed
469 470 471
   Inoperative semaphores can also cause crashes during heavy database
   access.
   
Bruce Momjian's avatar
Bruce Momjian committed
472
    3.6) Why can't I connect to my database from another machine?
473 474 475 476
    
   The default configuration allows only unix domain socket connections
   from the local machine. To enable TCP/IP connections, make sure the
   postmaster has been started with the -i option, and add an appropriate
Bruce Momjian's avatar
Bruce Momjian committed
477
   host entry to the file pgsql/data/pg_hba.conf.
478
   
Bruce Momjian's avatar
Bruce Momjian committed
479
    3.7) How do I tune the database engine for better performance?
480
    
Bruce Momjian's avatar
Bruce Momjian committed
481
   Certainly, indices can speed up queries. The EXPLAIN command allows
Bruce Momjian's avatar
Bruce Momjian committed
482 483 484
   you to see how PostgreSQL is interpreting your query, and which
   indices are being used.
   
Bruce Momjian's avatar
Bruce Momjian committed
485
   If you are doing a lot of INSERTs, consider doing them in a large
Bruce Momjian's avatar
Bruce Momjian committed
486 487 488 489 490
   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
Bruce Momjian's avatar
Bruce Momjian committed
491
   recreating indices when making large data changes.
Bruce Momjian's avatar
Bruce Momjian committed
492
   
Bruce Momjian's avatar
Bruce Momjian committed
493 494 495
   There are several tuning options. You can disable fsync() by starting
   the postmaster with a -o -F option. This will prevent fsync()'s from
   flushing to disk after every transaction.
Bruce Momjian's avatar
Bruce Momjian committed
496 497
   
   You can also use the postmaster -B option to increase the number of
Bruce Momjian's avatar
Bruce Momjian committed
498
   shared memory buffers used by the backend processes. If you make this
Bruce Momjian's avatar
Bruce Momjian committed
499
   parameter too high, the postmaster may not start because you've
Bruce Momjian's avatar
Bruce Momjian committed
500 501
   exceeded your kernel's limit on shared memory space. Each buffer is 8K
   and the default is 64 buffers.
Bruce Momjian's avatar
Bruce Momjian committed
502
   
Bruce Momjian's avatar
Bruce Momjian committed
503
   You can also use the backend -S option to increase the maximum amount
Bruce Momjian's avatar
Bruce Momjian committed
504 505
   of memory used by the backend process for temporary sorts. The -S
   value is measured in kilobytes, and the default is 512 (ie, 512K).
Bruce Momjian's avatar
Bruce Momjian committed
506
   
Bruce Momjian's avatar
Bruce Momjian committed
507
   You can also use the CLUSTER command to group data in tables to match
Bruce Momjian's avatar
Bruce Momjian committed
508
   an index. See the CLUSTER manual page for more details.
Bruce Momjian's avatar
Bruce Momjian committed
509
   
Bruce Momjian's avatar
Bruce Momjian committed
510
    3.8) What debugging features are available?
511
    
Bruce Momjian's avatar
Bruce Momjian committed
512 513 514
   PostgreSQL has several features that report status information that
   can be valuable for debugging purposes.
   
Bruce Momjian's avatar
Bruce Momjian committed
515
   First, by running configure with the --enable-cassert option, many
Bruce Momjian's avatar
Bruce Momjian committed
516 517 518 519 520 521
   assert()'s monitor the progress of the backend and halt the program
   when something unexpected occurs.
   
   Both postmaster and postgres have several debug options available.
   First, whenever you start the postmaster, make sure you send the
   standard output and error to a log file, like:
Bruce Momjian's avatar
Bruce Momjian committed
522 523
    cd /usr/local/pgsql
    ./bin/postmaster >server.log 2>&1 &
Bruce Momjian's avatar
Bruce Momjian committed
524 525

   This will put a server.log file in the top-level PostgreSQL directory.
Bruce Momjian's avatar
Bruce Momjian committed
526
   This file contains useful information about problems or errors
Bruce Momjian's avatar
Bruce Momjian committed
527 528
   encountered by the server. Postmaster has a -d option that allows even
   more detailed information to be reported. The -d option takes a number
Bruce Momjian's avatar
Bruce Momjian committed
529
   that specifies the debug level. Be warned that high debug level values
Bruce Momjian's avatar
Bruce Momjian committed
530
   generate large log files.
Bruce Momjian's avatar
Bruce Momjian committed
531
   
Bruce Momjian's avatar
Bruce Momjian committed
532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547
   If the 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 the postmaster, it is not
   running in an identical environment and locking/backend interaction
   problems may not be duplicated.
   
