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@@ -1,24 +1,15 @@ Frequently Asked Questions (FAQ) for PostgreSQL - Last updated: Fri Jun 2 11:32:13 EDT 2000 + Last updated: Tue Oct 17 00:21:20 EDT 2000 Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us) - The most recent version of this document can be viewed at the - postgreSQL Web site, http://www.PostgreSQL.org. + The most recent version of this document can be viewed at + http://www.PostgreSQL.org/docs/faq-english.html. - Linux-specific questions are answered in - http://www.PostgreSQL.org/docs/faq-linux.html. - - HPUX-specific questions are answered in - http://www.PostgreSQL.org/docs/faq-hpux.html. - - Solaris-specific questions are answered in - http://www.postgresql.org/docs/faq-solaris.html. - - Irix-specific questions are answered in - http://www.PostgreSQL.org/docs/faq-irix.html. + Platform-specific questions are answered at + http://www.PostgreSQL.org/docs/. _________________________________________________________________ General Questions @@ -28,9 +19,9 @@ 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 for PostgreSQL? - 1.7) What is the latest release of PostgreSQL? - 1.8) What documentation is available for 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? @@ -53,8 +44,8 @@ /usr/local/pgsql? 3.3) When I start the postmaster, I get a Bad System Call or core dumped message. Why? - 3.4) When I try to start the postmaster, I get IpcMemoryCreate - errors3. Why? + 3.4) When I try to start the postmaster, I get IpcMemoryCreate errors. + Why? 3.5) When I try to start the postmaster, I get IpcSemaphoreCreate errors. Why? 3.6) How do I prevent other hosts from accessing my PostgreSQL @@ -63,22 +54,22 @@ 3.8) Why can't I access the database as the root user? 3.9) All my servers crash under concurrent table access. Why? 3.10) How do I tune the database engine for better performance? - 3.11) What debugging features are available in PostgreSQL? - 3.12) I get 'Sorry, too many clients' when trying to connect. Why? - 3.13) What are the pg_psort.XXX files in my database directory? + 3.11) What debugging features are available? + 3.12) I get "Sorry, too many clients" when trying to connect. Why? + 3.13) What are the pg_sorttempNNN.NN files in my database directory? Operational Questions - 4.1) The system seems to be confused about commas, decimal points, and - date formats. + 4.1) Why is the system confused about commas, decimal points, and date + formats. 4.2) What is the exact difference between binary cursors and normal cursors? - 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? + 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? 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 - typical flat file? + typical text file? 4.8) How do I find out what indices or operations are defined in the database? 4.9) My queries are slow or don't make use of the indexes. Why? @@ -86,14 +77,14 @@ 4.11) What is an R-tree index? 4.12) What is Genetic Query Optimization? 4.13) How do I do regular expression searches and case-insensitive - regexp searching? + regular expression searches? 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? 4.16.1) How do I create a serial/auto-incrementing field? - 4.16.2) How do I get the value of a serial insert? + 4.16.2) How do I get the value of a SERIAL insert? 4.16.3) Don't currval() and nextval() lead to a race condition with - other concurrent backend processes? - 4.17) What is an oid? What is a tid? + other users? + 4.17) What is an OID? What is a TID? 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 exhausted?" @@ -108,13 +99,13 @@ 5.1) I wrote a user-defined function. When I run it in psql, why does it dump core? - 5.2) What does the message: NOTICE:PortalHeapMemoryFree: 0x402251d0 - not in alloc set! mean? - 5.3) How can I contribute some nifty new types and functions for + 5.2) What does the message "NOTICE:PortalHeapMemoryFree: 0x402251d0 + not in alloc set!" mean? + 5.3) How can I contribute some nifty new types and functions to PostgreSQL? 5.4) How do I write a C function to return a tuple? - 5.5) I have changed a source file. Why does the recompile does not see - the change? + 5.5) I have changed a source file. Why does the recompile not see the + change? _________________________________________________________________ General Questions @@ -127,14 +118,14 @@ replaces the PostQuel query language with an extended subset of SQL. PostgreSQL is free and the complete source is available. - PostgreSQL development is being 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 current and future development of PostgreSQL. + 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. The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen. Many - others have contributed to the porting, testing, debugging and + 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 @@ -149,7 +140,7 @@ 1.2) What's the copyright on PostgreSQL? - PostgreSQL is subject to the following COPYRIGHT. + PostgreSQL is subject to the following COPYRIGHT: PostgreSQL Data Base Management System @@ -204,36 +195,32 @@ A file win31.mak is included in the distribution for making a Win32 libpq library and psql. - The database server is now working on Windows NT using the Cygnus - Unix/NT porting library. See pgsql/doc/README.NT in the distribution. - - There is also a web page at - http://www.freebsd.org/~kevlo/postgres/portNT.html. There is another - port using U/Win at http://surya.wipro.com/uwin/ported.html. + The database server is now working on Windows NT using Cygwin, the + Cygnus Unix/NT porting library. See pgsql/doc/FAQ_NT in the + distribution. It does not work on MS Windows 9X because Cygwin does + not support the features we need on those platforms. 