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Backpatch FAQ changes to 8.0.X.

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Bruce Momjian
2005-01-30 04:22:53 +00:00
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@@ -1,7 +1,7 @@
Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Wed Jan 19 14:45:22 EST 2005
Last updated: Sat Jan 29 23:20:03 EST 2005
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
@@ -16,85 +16,72 @@
1.1) What is PostgreSQL? How is it pronounced?
1.2) What is the copyright on PostgreSQL?
1.3) What Unix platforms does PostgreSQL run on?
1.4) What non-Unix ports are available?
1.5) Where can I get PostgreSQL?
1.6) Where can I get support?
1.7) What is the latest release?
1.8) What documentation is available?
1.9) How do I find out about known bugs or missing features?
1.10) How can I learn SQL?
1.11) Is PostgreSQL Y2K compliant?
1.12) How do I join the development team?
1.13) How do I submit a bug report?
1.14) How does PostgreSQL compare to other DBMSs?
1.15) How can I financially assist PostgreSQL?
1.3) What platforms does PostgreSQL support?
1.4) Where can I get PostgreSQL?
1.5) Where can I get support?
1.6) What is the latest release?
1.7) What documentation is available?
1.8) How do I find out about known bugs or missing features?
1.9) How can I learn SQL?
1.10) How do I join the development team?
1.11) How do I submit a bug report?
1.12) How does PostgreSQL compare to other DBMSs?
1.13) How can I financially assist PostgreSQL?
User Client Questions
2.1) Are there ODBC drivers for PostgreSQL?
2.1) What interfaces are available for PostgreSQL?
2.2) What tools are available for using PostgreSQL with Web pages?
2.3) Does PostgreSQL have a graphical user interface?
2.4) What languages are available to communicate with PostgreSQL?
Administrative Questions
3.1) How do I install PostgreSQL somewhere other than
/usr/local/pgsql?
3.2) When I start postmaster, I get a Bad System Call or core dumped
message. Why?
3.3) When I try to start postmaster, I get IpcMemoryCreate errors.
Why?
3.4) When I try to start postmaster, I get IpcSemaphoreCreate errors.
Why?
3.5) How do I control connections from other hosts?
3.6) How do I tune the database engine for better performance?
3.7) What debugging features are available?
3.8) Why do I get "Sorry, too many clients" when trying to connect?
3.9) What is in the pgsql_tmp directory?
3.10) Why do I need to do a dump and restore to upgrade PostgreSQL
3.2) How do I control connections from other hosts?
3.3) How do I tune the database engine for better performance?
3.4) What debugging features are available?
3.5) Why do I get "Sorry, too many clients" when trying to connect?
3.6) What is in the pgsql_tmp directory?
3.7) Why do I need to do a dump and restore to upgrade PostgreSQL
releases?
3.11) What computer hardware should I use?
3.8) What computer hardware should I use?
Operational Questions
4.1) What is the difference between binary cursors and normal cursors?
4.2) How do I SELECT only the first few rows of a query? A random row?
4.3) How do I find out what tables, indexes, databases, and users are
4.1) How do I SELECT only the first few rows of a query? A random row?
4.2) How do I find out what tables, indexes, databases, and users are
defined? How do I see the queries used by psql to display them?
4.4) How do you remove a column from a table, or change its data type?
4.5) What is the maximum size for a row, a table, and a database?
4.6) How much database disk space is required to store data from a
4.3) How do you remove a column from a table, or change its data type?
4.4) What is the maximum size for a row, a table, and a database?
4.5) How much database disk space is required to store data from a
typical text file?
4.7) My queries are slow or don't make use of the indexes. Why?
4.8) How do I see how the query optimizer is evaluating my query?
4.9) What is an R-tree index?
4.10) What is the Genetic Query Optimizer?
4.11) How do I perform regular expression searches and
case-insensitive regular expression searches? How do I use an index
for case-insensitive searches?
4.12) In a query, how do I detect if a field is NULL?
4.13) What is the difference between the various character types?
4.14.0) How do I create a serial/auto-incrementing field?
4.14.1) How do I get the value of a SERIAL insert?
4.14.2) Doesn't currval() lead to a race condition with other users?
4.14.3) Why aren't my sequence numbers reused on transaction abort?
4.6) Why are my queries slow? Why don't they use my indexes?
4.7) How do I see how the query optimizer is evaluating my query?
4.8) How do I perform regular expression searches and case-insensitive
regular expression searches? How do I use an index for
case-insensitive searches?
4.9) In a query, how do I detect if a field is NULL?
4.10) What is the difference between the various character types?
4.11.0) How do I create a serial/auto-incrementing field?
4.11.1) How do I get the value of a SERIAL insert?
4.11.2) Doesn't currval() lead to a race condition with other users?
4.11.3) Why aren't my sequence numbers reused on transaction abort?
Why are there gaps in the numbering of my sequence/SERIAL column?
4.15) What is an OID? What is a TID?
4.16) What is the meaning of some of the terms used in PostgreSQL?
4.17) Why do I get the error "ERROR: Memory exhausted in
4.12) What is an OID? What is a TID?
4.13) What is the meaning of some of the terms used in PostgreSQL?
4.14) Why do I get the error "ERROR: Memory exhausted in
AllocSetAlloc()"?
4.18) How do I tell what PostgreSQL version I am running?
4.19) Why does my large-object operations get "invalid large obj
4.15) How do I tell what PostgreSQL version I am running?
4.16) Why does my large-object operations get "invalid large obj
descriptor"?
4.20) How do I create a column that will default to the current time?
4.21) Why are my subqueries using IN so slow?
4.22) How do I perform an outer join?
4.23) How do I perform queries using multiple databases?
4.24) How do I return multiple rows or columns from a function?
4.25) Why can't I reliably create/drop temporary tables in PL/PgSQL
4.17) How do I create a column that will default to the current time?
4.18) How do I perform an outer join?
4.19) How do I perform queries using multiple databases?
4.20) How do I return multiple rows or columns from a function?
4.21) Why can't I reliably create/drop temporary tables in PL/PgSQL
functions?
4.26) What encryption options are available?
4.22) What encryption options are available?
Extending PostgreSQL
@@ -172,14 +159,12 @@
no restrictions on how the source code may be used. We like it and
have no intention of changing it.
1.3) What Unix platforms does PostgreSQL run on?
1.3) What platforms does PostgreSQL support?
In general, any modern Unix-compatible platform should be able to run
PostgreSQL. The platforms that had received explicit testing at the
time of release are listed in the installation instructions.
1.4) What non-Unix ports are available?
Starting with version 8.0, PostgreSQL now runs natively on Microsoft
Windows NT-based operating systems like Win2000, WinXP, and Win2003. A
prepackaged installer is available at
@@ -191,12 +176,12 @@
http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgre
SQL&stype=all&sort=type&dir=%2F.
1.5) Where can I get PostgreSQL?
1.4) Where can I get PostgreSQL?
The primary anonymous ftp site for PostgreSQL is
ftp://ftp.PostgreSQL.org/pub. For mirror sites, see our main web site.
1.6) Where can I get support?
1.5) Where can I get support?
The main mailing list is: pgsql-general@PostgreSQL.org. It is
available for discussion of matters pertaining to PostgreSQL. To
@@ -241,13 +226,13 @@
A list of commercial support companies is available at
http://techdocs.postgresql.org/companies.php.
1.7) What is the latest release?
1.6) What is the latest release?
The latest release of PostgreSQL is version 8.0.0.
We plan to have major releases every six to eight months.
1.8) What documentation is available?
1.7) 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
@@ -267,12 +252,12 @@
Our web site contains even more documentation.
1.9) How do I find out about known bugs or missing features?
1.8) How do I find out about known bugs or missing features?