   If the 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 with the debugger and trace through
   the startup sequence.
Bruce Momjian's avatar
Bruce Momjian committed
548
   
Bruce Momjian's avatar
Bruce Momjian committed
549 550
   The postgres program has -s, -A, and -t options that can be very
   useful for debugging and performance measurements.
Bruce Momjian's avatar
Bruce Momjian committed
551 552 553 554
   
   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
Bruce Momjian's avatar
Bruce Momjian committed
555
   in the client's current directory.
Bruce Momjian's avatar
Bruce Momjian committed
556
   
Bruce Momjian's avatar
Bruce Momjian committed
557
    3.9) I get 'Sorry, too many clients' when trying to connect. Why?
558
    
Bruce Momjian's avatar
Bruce Momjian committed
559 560 561
   You need to increase the postmaster's limit on how many concurrent
   backend processes it can start.
   
Bruce Momjian's avatar
Bruce Momjian committed
562
   In PostgreSQL 6.5 and up, the default limit is 32 processes. You can
Bruce Momjian's avatar
Bruce Momjian committed
563
   increase it by restarting the postmaster with a suitable -N value.
Bruce Momjian's avatar
Bruce Momjian committed
564
   With the default configuration you can set -N as large as 1024. If you
Bruce Momjian's avatar
Bruce Momjian committed
565 566 567 568 569 570 571 572 573
   need more, increase MAXBACKENDS in include/config.h and rebuild. You
   can set the default value of -N at configuration time, if you like,
   using configure's --with-maxbackends switch.
   
   Note that if you make -N larger than 32, you 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
Bruce Momjian's avatar
Bruce Momjian committed
574 575 576
   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;
Bruce Momjian's avatar
Bruce Momjian committed
577
   and the maximum number of open files, NFILE and NINODE. The reason
Bruce Momjian's avatar
Bruce Momjian committed
578
   that PostgreSQL has a limit on the number of allowed backend processes
Bruce Momjian's avatar
Bruce Momjian committed
579
   is so your system won't run out of resources.
Bruce Momjian's avatar
Bruce Momjian committed
580
   
Bruce Momjian's avatar
Bruce Momjian committed
581 582 583
   In PostgreSQL versions prior to 6.5, the maximum number of backends
   was 64, and changing it required a rebuild after altering the
   MaxBackendId constant in include/storage/sinvaladt.h.
Bruce Momjian's avatar
Bruce Momjian committed
584
   
Bruce Momjian's avatar
Bruce Momjian committed
585
    3.10) What are the pg_sorttempNNN.NN files in my database directory?
586 587
    
   They are temporary files generated by the query executor. For example,
Bruce Momjian's avatar
Bruce Momjian committed
588
   if a sort needs to be done to satisfy an ORDER BY, and the sort
Bruce Momjian's avatar
Bruce Momjian committed
589 590
   requires more space than the backend's -S parameter allows, then
   temporary files are created to hold the extra data.
Bruce Momjian's avatar
Bruce Momjian committed
591
   
Bruce Momjian's avatar
Bruce Momjian committed
592 593 594
   The temporary files should be deleted automatically, but might not if
   a backend crashes during a sort. If you have no backends running at
   the time, it is safe to delete the pg_tempNNN.NN files.
595
     _________________________________________________________________
Bruce Momjian's avatar
Bruce Momjian committed
596
   
597 598
                           Operational Questions
                                      
Bruce Momjian's avatar
Bruce Momjian committed
599
    4.1) Why is system confused about commas, decimal points, and date formats.
600
    
Bruce Momjian's avatar
Bruce Momjian committed
601 602 603 604
   Check your locale configuration. PostgreSQL uses the locale setting of
   the user that ran the postmaster process. There are postgres and psql
   SET commands to control the date format. Set those accordingly for
   your operating environment.
Bruce Momjian's avatar
Bruce Momjian committed
605
   
606 607 608
    4.2) What is the exact difference between binary cursors and normal
    cursors?
    