1.5) Where can I get PostgreSQL? The primary anonymous ftp site for PostgreSQL is - ftp://ftp.postgreSQL.org/pub + ftp://ftp.PostgreSQL.org/pub. For mirror sites, see our main Web site. - For mirror sites, see our main web site. - - 1.6) Where can I get support for PostgreSQL? + 1.6) Where can I get support? - There is no official support for PostgreSQL from the University of - California, Berkeley. It is maintained through volunteer effort. + There is no support for PostgreSQL from the University of California, + Berkeley. It is maintained through volunteer effort. - The main mailing list is: pgsql-general@postgreSQL.org. It is + The main mailing list is: pgsql-general@PostgreSQL.org. It is available for discussion of matters pertaining to PostgreSQL. To - subscribe, send a mail with the lines in the body (not the subject - line) + subscribe, send mail with the following lines in the body (not the + subject line) subscribe end - to pgsql-general-request@postgreSQL.org. + 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: + email to: pgsql-general-digest-request@PostgreSQL.org with a body of: subscribe end @@ -241,14 +228,14 @@ has received around 30k of messages. The bugs mailing list is available. To subscribe to this list, send - email to bugs-request@postgreSQL.org with a BODY of: + 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 hackers-request@postgreSQL.org - with a BODY of: + subscribe to this list, send email to + pgsql-hackers-request@PostgreSQL.org with a body of: subscribe end @@ -256,43 +243,42 @@ Additional mailing lists and information about PostgreSQL can be found via the PostgreSQL WWW home page at: - http://postgreSQL.org + http://www.PostgreSQL.org There is also an IRC channel on EFNet, channel #PostgreSQL. I use the - unix command irc -c '#PostgreSQL' "$USER" irc.phoenix.net + unix command irc -c '#PostgreSQL' "$USER" irc.phoenix.net. Commercial support for PostgreSQL is available at - http://www.pgsql.com/ + http://www.pgsql.com/. - 1.7) What is the latest release of PostgreSQL? + 1.7) What is the latest release? The latest release of PostgreSQL is version 7.0.2. We plan to have major releases every four months. - 1.8) What documentation is available for PostgreSQL? + 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 manual on-line at http://www.postgresql.org/docs/postgres. - in the distribution. + browse the manual online at http://www.PostgreSQL.org/docs/postgres. - There is a PostgreSQL book availiable at - http://www.postgresql.org/docs/awbook.html + There is a PostgreSQL book available at + http://www.PostgreSQL.org/docs/awbook.html. psql has some nice \d commands to show information about types, operators, functions, aggregates, etc. - The web site contains even more documentation. + 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 for a - list of known bugs, missing features, and future plans. + 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 + The PostgreSQL book at http://www.PostgreSQL.org/docs/awbook.html teaches SQL. There is a nice tutorial at http://w3.one.net/~jhoffman/sqltut.htm and at http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM. @@ -300,9 +286,9 @@ 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 et al., - Addison Wesley. Others like The Complete Reference SQL, Groff et al., - McGraw-Hill. + 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? @@ -310,22 +296,23 @@ 1.12) How do I join the development team? - First, download the latest sources and read the PostgreSQL Developers - documentation on our web site, or in the distribution. Second, + 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. All of them have submitted so many - high-quality patches that it was a pain for the existing committers to - keep up, and we had confidence that patches they committed were likely - to be of high quality. + 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? - Fill out the "bug-template" file and send it to: bugs@postgreSQL.org + Fill out the "bug-template" file and send it to: + pgsql-bugs@PostgreSQL.org - Also check out our ftp site ftp://ftp.postgreSQL.org/pub to see if + 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 DBMS's? @@ -340,7 +327,7 @@ some features they don't have, like user-defined types, inheritance, rules, and multi-version concurrency control to reduce lock contention. We don't have outer joins, but are - working on them for our next release. + working on them. Performance PostgreSQL runs in two