PostgreSQL supports an extended subset of SQL-92. See our TODO list
for known bugs, missing features, and future plans.
1.10) How can I learn SQL?
1.9) How can I learn SQL?
The PostgreSQL book at http://www.PostgreSQL.org/docs/awbook.html
teaches SQL. There is another PostgreSQL book at
@@ -288,11 +273,7 @@
et al., Addison-Wesley. Others like The Complete Reference SQL, Groff
et al., McGraw-Hill.
1.11) Is PostgreSQL Y2K compliant?
Yes, we easily handle dates past the year 2000 AD, and before 2000 BC.
1.12) How do I join the development team?
1.10) How do I join the development team?
First, download the latest source and read the PostgreSQL Developers
documentation on our web site, or in the distribution. Second,
@@ -305,7 +286,7 @@
and we had confidence that patches they committed were of high
quality.
1.13) How do I submit a bug report?
1.11) How do I submit a bug report?
Visit the PostgreSQL bug form at
http://www.postgresql.org/support/submitbug.
@@ -313,7 +294,7 @@
Also check out our ftp site ftp://ftp.PostgreSQL.org/pub to see if
there is a more recent PostgreSQL version or patches.
1.14) How does PostgreSQL compare to other DBMSs?
1.12) How does PostgreSQL compare to other DBMSs?
There are several ways of measuring software: features, performance,
reliability, support, and price.
@@ -353,14 +334,14 @@
community, manuals, and the source code often make PostgreSQL
support superior to other DBMSs. There is commercial
per-incident support available for those who need it. (See FAQ
section 1.6.)
section 1.5.)
Price
We are free for all use, both commercial and non-commercial.
You can add our code to your product with no limitations,
except those outlined in our BSD-style license stated above.
1.15) How can I financially assist PostgreSQL?
1.13) How can I financially assist PostgreSQL?
PostgreSQL has had a first-class infrastructure since we started in
1996. This is all thanks to Marc Fournier, who has created and managed
@@ -386,21 +367,17 @@
User Client Questions
2.1) Are there ODBC drivers for PostgreSQL?
2.1) What interfaces are available for PostgreSQL?
There are two ODBC drivers available, PsqlODBC and OpenLink ODBC.
The PostgreSQL install includes only the C and embedded C interfaces.
All other interfaces are independent projects that are downloaded
separately; being separate allows them to have their own release
schedule and development teams.
You can download PsqlODBC from
http://gborg.postgresql.org/project/psqlodbc/projdisplay.php.
OpenLink ODBC can be gotten from http://www.openlinksw.com. It works
with their standard ODBC client software so you'll have PostgreSQL
ODBC available on every client platform they support (Win, Mac, Unix,
VMS).
They will probably be selling this product to people who need
commercial-quality support, but a freeware version will always be
available. Please send questions to postgres95@openlink.co.uk.
Some programming languages like PHP include an interface to
PostgreSQL. Interfaces for languages like Perl, TCL, Python, and many
others are available at http://gborg.postgresql.org in the
Drivers/Interfaces section and via Internet search.
2.2) What tools are available for using PostgreSQL with Web pages?
@@ -424,21 +401,6 @@
See http://techdocs.postgresql.org/guides/GUITools for a more detailed
list.
2.4) What languages are able to communicate with PostgreSQL?
Most popular programming languages contain an interface to PostgreSQL.
Check your programming language's list of extension modules.
The following interfaces are included in the PostgreSQL distribution:
* C (libpq)
* Embedded C (ecpg)
* Java (jdbc)
* Python (PyGreSQL)
* TCL (libpgtcl)
Additional interfaces are available at http://gborg.postgresql.org in
the Drivers/Interfaces section.
_________________________________________________________________
Administrative Questions
@@ -447,43 +409,7 @@
Specify the --prefix option when running configure.
3.2) When I start postmaster, I get a Bad System Call or core dumped
message. Why?
It could be a variety of problems, but first check to see that you
have System V extensions installed in your kernel. PostgreSQL requires
kernel support for shared memory and semaphores.
3.3) When I try to start postmaster, I get IpcMemoryCreate errors. Why?
You either do not have shared memory configured properly in your
kernel or you need to enlarge the shared memory available in the
kernel. The exact amount you need depends on your architecture and how
many buffers and backend processes you configure for postmaster. For
most systems, with default numbers of buffers and processes, you need
a minimum of ~1 MB. See the PostgreSQL Administrator's Guide/Server
Run-time Environment/Managing Kernel Resources section for more
detailed information about shared memory and semaphores.
3.4) When I try to start postmaster, I get IpcSemaphoreCreate errors. Why?
If the error message is IpcSemaphoreCreate: semget failed (No space
left on device) then your kernel is not configured with enough
semaphores. Postgres needs one semaphore per potential backend
process. A temporary solution is to start postmaster with a smaller
limit on the number of backend processes. Use -N with a parameter less
than the default of 32. A more permanent solution is to increase your
kernel's SEMMNS and SEMMNI parameters.
Inoperative semaphores can also cause crashes during heavy database
access.
If the error message is something else, you might not have semaphore
support configured in your kernel at all. See the PostgreSQL
Administrator's Guide for more detailed information about shared
memory and semaphores.
3.5) How do I control connections from other hosts?
3.2) How do I control connections from other hosts?
By default, PostgreSQL only allows connections from the local machine
using Unix domain sockets or TCP/IP connections. Other machines will
@@ -491,7 +417,7 @@
postgresql.conf and enable host-based authentication by modifying the
file $PGDATA/pg_hba.conf accordingly.
3.6) How do I tune the database engine for better performance?
3.3) How do I tune the database engine for better performance?
Certainly, indexes can speed up queries. The EXPLAIN ANALYZE command
allows you to see how PostgreSQL is interpreting your query, and which
@@ -523,7 +449,7 @@
You can also use the CLUSTER command to group data in tables to match
an index. See the CLUSTER manual page for more details.
3.7) What debugging features are available?
3.4) What debugging features are available?
PostgreSQL has several features that report status information that
can be valuable for debugging purposes.
@@ -578,7 +504,7 @@ log_*
in the client's current directory. Linux requires a compile with
-DLINUX_PROFILE for proper profiling.
3.8) Why do I get "Sorry, too many clients" when trying to connect?
3.5) Why do I get "Sorry, too many clients" when trying to connect?
You need to increase postmaster's limit on how many concurrent backend
processes it can start.
@@ -598,7 +524,7 @@ log_*
that PostgreSQL has a limit on the number of allowed backend processes
is so your system won't run out of resources.
3.9) What is in the pgsql_tmp directory?
3.6) What is in the pgsql_tmp directory?
This directory contains temporary files generated by the query
executor. For example, if a sort needs to be done to satisfy an ORDER
@@ -609,7 +535,7 @@ log_*
remain if a backend crashes during a sort. A stop and restart of the
postmaster will remove files from those directories.
3.10) Why do I need to do a dump and restore to upgrade between major
3.7) Why do I need to do a dump and restore to upgrade between major
PostgreSQL releases?
The PostgreSQL team makes only small changes between minor releases,
@@ -624,7 +550,7 @@ log_*
script can be used to upgrade without a dump/restore. The release
notes mention whether pg_upgrade is available for the release.
3.11) What computer hardware should I use?
3.8) What computer hardware should I use?
Because PC hardware is mostly compatible, people tend to believe that
all PC hardware is of equal quality. It is not. ECC RAM, SCSI, and
@@ -637,11 +563,7 @@ log_*
Operational Questions
4.1) What is the difference between binary cursors and normal cursors?
See the DECLARE manual page for a description.
4.2) How do I SELECT only the first few rows of a query? A random row?
4.1) How do I SELECT only the first few rows of a query? A random row?