Bruce Momjian's avatar
Bruce Momjian committed
609
   See the DECLARE manual page for a description.
Bruce Momjian's avatar
Bruce Momjian committed
610
   
Bruce Momjian's avatar
Bruce Momjian committed
611
    4.3) How do I SELECT only the first few rows of a query?
612
    
Bruce Momjian's avatar
Bruce Momjian committed
613
   See the FETCH manual page, or use SELECT ... LIMIT....
Bruce Momjian's avatar
Bruce Momjian committed
614
   
Bruce Momjian's avatar
Bruce Momjian committed
615 616 617 618 619
   The entire query may have to be evaluated, even if you only want the
   first few rows. Consider a query that has an ORDER BY. If there is an
   index that matches the ORDER BY, PostgreSQL may be able to evaluate
   only the first few records requested, or the entire query may have to
   be evaluated until the desired rows have been generated.
Bruce Momjian's avatar
Bruce Momjian committed
620
   
Bruce Momjian's avatar
Bruce Momjian committed
621
    4.4) How do I get a list of tables or other things I can see in psql?
622
    
Bruce Momjian's avatar
Bruce Momjian committed
623
   You can read the source code for psql in file
Bruce Momjian's avatar
Bruce Momjian committed
624 625 626
   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
Bruce Momjian's avatar
Bruce Momjian committed
627
   commands you give.
Bruce Momjian's avatar
Bruce Momjian committed
628
   
629 630
    4.5) How do you remove a column from a table?
    
Bruce Momjian's avatar
Bruce Momjian committed
631
   We do not support ALTER TABLE DROP COLUMN, but do this:
Bruce Momjian's avatar
Bruce Momjian committed
632 633 634 635 636
    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;
637 638 639

    4.6) What is the maximum size for a row, table, database?
    
Bruce Momjian's avatar
Bruce Momjian committed
640 641
   These are the limits:
Maximum size for a database?             unlimited (60GB databases exist)
Bruce Momjian's avatar
Bruce Momjian committed
642
Maximum size for a table?                16 TB
Bruce Momjian's avatar
Bruce Momjian committed
643
Maximum size for a row?                  unlimited in 7.1 and later
Bruce Momjian's avatar
Bruce Momjian committed
644
Maximum size for a field?                1GB in 7.1 and later
Bruce Momjian's avatar
Bruce Momjian committed
645
Maximum number of rows in a table?       unlimited
Bruce Momjian's avatar
Bruce Momjian committed
646
Maximum number of columns in a table?    250-1600 depending on column types
Bruce Momjian's avatar
Bruce Momjian committed
647 648 649
Maximum number of indexes on a table?    unlimited

   Of course, these are not actually unlimited, but limited to available
Bruce Momjian's avatar
Bruce Momjian committed
650 651
   disk space and memory/swap space. Performance may suffer when these
   values get unusually large.
Bruce Momjian's avatar
Bruce Momjian committed
652
   
Bruce Momjian's avatar
Bruce Momjian committed
653 654 655 656 657
   The maximum table size of 16TB does not require large file support
   from the operating system. Large tables are stored as multiple 1GB
   files.
   
   The maximum table size and maximum number of columns can be increased
Bruce Momjian's avatar
Bruce Momjian committed
658
   if the default block size is increased to 32k.
Bruce Momjian's avatar
Bruce Momjian committed
659
   
Bruce Momjian's avatar
Bruce Momjian committed
660
    4.7) How much database disk space is required to store data from a typical
Bruce Momjian's avatar
Bruce Momjian committed
661
    text file?
662
    
Bruce Momjian's avatar
Bruce Momjian committed
663
   A PostgreSQL database may need six-and-a-half times the disk space
Bruce Momjian's avatar
Bruce Momjian committed
664
   required to store the data in a flat file.
665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686
   
