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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.
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.
For further information see the documentation.
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.