See the FETCH manual page, or use SELECT ... LIMIT....
@@ -657,7 +579,7 @@ log_*
ORDER BY random()
LIMIT 1;
4.3) How do I find out what tables, indexes, databases, and users are
4.2) How do I find out what tables, indexes, databases, and users are
defined? How do I see the queries used by psql to display them?
Use the \dt command to see tables in psql. For a complete list of
@@ -676,7 +598,7 @@ log_*
many of the SELECTs needed to get information from the database system
tables.
4.4) How do you remove a column from a table, or change its data type?
4.3) How do you remove a column from a table, or change its data type?
DROP COLUMN functionality was added in release 7.3 with ALTER TABLE
DROP COLUMN. In earlier versions, you can do this:
@@ -702,7 +624,7 @@ log_*
You might then want to do VACUUM FULL tab to reclaim the disk space
used by the expired rows.
4.5) What is the maximum size for a row, a table, and a database?
4.4) What is the maximum size for a row, a table, and a database?
These are the limits:
Maximum size for a database? unlimited (32 TB databases exist)
@@ -724,7 +646,7 @@ log_*
The maximum table size and maximum number of columns can be quadrupled
by increasing the default block size to 32k.
4.6) How much database disk space is required to store data from a typical
4.5) How much database disk space is required to store data from a typical
text file?
A PostgreSQL database may require up to five times the disk space to
@@ -758,7 +680,7 @@ log_*
NULLs are stored as bitmaps, so they use very little space.
4.7) My queries are slow or don't make use of the indexes. Why?
4.6) Why are my queries slow? Why don't they use my indexes?
Indexes are not automatically used by every query. Indexes are only
used if the table is larger than a minimum size, and the query selects
@@ -800,7 +722,7 @@ log_*
[a-e].
* Case-insensitive searches such as ILIKE and ~* do not utilize
indexes. Instead, use functional indexes, which are described in
section 4.11.
section 4.10.
* The default C locale must be used during initdb because it is not
possible to know the next-greater character in a non-C locale. You
can create a special
@@ -813,40 +735,11 @@ LIKE
types exactly match the index's column types. This is particularly
true of int2, int8, and numeric column indexes.
4.8) How do I see how the query optimizer is evaluating my query?
4.7) How do I see how the query optimizer is evaluating my query?
See the EXPLAIN manual page.
4.9) What is an R-tree index?
An R-tree index is used for indexing spatial data. A hash index can't
handle range searches. A B-tree index only handles range searches in a
single dimension. R-trees can handle multi-dimensional data. For
example, if an R-tree index can be built on an attribute of type
point, the system can more efficiently answer queries such as "select
all points within a bounding rectangle."
The canonical paper that describes the original R-tree design is:
Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt of
Data, 45-57.
You can also find this paper in Stonebraker's "Readings in Database
Systems".
Built-in R-trees can handle polygons and boxes. In theory, R-trees can
be extended to handle higher number of dimensions. In practice,
extending R-trees requires a bit of work and we don't currently have
any documentation on how to do it.
4.10) What is the Genetic Query Optimizer?
The GEQO module speeds query optimization when joining many tables by
means of a Genetic Algorithm (GA). It allows the handling of large
join queries through nonexhaustive search.
4.11) How do I perform regular expression searches and case-insensitive
4.8) How do I perform regular expression searches and case-insensitive
regular expression searches? How do I use an index for case-insensitive
searches?
@@ -863,11 +756,11 @@ LIKE
functional index, it will be used:
CREATE INDEX tabindex ON tab (lower(col));
4.12) In a query, how do I detect if a field is NULL?
4.9) In a query, how do I detect if a field is NULL?
You test the column with IS NULL and IS NOT NULL.
4.13) What is the difference between the various character types?
4.10) What is the difference between the various character types?
Type Internal Name Notes
--------------------------------------------------
@@ -895,7 +788,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
particularly values that include NULL bytes. All the types described
here have similar performance characteristics.
4.14.1) How do I create a serial/auto-incrementing field?
4.11.1) How do I create a serial/auto-incrementing field?
PostgreSQL supports a SERIAL data type. It auto-creates a sequence.
For example, this:
@@ -916,11 +809,11 @@ BYTEA bytea variable-length byte array (null-byte safe)
However, if you need to dump and reload the database, you need to use
pg_dump's -o option or COPY WITH OIDS option to preserve the OIDs.
4.14.2) How do I get the value of a SERIAL insert?
4.11.2) How do I get the value of a SERIAL insert?
One approach is to retrieve the next SERIAL value from the sequence
object with the nextval() function before inserting and then insert it
explicitly. Using the example table in 4.14.1, an example in a
explicitly. Using the example table in 4.11.1, an example in a
pseudo-language would look like this:
new_id = execute("SELECT nextval('person_id_seq')");
execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
@@ -942,52 +835,36 @@ BYTEA bytea variable-length byte array (null-byte safe)
billion. In Perl, using DBI with the DBD::Pg module, the oid value is
made available via $sth->{pg_oid_status} after $sth->execute().
4.14.3) Doesn't currval() lead to a race condition with other users?
4.11.3) Doesn't currval() lead to a race condition with other users?
No. currval() returns the current value assigned by your backend, not
by all users.
4.14.4) Why aren't my sequence numbers reused on transaction abort? Why are
4.11.4) Why aren't my sequence numbers reused on transaction abort? Why are
there gaps in the numbering of my sequence/SERIAL column?
To improve concurrency, sequence values are given out to running
transactions as needed and are not locked until the transaction
completes. This causes gaps in numbering from aborted transactions.
4.15) What is an OID? What is a TID?
4.12) What is an OID? What is a TID?
OIDs are PostgreSQL's answer to unique row ids. Every row that is
created in PostgreSQL gets a unique OID. All OIDs generated during
initdb are less than 16384 (from include/access/transam.h). All
user-created OIDs are equal to or greater than this. By default, all
these OIDs are unique not only within a table or database, but unique
within the entire PostgreSQL installation.
Every row that is created in PostgreSQL gets a unique OID unless
created WITHOUT OIDS. OIDs are autotomatically assigned unique 4-byte
integers that are unique across the entire installation. However, they
overflow at 4 billion, and then the OIDs start being duplicated.
PostgreSQL uses OIDs to link its internal system tables together.
PostgreSQL uses OIDs in its internal system tables to link rows
between tables. These OIDs can be used to identify specific user rows
and used in joins. It is recommended you use column type OID to store
OID values. You can create an index on the OID field for faster
access.
OIDs are assigned to all new rows from a central area that is used by
all databases. If you want to change the OID to something else, or if
you want to make a copy of the table, with the original OIDs, there is
no reason you can't do it:
CREATE TABLE new_table(mycol int);
SELECT oid AS old_oid, mycol INTO tmp_table FROM old_table;
COPY tmp_table TO '/tmp/pgtable';
COPY new_table WITH OIDS FROM '/tmp/pgtable';
DROP TABLE tmp_table;
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.
To uniquely number columns in user tables, it is best to use SERIAL
rather than OIDs because SERIAL sequences are unique only within a
single table. and are therefore less likely to overflow. SERIAL8 is
available for storing eight-byte sequence values.
TIDs are used to identify specific physical rows with block and offset
values. TIDs change after rows are modified or reloaded. They are used
by index entries to point to physical rows.
4.16) What is the meaning of some of the terms used in PostgreSQL?
4.13) What is the meaning of some of the terms used in PostgreSQL?
Some of the source code and older documentation use terms that have
more common usage. Here are some:
@@ -1005,7 +882,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary
/glossary.html
4.17) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
4.14) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
You probably have run out of virtual memory on your system, or your
kernel has a low limit for certain resources. Try this before starting
@@ -1020,11 +897,11 @@ BYTEA bytea variable-length byte array (null-byte safe)
problem with the SQL client because the backend is returning too much
data, try it before starting the client.