   Consider a file of 300,000 lines with two integers on each line. The
   flat file is 2.4MB. The size of the PostgreSQL database file
   containing this data can be estimated at 14MB:
    36 bytes: each row header (approximate)
   + 8 bytes: two int fields @ 4 bytes each
   + 4 bytes: pointer on page to tuple
   ----------------------------------------
    48 bytes per row

   The data page size in PostgreSQL is 8192 bytes (8 KB), so:

   8192 bytes per page
   -------------------   =  171 rows per database page (rounded up)
     48 bytes per row

   300000 data rows
   --------------------  =  1755 database pages
      171 rows per page

1755 database pages * 8192 bytes per page  =  14,376,960 bytes (14MB)

Bruce Momjian's avatar
Bruce Momjian committed
687
   Indexes do not require as much overhead, but do contain the data that
688 689
   is being indexed, so they can be large also.
   
Bruce Momjian's avatar
Bruce Momjian committed
690
    4.8) How do I find out what tables or indexes are defined in the database?
691 692 693 694 695
    
   psql has a variety of backslash commands to show such information. Use
   \? to see them.
   
   Also try the file pgsql/src/tutorial/syscat.source. It illustrates
Bruce Momjian's avatar
Bruce Momjian committed
696
   many of the SELECTs needed to get information from the database system
697
   tables.
Bruce Momjian's avatar
Bruce Momjian committed
698
   
699 700
    4.9) My queries are slow or don't make use of the indexes. Why?
    
Bruce Momjian's avatar
Bruce Momjian committed
701 702 703 704 705 706
   PostgreSQL does not automatically maintain statistics. VACUUM must be
   run to update the statistics. After statistics are updated, the
   optimizer knows how many rows in the table, and can better decide if
   it should use indices. Note that the optimizer does not use indices in
   cases when the table is small because a sequential scan would be
   faster.
Bruce Momjian's avatar
Bruce Momjian committed
707
   
Bruce Momjian's avatar
Bruce Momjian committed
708
   For column-specific optimization statistics, use VACUUM ANALYZE.
Bruce Momjian's avatar
Bruce Momjian committed
709
   VACUUM ANALYZE is important for complex multijoin queries, so the
Bruce Momjian's avatar
Bruce Momjian committed
710 711
   optimizer can estimate the number of rows returned from each table,
   and choose the proper join order. The backend does not keep track of
Bruce Momjian's avatar
Bruce Momjian committed
712
   column statistics on its own, so VACUUM ANALYZE must be run to collect
713
   them periodically.
Bruce Momjian's avatar
Bruce Momjian committed
714
   
Bruce Momjian's avatar
Bruce Momjian committed
715 716 717
   Indexes are usually not used for ORDER BY operations: a sequential
   scan followed by an explicit sort is faster than an indexscan of all
   tuples of a large table, because it takes fewer disk accesses.
Bruce Momjian's avatar
Bruce Momjian committed
718
   
719
   When using wild-card operators such as LIKE or ~, indices can only be
Bruce Momjian's avatar
Bruce Momjian committed
720
   used if the beginning of the search is anchored to the start of the
Bruce Momjian's avatar
Bruce Momjian committed
721 722
   string. So, to use indices, LIKE searches should not begin with %, and
   ~(regular expression searches) should start with ^.
Bruce Momjian's avatar
Bruce Momjian committed
723
   
724 725
    4.10) How do I see how the query optimizer is evaluating my query?
    
Bruce Momjian's avatar
Bruce Momjian committed
726
   See the EXPLAIN manual page.
727 728 729
   
    4.11) What is an R-tree index?
    
Bruce Momjian's avatar
Bruce Momjian committed
730
   An R-tree index is used for indexing spatial data. A hash index can't
731 732 733
   handle range searches. A B-tree index only handles range searches in a
   single dimension. R-tree's can handle multi-dimensional data. For
   example, if an R-tree index can be built on an attribute of type
Bruce Momjian's avatar
Bruce Momjian committed
734 735
   point, the system can more efficiently answer queries such as "select
   all points within a bounding rectangle."
736
   
Bruce Momjian's avatar
Bruce Momjian committed
737
   The canonical paper that describes the original R-tree design is:
738
   
Bruce Momjian's avatar
Bruce Momjian committed
739
   Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
740 741 742 743
   Searching." Proc of the 1984 ACM SIGMOD Int'l Conf on Mgmt of Data,
   45-57.
   