modes. Normal fsync mode flushes every @@ -353,14 +340,13 @@ though in this mode, an OS crash could cause data corruption. We are working to provide an intermediate mode that suffers less performance overhead than full fsync mode, and will allow - data integrity within 30 seconds of an OS crash. The mode is - select-able by the database administrator. + data integrity within 30 seconds of an OS crash. In comparison to MySQL or leaner database systems, we are 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 - profiling and source code analysis. There is an interesting web + profiling and source code analysis. There is an interesting Web page comparing PostgreSQL to MySQL at http://openacs.org/why-not-mysql.html We handle each user connection by creating a Unix process. @@ -399,7 +385,7 @@ There are two ODBC drivers available, PsqlODBC and OpenLink ODBC. PsqlODBC is included in the distribution. More information about it - can be gotten from: ftp://ftp.postgresql.org/pub/odbc/index.html + can be gotten from ftp://ftp.PostgreSQL.org/pub/odbc/. OpenLink ODBC can be gotten from http://www.openlinksw.com. It works with their standard ODBC client software so you'll have PostgreSQL @@ -419,13 +405,12 @@ There is also one at http://www.phone.net/home/mwm/hotlist/. - For web integration, PHP is an excellent interface. It is at: + For Web integration, PHP is an excellent interface. It is at http://www.php.net - PHP is great for simple stuff, but for more complex cases, many use - the perl interface and CGI.pm. + For complex cases, many use the Perl interface and CGI.pm. - A WWW gateway based on WDB using perl can be downloaded from + A WWW gateway based on WDB using Perl can be downloaded from http://www.eol.ists.ca/~dunlop/wdb-p95 2.3) Does PostgreSQL have a graphical user interface? A report generator? @@ -433,7 +418,7 @@ We have a nice graphical user interface called pgaccess, which is shipped as part of the distribution. Pgaccess also has a report - generator. The web page is http://www.flex.ro/pgaccess + generator. The Web page is http://www.flex.ro/pgaccess We also include ecpg, which is an embedded SQL query language interface for C. @@ -441,27 +426,31 @@ 2.4) What languages are available to communicate with PostgreSQL? We have: - * C(libpq) - * C++(libpq++) - * Embedded C(ecpg) - * Java(jdbc) - * Perl(perl5) - * ODBC(odbc) - * Python(PyGreSQL) - * TCL(libpgtcl) - * C Easy API(libpgeasy) - * Embedded HTML(PHP from http://www.php.net) + * C (libpq) + * C++ (libpq++) + * Embedded C (ecpg) + * Java (jdbc) + * Perl (perl5) + * ODBC (odbc) + * Python (PyGreSQL) + * TCL (libpgtcl) + * C Easy API (libpgeasy) + * Embedded HTML (PHP from http://www.php.net) _________________________________________________________________ Administrative Questions 3.1) Why does initdb fail? + Try these: * check that you don't have any of the previous version's binaries in your path * check to see that you have the proper paths set * check that the postgres user owns the proper files + If you see an error message about oidvector, you definately have a + version mismatch. + 3.2) How do I install PostgreSQL somewhere other than /usr/local/pgsql? The simplest way is to specify the --prefix option when running @@ -473,15 +462,15 @@ 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 + have System V extensions installed in your kernel. PostgreSQL requires kernel support for shared memory and semaphores. 3.4) When I try to start the postmaster, I get IpcMemoryCreate errors. Why? - You either do not have shared memory configured properly in 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 postmaster to run with. + 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 the postmaster. For most systems, with default numbers of buffers and processes, you need a minimum of ~1MB. @@ -512,14 +501,13 @@ 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 - host entry to the file pgsql/data/pg_hba.conf. See the pg_hba.conf - manual page. + host entry to the file pgsql/data/pg_hba.conf. 3.8) Why can't I access the database as the root user? You should not create database users with user id 0 (root). They will be unable to access the database. This is a security precaution - because of the ability of any user to dynamically link object modules + because of the ability of users to dynamically link object modules into the database engine. 3.9) All my servers crash under concurrent table access. Why? @@ -534,20 +522,20 @@ indices are being used. If you are doing a lot of INSERTs, consider doing them in a large - batch using the COPY command. This is much faster than single - 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 + 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 indices when making large data changes. - There are several tuning things that can be done. 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. + 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. 