4.18) How do I tell what PostgreSQL version I am running?
4.15) How do I tell what PostgreSQL version I am running?
From psql, type SELECT version();
4.19) Why does my large-object operations get "invalid large obj
4.16) Why does my large-object operations get "invalid large obj
descriptor"?
You need to put BEGIN WORK and COMMIT around any use of a large object
@@ -1039,33 +916,12 @@ BYTEA bytea variable-length byte array (null-byte safe)
If you are using a client interface like ODBC you may need to set
auto-commit off.
4.20) How do I create a column that will default to the current time?
4.17) How do I create a column that will default to the current time?
Use CURRENT_TIMESTAMP:
CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
4.21) Why are my subqueries using IN so slow?
In versions prior to 7.4, subqueries were joined to outer queries by
sequentially scanning the result of the subquery for each row of the
outer query. If the subquery returns only a few rows and the outer
query returns many rows, IN is fastest. To speed up other queries,
replace IN with EXISTS:
SELECT *
FROM tab
WHERE col IN (SELECT subcol FROM subtab);
to:
SELECT *
FROM tab
WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
For this to be fast, subcol should be an indexed column.
In version 7.4 and later, IN actually uses the same sophisticated join
techniques as normal queries, and is prefered to using EXISTS.
4.22) How do I perform an outer join?
4.18) How do I perform an outer join?
PostgreSQL supports outer joins using the SQL standard syntax. Here
are two examples:
@@ -1095,7 +951,7 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
ORDER BY col1
4.23) How do I perform queries using multiple databases?
4.19) How do I perform queries using multiple databases?
There is no way to query a database other than the current one.
Because PostgreSQL loads database-specific system catalogs, it is
@@ -1105,12 +961,12 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
course, a client can make simultaneous connections to different
databases and merge the results on the client side.
4.24) How do I return multiple rows or columns from a function?
4.20) How do I return multiple rows or columns from a function?
In 7.3, you can easily return multiple rows or columns from a
function, http://techdocs.postgresql.org/guides/SetReturningFunctions.
4.25) Why can't I reliably create/drop temporary tables in PL/PgSQL
4.21) Why can't I reliably create/drop temporary tables in PL/PgSQL
functions?
PL/PgSQL caches function contents, and an unfortunate side effect is
@@ -1121,7 +977,7 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
table access in PL/PgSQL. This will cause the query to be reparsed
every time.
4.26) What encryption options are available?
4.22) What encryption options are available?
* contrib/pgcrypto contains many encryption functions for use in SQL
queries.

416
doc/src/FAQ/FAQ.html
View File

@@ -10,7 +10,7 @@
alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Wed Jan 19 14:45:22 EST 2005</P>
<P>Last updated: Sat Jan 29 23:20:03 EST 2005</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
@@ -28,117 +28,100 @@
<H2 align="center">General Questions</H2>
<A href="#1.1">1.1</A>) What is PostgreSQL? How is it pronounced?<BR>
<A href="#1.2">1.2</A>) What is the copyright on PostgreSQL?<BR>
<A href="#1.3">1.3</A>) What Unix platforms does PostgreSQL run
on?<BR>
<A href="#1.4">1.4</A>) What non-Unix ports are available?<BR>
<A href="#1.5">1.5</A>) Where can I get PostgreSQL?<BR>
<A href="#1.6">1.6</A>) Where can I get support?<BR>
<A href="#1.7">1.7</A>) What is the latest release?<BR>
<A href="#1.8">1.8</A>) What documentation is available?<BR>
<A href="#1.9">1.9</A>) How do I find out about known bugs or
<A href="#1.3">1.3</A>) What platforms does PostgreSQL support?<BR>
<A href="#1.4">1.4</A>) Where can I get PostgreSQL?<BR>
<A href="#1.5">1.5</A>) Where can I get support?<BR>
<A href="#1.6">1.6</A>) What is the latest release?<BR>
<A href="#1.7">1.7</A>) What documentation is available?<BR>
<A href="#1.8">1.8</A>) How do I find out about known bugs or
missing features?<BR>
<A href="#1.10">1.10</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
<A href="#1.11">1.11</A>) Is PostgreSQL Y2K compliant?<BR>
<A href="#1.12">1.12</A>) How do I join the development team?<BR>
<A href="#1.13">1.13</A>) How do I submit a bug report?<BR>
<A href="#1.14">1.14</A>) How does PostgreSQL compare to other
<A href="#1.9">1.9</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
<A href="#1.10">1.10</A>) How do I join the development team?<BR>
<A href="#1.11">1.11</A>) How do I submit a bug report?<BR>
<A href="#1.12">1.12</A>) How does PostgreSQL compare to other
<SMALL>DBMS</SMALL>s?<BR>
<A href="#1.15">1.15</A>) How can I financially assist
<A href="#1.13">1.13</A>) How can I financially assist
PostgreSQL?<BR>
<H2 align="center">User Client Questions</H2>
<A href="#2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers for
<A href="#2.1">2.1</A>) What interfaces are available for
PostgreSQL?<BR>
<A href="#2.2">2.2</A>) What tools are available for using
PostgreSQL with Web pages?<BR>
<A href="#2.3">2.3</A>) Does PostgreSQL have a graphical user
interface?<BR>
<A href="#2.4">2.4</A>) What languages are available to
communicate with PostgreSQL?<BR>
<H2 align="center">Administrative Questions</H2>
<A href="#3.1">3.1</A>) How do I install PostgreSQL somewhere other
than <I>/usr/local/pgsql</I>?<BR>
<A href="#3.2">3.2</A>) When I start <I>postmaster</I>, I get a
<I>Bad System Call</I> or core dumped message. Why?<BR>
<A href="#3.3">3.3</A>) When I try to start <I>postmaster</I>, I
get <I>IpcMemoryCreate</I> errors. Why?<BR>
<A href="#3.4">3.4</A>) When I try to start <I>postmaster</I>, I
get <I>IpcSemaphoreCreate</I> errors. Why?<BR>
<A href="#3.5">3.5</A>) How do I control connections from other
<A href="#3.2">3.2</A>) How do I control connections from other
hosts?<BR>
<A href="#3.6">3.6</A>) How do I tune the database engine for
<A href="#3.3">3.3</A>) How do I tune the database engine for
better performance?<BR>
<A href="#3.7">3.7</A>) What debugging features are available?<BR>
<A href="#3.8">3.8</A>) Why do I get <I>"Sorry, too many
<A href="#3.4">3.4</A>) What debugging features are available?<BR>
<A href="#3.5">3.5</A>) Why do I get <I>"Sorry, too many
clients"</I> when trying to connect?<BR>
<A href="#3.9">3.9</A>) What is in the <I>pgsql_tmp</I>
<A href="#3.6">3.6</A>) What is in the <I>pgsql_tmp</I>
directory?<BR>
<A href="#3.10">3.10</A>) Why do I need to do a dump and restore
<A href="#3.7">3.7</A>) Why do I need to do a dump and restore
to upgrade PostgreSQL releases?<BR>
<A href="#3.11">3.11</A>) What computer hardware should I use?<BR>
<A href="#3.8">3.8</A>) What computer hardware should I use?<BR>
<H2 align="center">Operational Questions</H2>
<A href="#4.1">4.1</A>) What is the difference between binary
cursors and normal cursors?<BR>
<A href="#4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
<A href="#4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the
first few rows of a query? A random row?<BR>
<A href="#4.3">4.3</A>) How do I find out what tables, indexes,
<A href="#4.2">4.2</A>) How do I find out what tables, indexes,
databases, and users are defined? How do I see the queries used
by <I>psql</I> to display them?<BR>
<A href="#4.4">4.4</A>) How do you remove a column from a
<A href="#4.3">4.3</A>) How do you remove a column from a
table, or change its data type?<BR>
<A href="#4.5">4.5</A>) What is the maximum size for a row, a
<A href="#4.4">4.4</A>) What is the maximum size for a row, a
table, and a database?<BR>