   You can also find this paper in Stonebraker's "Readings in Database
Bruce Momjian's avatar
Bruce Momjian committed
744
   Systems".
745
   
Bruce Momjian's avatar
Bruce Momjian committed
746
   Built-in R-trees can handle polygons and boxes. In theory, R-trees can
747
   be extended to handle higher number of dimensions. In practice,
Bruce Momjian's avatar
Bruce Momjian committed
748
   extending R-trees requires a bit of work and we don't currently have
749 750
   any documentation on how to do it.
   
Bruce Momjian's avatar
Bruce Momjian committed
751
    4.12) What is the Genetic Query Optimizer?
752
    
Bruce Momjian's avatar
Bruce Momjian committed
753 754
   The GEQO module speeds query optimization when joining many tables by
   means of a Genetic Algorithm (GA). It allows the handling of large
Bruce Momjian's avatar
Bruce Momjian committed
755
   join queries through nonexhaustive search.
Bruce Momjian's avatar
Bruce Momjian committed
756
   
Bruce Momjian's avatar
Bruce Momjian committed
757 758
    4.13) How do I perform regular expression searches and case-insensitive
    regular expression searches?
759
    
Bruce Momjian's avatar
Bruce Momjian committed
760
   The ~ operator does regular expression matching, and ~* does
Bruce Momjian's avatar
Bruce Momjian committed
761 762 763
   case-insensitive regular expression matching. The case-insensitive
   variant of LIKE is called ILIKE.
   
764 765
    4.14) In a query, how do I detect if a field is NULL?
    
Bruce Momjian's avatar
Bruce Momjian committed
766
   You test the column with IS NULLIS NOT NULL.
767 768 769
   
    4.15) What is the difference between the various character types?
    
Bruce Momjian's avatar
Bruce Momjian committed
770
Type            Internal Name   Notes
Marc G. Fournier's avatar
Marc G. Fournier committed
771
--------------------------------------------------
Bruce Momjian's avatar
Bruce Momjian committed
772
"char"          char            1 character
Bruce Momjian's avatar
Bruce Momjian committed
773 774
CHAR(#)         bpchar          blank padded to the specified fixed length
VARCHAR(#)      varchar         size specifies maximum length, no padding
775 776
TEXT            text            no specific upper limit on length
BYTEA           bytea           variable-length byte array (null-safe)
Bruce Momjian's avatar
Bruce Momjian committed
777

Bruce Momjian's avatar
Bruce Momjian committed
778 779
   You will see the internal name when examining system catalogs and in
   some error messages.
Bruce Momjian's avatar
Bruce Momjian committed
780
   
Bruce Momjian's avatar
Bruce Momjian committed
781
   The last four types above are "varlena" types (i.e., the first four
Bruce Momjian's avatar
Bruce Momjian committed
782 783
   bytes on disk are the length, followed by the data). Thus the actual
   space used is slightly greater than the declared size. However, these
784 785
   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.
Bruce Momjian's avatar
Bruce Momjian committed
786
   
Bruce Momjian's avatar
Bruce Momjian committed
787 788
    4.16.1) How do I create a serial/auto-incrementing field?
    
Bruce Momjian's avatar
Bruce Momjian committed
789
   PostgreSQL supports a SERIAL data type. It auto-creates a sequence and
Bruce Momjian's avatar
Bruce Momjian committed
790
   index on the column. For example, this:
Bruce Momjian's avatar
Bruce Momjian committed
791 792 793 794
    CREATE TABLE person (
        id   SERIAL,
        name TEXT
    );
Bruce Momjian's avatar
Bruce Momjian committed
795

Bruce Momjian's avatar
Bruce Momjian committed
796
   is automatically translated into this:
Bruce Momjian's avatar
Bruce Momjian committed
797 798 799 800 801 802
    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 );
Bruce Momjian's avatar
Bruce Momjian committed
803 804