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 up because you've + parameter too high, the postmaster may not start because you've exceeded your kernel's limit on shared memory space. Each buffer is 8K and the default is 64 buffers. @@ -555,10 +543,10 @@ of memory used by the backend process for temporary sorts. The -S value is measured in kilobytes, and the default is 512 (ie, 512K). - You can also use the CLUSTER command to group data in base tables to - match an index. See the cluster(l) manual page for more details. + You can also use the CLUSTER command to group data in tables to match + an index. See the CLUSTER manual page for more details. - 3.11) What debugging features are available in PostgreSQL? + 3.11) What debugging features are available? PostgreSQL has several features that report status information that can be valuable for debugging purposes. @@ -610,9 +598,9 @@ You need to increase the postmaster's limit on how many concurrent backend processes it can start. - In Postgres 6.5 and up, the default limit is 32 processes. You can + In PostgreSQL 6.5 and up, the default limit is 32 processes. You can increase it by restarting the postmaster with a suitable -N value. - With the default configuration you can set -N as large as 1024; if you + With the default configuration you can set -N as large as 1024. If you 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. @@ -622,38 +610,37 @@ 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, + 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 Postgres has a limit on the number of allowed backend processes - is so that you can ensure that your system won't run out of resources. + that PostgreSQL has a limit on the number of allowed backend processes + is so your system won't run out of resources. - In Postgres 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. + 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. - 3.13) What are the pg_tempNNN.NN files in my database directory? + 3.13) What are the pg_sorttempNNN.NN files in my database directory? They are 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 temp - files are created to hold the extra data. + requires more space than the backend's -S parameter allows, then + temporary files are created to hold the extra data. - The temp files should go away automatically, but might not if a - backend crashes during a sort. If you have no transactions running at + 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. _________________________________________________________________ Operational Questions - 4.1) The system seems to be confused about commas, decimal points, and date - formats. + 4.1) Why is system confused about commas, decimal points, and date formats. - Check your locale configuration. PostgreSQL uses the locale settings - 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. + 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. 4.2) What is the exact difference between binary cursors and normal cursors? @@ -670,12 +657,13 @@ only the first few records requested, or the entire query may have to be evaluated until the desired rows have been generated. - 4.4) How do I get a list of tables, or other information I see in psql? + 4.4) How do I get a list of tables or other things I can see in psql? - You can read the source code for psql, file pgsql/src/bin/psql/psql.c. - It contains SQL commands that generate the output for psql's backslash - commands. You can also start psql with the -E option so that it will - print out the queries it uses to execute the commands you give. + You can read the source code for psql in file + pgsql/src/bin/psql/psql.c. It contains SQL commands that generate the + output for psql's backslash commands. You can also start psql with the + -E option so it will print out the queries it uses to execute the + commands you give. 4.5) How do you remove a column from a table? @@ -693,7 +681,7 @@ Maximum size for a database? unlimited (60GB databases exist) Maximum size for a table? unlimited on all operating systems Maximum size for a row? 8k, configurable to 32k Maximum number of rows in a table? unlimited -Maximum number of columns table? unlimited +Maximum number of columns in a table? unlimited Maximum number of indexes on a table? unlimited Of course, these are not actually unlimited, but limited to available @@ -703,13 +691,13 @@ Maximum number of indexes on a table? unlimited BLCKSZ. To use attributes larger than 8K, you can also use the large object interface. - Row length limit will be removed in 7.1. + The row length limit will be removed in 7.1. - 4.7)How much database disk space is required to store data from a typical - flat file? + 4.7) How much database disk space is required to store data from a typical + text file? - A Postgres database can require about six and a half times the disk - space required to store the data in a flat file. + A PostgreSQL database may need six-and-a-half times the disk space + required to store the data in a flat file. 