<A href="#4.6">4.6</A>) How much database disk space is required
<A href="#4.5">4.5</A>) How much database disk space is required
to store data from a typical text file?<BR>
<A href="#4.7">4.7</A>) My queries are slow or don't make use of
the indexes. Why?<BR>
<A href="#4.8">4.8</A>) How do I see how the query optimizer is
<A href="#4.6">4.6</A>) Why are my queries slow? Why don't they
use my indexes?<BR>
<A href="#4.7">4.7</A>) How do I see how the query optimizer is
evaluating my query?<BR>
<A href="#4.9">4.9</A>) What is an R-tree index?<BR>
<A href="#4.10">4.10</A>) What is the Genetic Query Optimizer?<BR>
<A href="#4.11">4.11</A>) How do I perform regular expression
<A href="#4.8">4.8</A>) How do I perform regular expression
searches and case-insensitive regular expression searches? How do I
use an index for case-insensitive searches?<BR>
<A href="#4.12">4.12</A>) In a query, how do I detect if a field
<A href="#4.9">4.9</A>) In a query, how do I detect if a field
is <SMALL>NULL</SMALL>?<BR>
<A href="#4.13">4.13</A>) What is the difference between the
<A href="#4.10">4.10</A>) What is the difference between the
various character types?<BR>
<A href="#4.14.0">4.14.0</A>) How do I create a
<A href="#4.11.0">4.11.0</A>) How do I create a
serial/auto-incrementing field?<BR>
<A href="#4.14.1">4.14.1</A>) How do I get the value of a
<A href="#4.11.1">4.11.1</A>) How do I get the value of a
<SMALL>SERIAL</SMALL> insert?<BR>
<A href="#4.14.2">4.14.2</A>) Doesn't <I>currval()</I>
<A href="#4.11.2">4.11.2</A>) Doesn't <I>currval()</I>
lead to a race condition with other users?<BR>
<A href="#4.14.3">4.14.3</A>) Why aren't my sequence numbers
<A href="#4.11.3">4.11.3</A>) Why aren't my sequence numbers
reused on transaction abort? Why are there gaps in the numbering of
my sequence/SERIAL column?<BR>
<A href="#4.15">4.15</A>) What is an <SMALL>OID</SMALL>? What is a
<A href="#4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is a
<SMALL>TID</SMALL>?<BR>
<A href="#4.16">4.16</A>) What is the meaning of some of the terms
<A href="#4.13">4.13</A>) What is the meaning of some of the terms
used in PostgreSQL?<BR>
<A href="#4.17">4.17</A>) Why do I get the error <I>"ERROR: Memory
<A href="#4.14">4.14</A>) Why do I get the error <I>"ERROR: Memory
exhausted in AllocSetAlloc()"</I>?<BR>
<A href="#4.18">4.18</A>) How do I tell what PostgreSQL version I
<A href="#4.15">4.15</A>) How do I tell what PostgreSQL version I
am running?<BR>
<A href="#4.19">4.19</A>) Why does my large-object operations get
<A href="#4.16">4.16</A>) Why does my large-object operations get
<I>"invalid large obj descriptor"</I>?<BR>
<A href="#4.20">4.20</A>) How do I create a column that will
<A href="#4.17">4.17</A>) How do I create a column that will
default to the current time?<BR>
<A href="#4.21">4.21</A>) Why are my subqueries using
<CODE><SMALL>IN</SMALL></CODE> so slow?<BR>
<A href="#4.22">4.22</A>) How do I perform an outer join?<BR>
<A href="#4.23">4.23</A>) How do I perform queries using multiple
<A href="#4.18">4.18</A>) How do I perform an outer join?<BR>
<A href="#4.19">4.19</A>) How do I perform queries using multiple
databases?<BR>
<A href="#4.24">4.24</A>) How do I return multiple rows or columns
<A href="#4.20">4.20</A>) How do I return multiple rows or columns
from a function?<BR>
<A href="#4.25">4.25</A>) Why can't I reliably create/drop
<A href="#4.21">4.21</A>) Why can't I reliably create/drop
temporary tables in PL/PgSQL functions?<BR>
<A href="#4.26">4.26</A>) What encryption options are available?<BR>
<A href="#4.22">4.22</A>) What encryption options are available?<BR>
<H2 align="center">Extending PostgreSQL</H2>
@@ -225,16 +208,13 @@
It has no restrictions on how the source code may be used. We like
it and have no intention of changing it.</P>
<H4><A name="1.3">1.3</A>) What Unix platforms does PostgreSQL run
on?</H4>
<H4><A name="1.3">1.3</A>) What platforms does PostgreSQL support?</H4>
<P>In general, any modern Unix-compatible platform should be able to
run PostgreSQL. The platforms that had received explicit testing at
the time of release are listed in the installation
instructions.</P>
<H4><A name="1.4">1.4</A>) What non-Unix ports are available?</H4>
<P>Starting with version 8.0, PostgreSQL now runs natively on
Microsoft Windows NT-based operating systems like Win2000, WinXP,
and Win2003. A prepackaged installer is available at <a href=
@@ -248,13 +228,13 @@
"http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgreSQL&stype=all&sort=type&dir=%2F">
http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgreSQL&stype=all&sort=type&dir=%2F</a>.</p>
<H4><A name="1.5">1.5</A>) Where can I get PostgreSQL?</H4>
<H4><A name="1.4">1.4</A>) Where can I get PostgreSQL?</H4>
<P>The primary anonymous ftp site for PostgreSQL is <A href=
"ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A>.
For mirror sites, see our main web site.</P>
<H4><A name="1.6">1.6</A>) Where can I get support?</H4>
<H4><A name="1.5">1.5</A>) Where can I get support?</H4>
<P>The main mailing list is: <A href=
"mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>.
@@ -316,13 +296,13 @@
<P>A list of commercial support companies is available at <A href=
"http://techdocs.postgresql.org/companies.php">http://techdocs.postgresql.org/companies.php</A>.</P>
<H4><A name="1.7">1.7</A>) What is the latest release?</H4>
<H4><A name="1.6">1.6</A>) What is the latest release?</H4>
<P>The latest release of PostgreSQL is version 8.0.0.</P>
<P>We plan to have major releases every six to eight months.</P>
<H4><A name="1.8">1.8</A>) What documentation is available?</H4>
<H4><A name="1.7">1.7</A>) What documentation is available?</H4>
<P>Several manuals, manual pages, and some small test examples are
included in the distribution. See the <I>/doc</I> directory. You
@@ -346,14 +326,14 @@
<P>Our web site contains even more documentation.</P>
<H4><A name="1.9">1.9</A>) How do I find out about known bugs or
<H4><A name="1.8">1.8</A>) How do I find out about known bugs or
missing features?</H4>
<P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
See our <A href="http://developer.PostgreSQL.org/todo.php">TODO</A>
list for known bugs, missing features, and future plans.</P>
<H4><A name="1.10">1.10</A>) How can I learn
<H4><A name="1.9">1.9</A>) How can I learn
<SMALL>SQL</SMALL>?</H4>
<P>The PostgreSQL book at <A href=
@@ -377,12 +357,7 @@
Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
<H4><A name="1.11">1.11</A>) Is PostgreSQL Y2K compliant?</H4>
<P>Yes, we easily handle dates past the year 2000 AD, and before
2000 BC.</P>
<H4><A name="1.12">1.12</A>) How do I join the development
<H4><A name="1.10">1.10</A>) How do I join the development
team?</H4>
<P>First, download the latest source and read the PostgreSQL
@@ -397,7 +372,7 @@
committers to keep up, and we had confidence that patches they
committed were of high quality.</P>
<H4><A name="1.13">1.13</A>) How do I submit a bug report?</H4>
<H4><A name="1.11">1.11</A>) How do I submit a bug report?</H4>
<P>Visit the PostgreSQL bug form at <A href=
"http://www.postgresql.org/support/submitbug">
@@ -407,7 +382,7 @@
"ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
see if there is a more recent PostgreSQL version or patches.</P>
<H4><A name="1.14">1.14</A>) How does PostgreSQL compare to other
<H4><A name="1.12">1.12</A>) How does PostgreSQL compare to other
<SMALL>DBMS</SMALL>s?</H4>
<P>There are several ways of measuring software: features,
@@ -460,7 +435,7 @@
community, manuals, and the source code often make PostgreSQL
support superior to other <SMALL>DBMS</SMALL>s. There is
commercial per-incident support available for those who need it.