   See the create_sequence manual page for more information about
Bruce Momjian's avatar
Bruce Momjian committed
805
   sequences. You can also use each row's OID field as a unique value.
Bruce Momjian's avatar
Bruce Momjian committed
806
   However, if you need to dump and reload the database, you need to use
Bruce Momjian's avatar
Bruce Momjian committed
807
   pg_dump's -o option or COPY WITH OIDS option to preserve the OIDs.
Bruce Momjian's avatar
Bruce Momjian committed
808
   
Bruce Momjian's avatar
Bruce Momjian committed
809
   Numbering Rows.
Bruce Momjian's avatar
Bruce Momjian committed
810
   
Bruce Momjian's avatar
Bruce Momjian committed
811
    4.16.2) How do I get the value of a SERIAL insert?
812
    
Bruce Momjian's avatar
Bruce Momjian committed
813 814 815 816
   One approach is to to retrieve the next SERIAL value from the sequence
   object with the nextval() function before inserting and then insert it
   explicitly. Using the example table in 4.16.1, that might look like
   this:
Bruce Momjian's avatar
Bruce Momjian committed
817 818
    $newSerialID = nextval('person_id_seq');
    INSERT INTO person (id, name) VALUES ($newSerialID, 'Blaise Pascal');
Bruce Momjian's avatar
Bruce Momjian committed
819 820 821

   You would then also have the new value stored in $newSerialID for use
   in other queries (e.g., as a foreign key to the person table). Note
Bruce Momjian's avatar
Bruce Momjian committed
822
   that the name of the automatically created SEQUENCE object will be
Bruce Momjian's avatar
Bruce Momjian committed
823 824 825
   named <table>_<serialcolumn>_seq, where table and serialcolumn are the
   names of your table and your SERIAL column, respectively.
   
Bruce Momjian's avatar
Bruce Momjian committed
826 827
   Alternatively, you could retrieve the assigned SERIAL value with the
   currval() function after it was inserted by default, e.g.,
Bruce Momjian's avatar
Bruce Momjian committed
828 829
    INSERT INTO person (name) VALUES ('Blaise Pascal');
    $newID = currval('person_id_seq');
Bruce Momjian's avatar
Bruce Momjian committed
830

Bruce Momjian's avatar
Bruce Momjian committed
831
   Finally, you could use the OID returned from the INSERT statement to
Bruce Momjian's avatar
Bruce Momjian committed
832
   look up the default value, though this is probably the least portable
Bruce Momjian's avatar
Bruce Momjian committed
833
   approach. In Perl, using DBI with Edmund Mergl's DBD::Pg module, the
Bruce Momjian's avatar
Bruce Momjian committed
834 835 836
   oid value is made available via $sth->{pg_oid_status} after
   $sth->execute().
   
Bruce Momjian's avatar
Bruce Momjian committed
837
    4.16.3) Don't currval() and nextval() lead to a race condition with other
Bruce Momjian's avatar
Bruce Momjian committed
838
    users?
Bruce Momjian's avatar
Bruce Momjian committed
839
    
Bruce Momjian's avatar
Bruce Momjian committed
840
   No. This is handled by the backends.
Bruce Momjian's avatar
Bruce Momjian committed
841
   
Bruce Momjian's avatar
Bruce Momjian committed
842
    4.17) What is an OID? What is a TID?
843
    
Bruce Momjian's avatar
Bruce Momjian committed
844
   OIDs are PostgreSQL's answer to unique row ids. Every row that is
Bruce Momjian's avatar
Bruce Momjian committed
845
   created in PostgreSQL gets a unique OID. All OIDs generated during
Bruce Momjian's avatar
Bruce Momjian committed
846
   initdb are less than 16384 (from backend/access/transam.h). All
Bruce Momjian's avatar
Bruce Momjian committed
847 848
   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
Bruce Momjian's avatar
Bruce Momjian committed
849
   within the entire PostgreSQL installation.
Bruce Momjian's avatar
Bruce Momjian committed
850
   
Bruce Momjian's avatar
Bruce Momjian committed
851 852 853 854
   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
Bruce Momjian's avatar
Bruce Momjian committed
855
   access.
Bruce Momjian's avatar
Bruce Momjian committed
856
   