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 @@ -732,7 +720,7 @@ Maximum number of indexes on a table? unlimited 1755 database pages * 8192 bytes per page = 14,376,960 bytes (14MB) - Indexes do not contain as much overhead, but do contain the data that + Indexes do not require as much overhead, but do contain the data that is being indexed, so they can be large also. 4.8) How do I find out what indices or operations are defined in the @@ -747,15 +735,15 @@ Maximum number of indexes on a table? unlimited 4.9) My queries are slow or don't make use of the indexes. Why? - PostgreSQL does not automatically maintain statistics. One has to make - an explicit VACUUM call 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. + 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. For column-specific optimization statistics, use VACUUM ANALYZE. - VACUUM ANALYZE is important for complex multi-join queries, so the + VACUUM ANALYZE is important for complex multijoin queries, so the optimizer can estimate the number of rows returned from each table, and choose the proper join order. The backend does not keep track of column statistics on its own, so VACUUM ANALYZE must be run to collect @@ -776,41 +764,38 @@ Maximum number of indexes on a table? unlimited 4.11) What is an R-tree index? - An r-tree index is used for indexing spatial data. A hash index can't + 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-tree's 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 efficient answer queries like select all - points within a bounding rectangle. + 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: + The canonical paper that describes the original R-tree design is: - Guttman, A. "R-Trees: A Dynamic Index Structure for Spatial + Guttman, A. "R-trees: A Dynamic Index Structure for Spatial 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 - Systems" + Systems". - Builtin R-Trees can handle polygons and boxes. In theory, R-trees can + 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 require a bit of work and we don't currently have + extending R-trees requires a bit of work and we don't currently have any documentation on how to do it. 4.12) What is Genetic Query Optimization? - The GEQO module in PostgreSQL is intended to solve the query - optimization problem of joining many tables by means of a Genetic - Algorithm (GA). It allows the handling of large join queries through - non-exhaustive search. - - For further information see the documentation. + 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. - 4.13) How do I do regular expression searches and case-insensitive regexp - searching? + 4.13) How do I do regular expression searches and case-insensitive regular + expression searches? - The ~ operator does regular-expression matching, and ~* does - case-insensitive regular-expression matching. There is no + The ~ operator does regular expression matching, and ~* does + case-insensitive regular expression matching. There is no case-insensitive variant of the LIKE operator, but you can get the effect of case-insensitive LIKE with this: WHERE lower(textfield) LIKE lower(pattern) @@ -832,7 +817,7 @@ BYTEA bytea variable-length array of bytes You will see the internal name when examining system catalogs and in some error messages. - The last four types above are "varlena" types (i.e. the first four + The last four types above are "varlena" types (i.e., the first four bytes are the length, followed by the data). char(#) allocates the maximum number of bytes no matter how much data is stored in the field. text, varchar(#), and bytea all have variable length on the @@ -842,14 +827,14 @@ BYTEA bytea variable-length array of bytes 4.16.1) How do I create a serial/auto-incrementing field? - PostgreSQL supports SERIAL data type. It auto-creates a sequence and - index on the column. For example, this... + PostgreSQL supports a SERIAL data type. It auto-creates a sequence and + index on the column. For example, this: CREATE TABLE person ( id SERIAL, name TEXT ); - ...is automatically translated into this... + is automatically translated into this: CREATE SEQUENCE person_id_seq; CREATE TABLE person ( id INT4 NOT NULL DEFAULT nextval('person_id_seq'), @@ -858,69 +843,73 @@ BYTEA bytea variable-length array of bytes CREATE UNIQUE INDEX person_id_key ON person ( id ); See the create_sequence manual page for more information about - sequences. You can also use each row's oid field as a unique value. + 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. + pg_dump's -o option or COPY WITH OIDS option to preserve the OIDs. - For more details, see Bruce Momjian's chapter on Numbering Rows. + Numbering Rows. - 4.16.2) How do I get the back the generated SERIAL value after an insert? + 4.16.2) How do I get the value of a SERIAL insert? - Probably the simplest 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: + 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: $newSerialID = nextval('person_id_seq'); INSERT INTO person (id, name) VALUES ($newSerialID, 'Blaise Pascal'); 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 - that the name of the automatically-created SEQUENCE object will be + 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. - Similarly, you could retrieve the just-assigned SERIAL value with the + Alternatively, you could retrieve the assigned SERIAL value with the currval() function after it was inserted by default, e.g., INSERT INTO person (name) VALUES ('Blaise Pascal'); $newID = currval('person_id_seq'); - Finally, you could use the oid returned from the INSERT statement to - lookup the default value, though this is probably the least portable - approach. In perl, using DBI with Edmund Mergl's DBD::Pg module, the + 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. 