(See <A href="#1.6">FAQ section 1.6</A>.)<BR>
(See <A href="#1.5">FAQ section 1.5</A>.)<BR>
<BR>
</DD>
@@ -473,7 +448,7 @@
</DD>
</DL>
<H4><A name="1.15">1.15</A>) How can I financially assist
<H4><A name="1.13">1.13</A>) How can I financially assist
PostgreSQL?</H4>
<P>PostgreSQL has had a first-class infrastructure since we started
@@ -503,26 +478,20 @@
<H2 align="center">User Client Questions</H2>
<H4><A name="2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers
for PostgreSQL?</H4>
<H4><A name="2.1">2.1</A>) What interfaces are available for
PostgreSQL?</H4>
<P>There are two <SMALL>ODBC</SMALL> drivers available, PsqlODBC
and OpenLink <SMALL>ODBC</SMALL>.</P>
<P>The PostgreSQL install includes only the <SMALL>C</SMALL> and embedded
<SMALL>C</SMALL> interfaces. All other interfaces are independent projects
that are downloaded separately; being separate allows them to have their
own release schedule and development teams.</P>
<P>You can download PsqlODBC from <A href=
"http://gborg.postgresql.org/project/psqlodbc/projdisplay.php">
http://gborg.postgresql.org/project/psqlodbc/projdisplay.php</A>.</P>
<P>OpenLink <SMALL>ODBC</SMALL> can be gotten from <A href=
"http://www.openlinksw.com/">http://www.openlinksw.com</A>. It
works with their standard <SMALL>ODBC</SMALL> client software so
you'll have PostgreSQL <SMALL>ODBC</SMALL> available on every
client platform they support (Win, Mac, Unix, VMS).</P>
<P>They will probably be selling this product to people who need
commercial-quality support, but a freeware version will always be
available. Please send questions to <A href=
"mailto:postgres95@openlink.co.uk">postgres95@openlink.co.uk</A>.</P>
<P>Some programming languages like <SMALL>PHP</SMALL> include an
interface to PostgreSQL. Interfaces for languages like Perl,
<SMALL>TCL</SMALL>, Python, and many others are available at
<a href="http://gborg.postgresql.org">http://gborg.postgresql.org</A>
in the <I>Drivers/Interfaces</I> section and via Internet search.
</P>
<H4><A name="2.2">2.2</A>) What tools are available for using
PostgreSQL with Web pages?</H4>
@@ -552,34 +521,9 @@
<P>See <a href="http://techdocs.postgresql.org/guides/GUITools">http://techdocs.postgresql.org/guides/GUITools</a> for a more detailed list.</P>
<H4><A name="2.4">2.4</A>) What languages are able to communicate with
PostgreSQL?</H4>
<P>Most popular programming languages contain an interface to
PostgreSQL. Check your programming language's list of extension
modules.</P>
<P>The following interfaces are included in the PostgreSQL
distribution:</P>
<UL>
<LI>C (libpq)</LI>
<LI>Embedded C (ecpg)</LI>
<LI>Java (jdbc)</LI>
<LI>Python (PyGreSQL)</LI>
<LI>TCL (libpgtcl)</LI>
</UL>
<P>Additional interfaces are available at
<a href="http://gborg.postgresql.org">http://gborg.postgresql.org</A>
in the <I>Drivers/Interfaces</I> section.
</P>
<HR>
<H2 align="center">Administrative Questions</H2>
<H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
@@ -588,47 +532,7 @@
<P>Specify the <I>--prefix</I> option when running
<I>configure</I>.</P>
<H4><A name="3.2">3.2</A>) When I start <I>postmaster</I>, I get a
<I>Bad System Call</I> or core dumped message. Why?</H4>
<P>It could be a variety of problems, but first check to see that
you have System V extensions installed in your kernel. PostgreSQL
requires kernel support for shared memory and semaphores.</P>
<H4><A name="3.3">3.3</A>) When I try to start <I>postmaster</I>, I
get <I>IpcMemoryCreate</I> errors. Why?</H4>
<P>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
<I>postmaster</I>. For most systems, with default numbers of
buffers and processes, you need a minimum of ~1 MB. See the <A
href="http://www.postgresql.org/docs/current/static/kernel-resources.html">PostgreSQL
Administrator's Guide/Server Run-time Environment/Managing Kernel Resources</A>
section for more detailed information about shared memory and semaphores.</P>
<H4><A name="3.4">3.4</A>) When I try to start <I>postmaster</I>, I
get <I>IpcSemaphoreCreate</I> errors. Why?</H4>
<P>If the error message is <I>IpcSemaphoreCreate: semget failed (No
space left on device)</I> then your kernel is not configured with
enough semaphores. Postgres needs one semaphore per potential
backend process. A temporary solution is to start <I>postmaster</I>
with a smaller limit on the number of backend processes. Use
<I>-N</I> with a parameter less than the default of 32. A more
permanent solution is to increase your kernel's
<SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI</SMALL> parameters.</P>
<P>Inoperative semaphores can also cause crashes during heavy
database access.</P>
<P>If the error message is something else, you might not have
semaphore support configured in your kernel at all. See the
PostgreSQL Administrator's Guide for more detailed information
about shared memory and semaphores.</P>
<H4><A name="3.5">3.5</A>) How do I control connections from other
<H4><A name="3.2">3.2</A>) How do I control connections from other
hosts?</H4>
<P>By default, PostgreSQL only allows connections from the local
@@ -638,7 +542,7 @@
host-based authentication by modifying the file
<I>$PGDATA/pg_hba.conf</I> accordingly.</P>
<H4><A name="3.6">3.6</A>) How do I tune the database engine for
<H4><A name="3.3">3.3</A>) How do I tune the database engine for
better performance?</H4>
<P>Certainly, indexes can speed up queries. The
@@ -677,7 +581,7 @@
data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
manual page for more details.</P>
<H4><A name="3.7">3.7</A>) What debugging features are
<H4><A name="3.4">3.4</A>) What debugging features are
available?</H4>
<P>PostgreSQL has several features that report status information
@@ -735,7 +639,7 @@
file will be put in the client's current directory. Linux requires
a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>
<H4><A name="3.8">3.8</A>) Why do I get <I>"Sorry, too many
<H4><A name="3.5">3.5</A>) Why do I get <I>"Sorry, too many
clients"</I> when trying to connect?</H4>
<P>You need to increase <I>postmaster</I>'s limit on how many
@@ -760,7 +664,7 @@
the number of allowed backend processes is so your system won't run
out of resources.</P>
<H4><A name="3.9">3.9</A>) What is in the <I>pgsql_tmp</I> directory?</H4>
<H4><A name="3.6">3.6</A>) What is in the <I>pgsql_tmp</I> directory?</H4>
<P>This directory contains temporary files generated by the query
executor. For example, if a sort needs to be done to satisfy an
@@ -772,7 +676,7 @@
remain if a backend crashes during a sort. A stop and restart of the
<I>postmaster</I> will remove files from those directories.</P>
<H4><A name="3.10">3.10</A>) Why do I need to do a dump and restore
<H4><A name="3.7">3.7</A>) Why do I need to do a dump and restore
to upgrade between major PostgreSQL releases?</H4>
<P>The PostgreSQL team makes only small changes between minor releases,