Bruce Momjian's avatar
Bruce Momjian committed
857
   Oids are assigned to all new rows from a central area that is used by
Bruce Momjian's avatar
Bruce Momjian committed
858 859
   all databases. If you want to change the OID to something else, or if
   you want to make a copy of the table, with the original OID's, there
Bruce Momjian's avatar
Bruce Momjian committed
860
   is no reason you can't do it:
Bruce Momjian's avatar
Bruce Momjian committed
861
        CREATE TABLE new_table(old_oid oid, mycol int);
Bruce Momjian's avatar
Bruce Momjian committed
862
        SELECT old_oid, mycol INTO new FROM old;
Bruce Momjian's avatar
Bruce Momjian committed
863 864 865
        COPY new TO '/tmp/pgtable';
        DELETE FROM new;
        COPY new WITH OIDS FROM '/tmp/pgtable';
Bruce Momjian's avatar
Bruce Momjian committed
866

Bruce Momjian's avatar
Bruce Momjian committed
867
   OIDs are stored as 4-byte integers, and will overflow at 4 billion. No
Bruce Momjian's avatar
Bruce Momjian committed
868
   one has reported this ever happening, and we plan to have the limit
Bruce Momjian's avatar
Bruce Momjian committed
869 870
   removed before anyone does.
   
Bruce Momjian's avatar
Bruce Momjian committed
871
   TIDs are used to identify specific physical rows with block and offset
Bruce Momjian's avatar
Bruce Momjian committed
872 873
   values. Tids change after rows are modified or reloaded. They are used
   by index entries to point to physical rows.
Bruce Momjian's avatar
Bruce Momjian committed
874
   
875 876
    4.18) What is the meaning of some of the terms used in PostgreSQL?
    
Bruce Momjian's avatar
Bruce Momjian committed
877 878
   Some of the source code and older documentation use terms that have
   more common usage. Here are some:
Bruce Momjian's avatar
Bruce Momjian committed
879
     * table, relation, class
Bruce Momjian's avatar
Bruce Momjian committed
880
     * row, record, tuple
Bruce Momjian's avatar
Bruce Momjian committed
881
     * column, field, attribute
Bruce Momjian's avatar
Bruce Momjian committed
882 883 884
     * retrieve, select
     * replace, update
     * append, insert
Bruce Momjian's avatar
Bruce Momjian committed
885
     * OID, serial value
Bruce Momjian's avatar
Bruce Momjian committed
886 887 888
     * portal, cursor
     * range variable, table name, table alias
       
Bruce Momjian's avatar
Bruce Momjian committed
889 890 891
   A list of general database terms can be found at:
   http://www.comptechnews.com/~reaster/dbdesign.html
   
892 893
    4.19) Why do I get the error "FATAL: palloc failure: memory exhausted?"
    
Bruce Momjian's avatar
Bruce Momjian committed
894 895 896
   It is possible you have run out of virtual memory on your system, or
   your kernel has a low limit for certain resources. Try this before
   starting the postmaster:
Bruce Momjian's avatar
Bruce Momjian committed
897 898
    ulimit -d 65536
    limit datasize 64m
Bruce Momjian's avatar
Bruce Momjian committed
899 900 901

   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
Bruce Momjian's avatar
Bruce Momjian committed
902
   query to complete. This command applies to the current process, and
Bruce Momjian's avatar
Bruce Momjian committed
903 904 905
   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.
Bruce Momjian's avatar
Bruce Momjian committed
906
   
907 908
    4.20) How do I tell what PostgreSQL version I am running?
    
Bruce Momjian's avatar
Bruce Momjian committed
909
   From psql, type select version();
Bruce Momjian's avatar
Bruce Momjian committed
910 911 912 913 914 915
   
    4.21) My large-object operations get invalid large obj descriptor. Why?
    
   You need to put BEGIN WORK and COMMIT around any use of a large object
   handle, that is, surrounding lo_open ... lo_close.
   
Bruce Momjian's avatar
Bruce Momjian committed
916 917 918 919 920
   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.
Bruce Momjian's avatar
Bruce Momjian committed
921 922 923 924 925 926
   
   If you are using a client interface like ODBC you may need to set
   auto-commit off.
   
    4.22) How do I create a column that will default to the current time?
    