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(). 4.16.3) Don't currval() and nextval() lead to a race condition with other - concurrent backend processes? + users? - No. That has been handled by the backends. + No. This is handled by the backends. - 4.17) What is an oid? What is a tid? + 4.17) 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 + 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 backend/access/transam.h). All - user-created oids are equal or greater that this. By default, all - these oids are unique not only within a table, or database, but unique + 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. See the sql(l) manual page to see the other internal - columns. You can create an index on the oid field for faster access. + 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 oid's, there + 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 is no reason you can't do it: CREATE TABLE new_table(old_oid oid, mycol int); - SELECT INTO new SELECT old_oid, mycol FROM old; + SELECT old_oid, mycol INTO new FROM old; COPY new TO '/tmp/pgtable'; DELETE FROM new; COPY new WITH OIDS FROM '/tmp/pgtable'; - Tids are used to identify specific physical rows with block and offset + 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. @@ -934,10 +923,13 @@ BYTEA bytea variable-length array of bytes * retrieve, select * replace, update * append, insert - * oid, serial value + * OID, serial value * portal, cursor * range variable, table name, table alias + A list of general database terms can be found at: + http://www.comptechnews.com/~reaster/dbdesign.html + 4.19) Why do I get the error "FATAL: palloc failure: memory exhausted?" It is possible you have run out of virtual memory on your system, or @@ -962,12 +954,11 @@ BYTEA bytea variable-length array of bytes You need to put BEGIN WORK and COMMIT around any use of a large object handle, that is, surrounding lo_open ... lo_close. - Current PostgreSQL enforces the rule by closing large object handles - at transaction commit, which will be instantly upon completion of the - lo_open command if you are not inside a transaction. 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. + Currently PostgreSQL enforces the rule by closing large object handles + at transaction commit. So the first attempt to do anything with the + handle will draw invalid large obj descriptor. So code that used to + work (at least most of the time) will now generate that error message + if you fail to use a transaction. If you are using a client interface like ODBC you may need to set auto-commit off. @@ -975,13 +966,13 @@ BYTEA bytea variable-length array of bytes 4.22) How do I create a column that will default to the current time? Use now(): - CREATE TABLE test (x int, modtime timestamp default now() ); + CREATE TABLE test (x int, modtime timestamp DEFAULT now() ); 4.23) Why are my subqueries using IN so slow? - Currently, we join subqueries to outer queries by sequential scanning - the result of the subquery for each row of the outer query. A - workaround is to replace IN with EXISTS. For example, change: + Currently, we join subqueries to outer queries by sequentially + scanning the result of the subquery for each row of the outer query. A + workaround is to replace IN with EXISTS: SELECT * FROM tab WHERE col1 IN (SELECT col2 FROM TAB2) @@ -1015,16 +1006,15 @@ BYTEA bytea variable-length array of bytes dump core? The problem could be a number of things. Try testing your user-defined - function in a stand alone test program first. + function in a stand-alone test program first. - 5.2) What does the message: NOTICE:PortalHeapMemoryFree: 0x402251d0 not in - alloc set! mean? + 5.2) What does the message "NOTICE:PortalHeapMemoryFree: 0x402251d0 not in + alloc set!" mean? You are pfree'ing something that was not palloc'ed. Beware of mixing malloc/free and palloc/pfree. - 5.3) How can I contribute some nifty new types and functions for - PostgreSQL? + 5.3) How can I contribute some nifty new types and functions to PostgreSQL? Send your extensions to the pgsql-hackers mailing list, and they will eventually end up in the contrib/ subdirectory. @@ -1034,9 +1024,8 @@ BYTEA bytea variable-length array of bytes This requires wizardry so extreme that the authors have never tried it, though in principle it can be done. - 5.5) I have changed a source file. Why does the recompile does not see the + 5.5) I have changed a source file. Why does the recompile not see the change? The Makefiles do not have the proper dependencies for include files. - You have to do a make clean and then another make. You have to do a - make clean and then another make. + You have to do a make clean and then another make. |