@@ -788,7 +692,7 @@
The release notes mention whether <I>pg_upgrade</I> is available for the
release.</P>
<H4><A name="3.11">3.11</A>) What computer hardware should I use?</H4>
<H4><A name="3.8">3.8</A>) What computer hardware should I use?</H4>
<P>Because PC hardware is mostly compatible, people tend to believe that
all PC hardware is of equal quality. It is not. ECC RAM, SCSI, and
@@ -802,13 +706,7 @@
<H2 align="center">Operational Questions</H2>
<H4><A name="4.1">4.1</A>) What is the difference between binary
cursors and normal cursors?</H4>
<P>See the <SMALL>DECLARE</SMALL> manual page for a
description.</P>
<H4><A name="4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
<H4><A name="4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the
first few rows of a query? A random row?</H4>
<P>See the <SMALL>FETCH</SMALL> manual page, or use
@@ -829,7 +727,7 @@
LIMIT 1;
</PRE>
<H4><A name="4.3">4.3</A>) How do I find out what tables, indexes,
<H4><A name="4.2">4.2</A>) How do I find out what tables, indexes,
databases, and users are defined? How do I see the queries used
by <I>psql</I> to display them?</H4>
@@ -850,7 +748,7 @@
illustrates many of the <SMALL>SELECT</SMALL>s needed to get
information from the database system tables.</P>
<H4><A name="4.4">4.4</A>) How do you remove a column from a
<H4><A name="4.3">4.3</A>) How do you remove a column from a
table, or change its data type?</H4>
<P><SMALL>DROP COLUMN</SMALL> functionality was added in release 7.3
@@ -881,7 +779,7 @@
<P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the
disk space used by the expired rows.</P>
<H4><A name="4.5">4.5</A>) What is the maximum size for a row, a
<H4><A name="4.4">4.4</A>) What is the maximum size for a row, a
table, and a database?</H4>
<P>These are the limits:</P>
@@ -907,7 +805,7 @@
<P>The maximum table size and maximum number of columns can be
quadrupled by increasing the default block size to 32k.</P>
<H4><A name="4.6">4.6</A>) How much database disk space is required
<H4><A name="4.5">4.5</A>) How much database disk space is required
to store data from a typical text file?</H4>
<P>A PostgreSQL database may require up to five times the disk
@@ -944,8 +842,8 @@
<P><SMALL>NULL</SMALL>s are stored as bitmaps, so they
use very little space.</P>
<H4><A name="4.7">4.7</A>) My queries are slow or don't make use of
the indexes. Why?</H4>
<H4><A name="4.6">4.6</A>) Why are my queries slow? Why don't they
use my indexes?</H4>
<P>Indexes are not automatically used by every query. Indexes are only
used if the table is larger than a minimum size, and the query
@@ -995,7 +893,7 @@
e.g. [a-e].</LI>
<LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
<I>~*</I> do not utilize indexes. Instead, use functional
indexes, which are described in section <a href="#4.11">4.11</a>.</LI>
indexes, which are described in section <a href="#4.10">4.10</a>.</LI>
<LI>The default <I>C</I> locale must be used during
<i>initdb</i> because it is not possible to know the next-greater
character in a non-C locale. You can create a special
@@ -1008,45 +906,12 @@
types exactly match the index's column types. This is particularly
true of int2, int8, and numeric column indexes.</P>
<H4><A name="4.8">4.8</A>) How do I see how the query optimizer is
<H4><A name="4.7">4.7</A>) How do I see how the query optimizer is
evaluating my query?</H4>
<P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
<H4><A name="4.9">4.9</A>) What is an R-tree index?</H4>
<P>An R-tree index is used for indexing spatial data. A hash index
can't handle range searches. A B-tree index only handles range
searches in a single dimension. R-trees can handle
multi-dimensional data. For example, if an R-tree index can be
built on an attribute of type <I>point</I>, the system can more
efficiently answer queries such as "select all points within a
bounding rectangle."</P>
<P>The canonical paper that describes the original R-tree design
is:</P>
<P>Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt
of Data, 45-57.</P>
<P>You can also find this paper in Stonebraker's "Readings in
Database Systems".</P>
<P>Built-in R-trees can handle polygons and boxes. In theory,
R-trees can be extended to handle higher number of dimensions. In
practice, extending R-trees requires a bit of work and we don't
currently have any documentation on how to do it.</P>
<H4><A name="4.10">4.10</A>) What is the Genetic Query
Optimizer?</H4>
<P>The <SMALL>GEQO</SMALL> 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.</P>
<H4><A name="4.11">4.11</A>) How do I perform regular expression
<H4><A name="4.8">4.8</A>) How do I perform regular expression
searches and case-insensitive regular expression searches? How do I
use an index for case-insensitive searches?</H4>
@@ -1069,13 +934,13 @@
CREATE INDEX tabindex ON tab (lower(col));
</PRE>
<H4><A name="4.12">4.12</A>) In a query, how do I detect if a field
<H4><A name="4.9">4.9</A>) In a query, how do I detect if a field
is <SMALL>NULL</SMALL>?</H4>
<P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
NOT NULL</SMALL>.</P>
<H4><A name="4.13">4.13</A>) What is the difference between the
<H4><A name="4.10">4.10</A>) What is the difference between the
various character types?</H4>
<PRE>
Type Internal Name Notes
@@ -1107,7 +972,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
particularly values that include <SMALL>NULL</SMALL> bytes. All the
types described here have similar performance characteristics.</P>
<H4><A name="4.14.1">4.14.1</A>) How do I create a
<H4><A name="4.11.1">4.11.1</A>) How do I create a
serial/auto-incrementing field?</H4>
<P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
@@ -1135,13 +1000,13 @@ BYTEA bytea variable-length byte array (null-byte safe)
you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
<H4><A name="4.14.2">4.14.2</A>) How do I get the value of a
<H4><A name="4.11.2">4.11.2</A>) How do I get the value of a
<SMALL>SERIAL</SMALL> insert?</H4>
<P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
from the sequence object with the <I>nextval()</I> function
<I>before</I> inserting and then insert it explicitly. Using the
example table in <A href="#4.14.1">4.14.1</A>, an example in a
example table in <A href="#4.11.1">4.11.1</A>, an example in a
pseudo-language would look like this:</P>
<PRE>
new_id = execute("SELECT nextval('person_id_seq')");
@@ -1164,7 +1029,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
new_id = execute("SELECT currval('person_id_seq')");
</PRE>
<P>Finally, you could use the <A href="#4.15"><SMALL>OID</SMALL></A>
<P>Finally, you could use the <A href="#4.12"><SMALL>OID</SMALL></A>
returned from the <SMALL>INSERT</SMALL> statement to look up the
default value, though this is probably the least portable approach,
and the oid value will wrap around when it reaches 4 billion.