Bruce Momjian's avatar
Bruce Momjian committed
927
   Use now():
Bruce Momjian's avatar
Bruce Momjian committed
928
CREATE TABLE test (x int, modtime timestamp DEFAULT now() );
Bruce Momjian's avatar
Bruce Momjian committed
929 930 931

    4.23) Why are my subqueries using IN so slow?
    
Bruce Momjian's avatar
Bruce Momjian committed
932 933
   Currently, we join subqueries to outer queries by sequentially
   scanning the result of the subquery for each row of the outer query. A
Bruce Momjian's avatar
Bruce Momjian committed
934
   workaround is to replace IN with EXISTS:
Bruce Momjian's avatar
Bruce Momjian committed
935 936 937
SELECT *
    FROM tab
    WHERE col1 IN (SELECT col2 FROM TAB2)
Bruce Momjian's avatar
Bruce Momjian committed
938 939

   to:
Bruce Momjian's avatar
Bruce Momjian committed
940 941 942
SELECT *
    FROM tab
    WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 = col2)
Bruce Momjian's avatar
Bruce Momjian committed
943

Bruce Momjian's avatar
Bruce Momjian committed
944
   We hope to fix this limitation in a future release.
Bruce Momjian's avatar
Bruce Momjian committed
945
   
Bruce Momjian's avatar
Bruce Momjian committed
946
    4.24) How do I perform an outer join?
Bruce Momjian's avatar
Bruce Momjian committed
947
    
Bruce Momjian's avatar
Bruce Momjian committed
948 949
   PostgreSQL 7.1 and later supports outer joins using the SQL standard
   syntax. Here are two examples:
Bruce Momjian's avatar
Bruce Momjian committed
950 951 952 953
    SELECT *
    FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);

   or
Bruce Momjian's avatar
Bruce Momjian committed
954 955 956
    SELECT *
    FROM t1 LEFT OUTER JOIN t2 USING (col);

Bruce Momjian's avatar
Bruce Momjian committed
957 958 959 960 961 962
   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.
Bruce Momjian's avatar
Bruce Momjian committed
963
   
Bruce Momjian's avatar
Bruce Momjian committed
964 965 966
   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:
Bruce Momjian's avatar
Bruce Momjian committed
967 968 969 970 971 972 973
    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)
Bruce Momjian's avatar
Bruce Momjian committed
974
    ORDER BY col1
Bruce Momjian's avatar
Bruce Momjian committed
975 976
     _________________________________________________________________
   
977 978 979 980 981
                            Extending PostgreSQL
                                      
    5.1) I wrote a user-defined function. When I run it in psql, why does it
    dump core?
    
Bruce Momjian's avatar
Bruce Momjian committed
982
   The problem could be a number of things. Try testing your user-defined
Bruce Momjian's avatar
Bruce Momjian committed
983
   function in a stand-alone test program first.
Bruce Momjian's avatar
Bruce Momjian committed
984
   
Bruce Momjian's avatar
Bruce Momjian committed
985 986
    5.2) What does the message "NOTICE:PortalHeapMemoryFree: 0x402251d0 not in
    alloc set!" mean?
987
    
Bruce Momjian's avatar
Bruce Momjian committed
988 989
   You are pfree'ing something that was not palloc'ed. Beware of mixing
   malloc/free and palloc/pfree.
Bruce Momjian's avatar
Bruce Momjian committed
990
   
Bruce Momjian's avatar
Bruce Momjian committed
991
    5.3) How can I contribute some nifty new types and functions to PostgreSQL?
992 993 994
    
   Send your extensions to the pgsql-hackers mailing list, and they will
   eventually end up in the contrib/ subdirectory.
Bruce Momjian's avatar
Bruce Momjian committed
995
   
996 997 998 999
    5.4) How do I write a C function to return a tuple?
    
   This requires wizardry so extreme that the authors have never tried
   it, though in principle it can be done.
Bruce Momjian's avatar
Bruce Momjian committed
1000
   
Bruce Momjian's avatar
Bruce Momjian committed
1001
    5.5) I have changed a source file. Why does the recompile not see the
1002 1003 1004
    change?
    
   The Makefiles do not have the proper dependencies for include files.
Bruce Momjian's avatar
Bruce Momjian committed
1005 1006 1007
   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.