@@ -1172,13 +1037,13 @@ BYTEA bytea variable-length byte array (null-byte safe)
available via <I>$sth-&gt;{pg_oid_status}</I> after
<I>$sth-&gt;execute()</I>.</P>
<H4><A name="4.14.3">4.14.3</A>) Doesn't <I>currval()</I>
<H4><A name="4.11.3">4.11.3</A>) Doesn't <I>currval()</I>
lead to a race condition with other users?</H4>
<P>No. <I>currval()</I> returns the current value assigned by your
backend, not by all users.</P>
<H4><A name="4.14.4">4.14.4</A>) Why aren't my sequence numbers
<H4><A name="4.11.4">4.11.4</A>) Why aren't my sequence numbers
reused on transaction abort? Why are there gaps in the numbering of
my sequence/SERIAL column?</H4>
@@ -1187,47 +1052,30 @@ BYTEA bytea variable-length byte array (null-byte safe)
completes. This causes gaps in numbering from aborted
transactions.</P>
<H4><A name="4.15">4.15</A>) What is an <SMALL>OID</SMALL>? What is
<H4><A name="4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is
a <SMALL>TID</SMALL>?</H4>
<P><SMALL>OID</SMALL>s are PostgreSQL's answer to unique row ids.
Every row that is created in PostgreSQL gets a unique
<SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during
<I>initdb</I> are less than 16384 (from
<I>include/access/transam.h</I>). All user-created
<SMALL>OID</SMALL>s are equal to or greater than this. By default,
all these <SMALL>OID</SMALL>s are unique not only within a table or
database, but unique within the entire PostgreSQL installation.</P>
<P>Every row that is created in PostgreSQL gets a unique
<SMALL>OID</SMALL> unless created <SMALL>WITHOUT OIDS</SMALL>.
O<SMALL>ID</SMALL>s are autotomatically assigned unique 4-byte
integers that are unique across the entire installation. However,
they overflow at 4 billion, and then the O<SMALL>ID</SMALL>s start
being duplicated. PostgreSQL uses <SMALL>OID</SMALL>s to link its
internal system tables together.</P>
<P>PostgreSQL uses <SMALL>OID</SMALL>s in its internal system
tables to link rows between tables. These <SMALL>OID</SMALL>s can
be used to identify specific user rows and used in joins. It is
recommended you use column type <SMALL>OID</SMALL> to store
<SMALL>OID</SMALL> values. You can create an index on the
<SMALL>OID</SMALL> field for faster access.</P>
<P>O<SMALL>ID</SMALL>s are assigned to all new rows from a central
area that is used by all databases. If you want to change the
<SMALL>OID</SMALL> to something else, or if you want to make a copy
of the table, with the original <SMALL>OID</SMALL>s, there is no
reason you can't do it:</P>
<PRE>
CREATE TABLE new_table(mycol int);
SELECT oid AS old_oid, mycol INTO tmp_table FROM old_table;
COPY tmp_table TO '/tmp/pgtable';
COPY new_table WITH OIDS FROM '/tmp/pgtable';
DROP TABLE tmp_table;
</PRE>
<P>O<SMALL>ID</SMALL>s 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.</P>
<P>To uniquely number columns in user tables, it is best to use
<SMALL>SERIAL</> rather than O<SMALL>ID</SMALL>s because
<SMALL>SERIAL<SMALL> sequences are unique only within a single
table. and are therefore less likely to overflow.
<SMALL>SERIAL8</SMALL> is available for storing eight-byte sequence
values.</P>
<P>T<SMALL>ID</SMALL>s are used to identify specific physical rows
with block and offset values. T<SMALL>ID</SMALL>s change after rows
are modified or reloaded. They are used by index entries to point
to physical rows.</P>
<H4><A name="4.16">4.16</A>) What is the meaning of some of the
<H4><A name="4.13">4.13</A>) What is the meaning of some of the
terms used in PostgreSQL?</H4>
<P>Some of the source code and older documentation use terms that
@@ -1256,7 +1104,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
<P>A list of general database terms can be found at: <A href=
"http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html">http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html</A></P>
<H4><A name="4.17">4.17</A>) Why do I get the error <I>"ERROR:
<H4><A name="4.14">4.14</A>) Why do I get the error <I>"ERROR:
Memory exhausted in AllocSetAlloc()"</I>?</H4>
<P>You probably have run out of virtual memory on your system,
@@ -1275,12 +1123,12 @@ BYTEA bytea variable-length byte array (null-byte safe)
backend is returning too much data, try it before starting the
client.
<H4><A name="4.18">4.18</A>) How do I tell what PostgreSQL version
<H4><A name="4.15">4.15</A>) How do I tell what PostgreSQL version
I am running?</H4>
<P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
<H4><A name="4.19">4.19</A>) Why does my large-object operations
<H4><A name="4.16">4.16</A>) Why does my large-object operations
get <I>"invalid large obj descriptor"</I>?</H4>
<P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
@@ -1296,7 +1144,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
<P>If you are using a client interface like <SMALL>ODBC</SMALL> you
may need to set <CODE>auto-commit off.</CODE></P>
<H4><A name="4.20">4.20</A>) How do I create a column that will
<H4><A name="4.17">4.17</A>) How do I create a column that will
default to the current time?</H4>
<P>Use <I>CURRENT_TIMESTAMP</I>:</P>
@@ -1305,31 +1153,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
</CODE>
</PRE>
<H4><A name="4.21">4.21</A>) Why are my subqueries using
<CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
<P>In versions prior to 7.4, subqueries were joined to outer queries
by sequentially scanning the result of the subquery for each row of
the outer query. If the subquery returns only a few rows and the outer
query returns many rows, <CODE><SMALL>IN</SMALL></CODE> is fastest. To
speed up other queries, replace <CODE>IN</CODE> with
<CODE>EXISTS</CODE>:</P>
<PRE> SELECT *
FROM tab
WHERE col IN (SELECT subcol FROM subtab);
</PRE>
to:
<PRE> SELECT *
FROM tab
WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
</PRE>
For this to be fast, <CODE>subcol</CODE> should be an indexed column.
<P>In version 7.4 and later, <CODE>IN</CODE> actually uses the same
sophisticated join techniques as normal queries, and is prefered
to using <CODE>EXISTS</CODE>.
<H4><A name="4.22">4.22</A>) How do I perform an outer join?</H4>
<H4><A name="4.18">4.18</A>) How do I perform an outer join?</H4>
<P>PostgreSQL supports outer joins using the SQL standard syntax.
Here are two examples:</P>
@@ -1369,7 +1193,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
ORDER BY col1
</PRE>
<H4><A name="4.23">4.23</A>) How do I perform queries using
<H4><A name="4.19">4.19</A>) How do I perform queries using
multiple databases?</H4>
<P>There is no way to query a database other than the current one.
@@ -1381,7 +1205,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
connections to different databases and merge the results on the
client side.</P>
<H4><A name="4.24">4.24</A>) How do I return multiple rows or
<H4><A name="4.20">4.20</A>) How do I return multiple rows or
columns from a function?</H4>
<P>In 7.3, you can easily return multiple rows or columns from a
@@ -1389,7 +1213,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
<a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
http://techdocs.postgresql.org/guides/SetReturningFunctions</a>.
<H4><A name="4.25">4.25</A>) Why can't I reliably create/drop
<H4><A name="4.21">4.21</A>) Why can't I reliably create/drop
temporary tables in PL/PgSQL functions?</H4>
<P>PL/PgSQL caches function contents, and an unfortunate side effect
is that if a PL/PgSQL function accesses a temporary table, and that
@@ -1399,7 +1223,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
<SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
will cause the query to be reparsed every time.</P>
<H4><A name="4.26">4.26</A>) What encryption options are available?
<H4><A name="4.22">4.22</A>) What encryption options are available?
</H4>
<UL>
<LI><I>contrib/pgcrypto</I> contains many encryption functions for