Frequently Asked Questions (FAQ) for PostgreSQL
- Last updated: Sat Jan 29 23:06:02 EST 2005
+ Last updated: Sat Jan 29 23:15:42 EST 2005
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
typical text file?
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) What is an R-tree index?
- 4.9) What is the Genetic Query Optimizer?
- 4.10) How do I perform regular expression searches and
- case-insensitive regular expression searches? How do I use an index
- for case-insensitive searches?
- 4.11) In a query, how do I detect if a field is NULL?
- 4.12) What is the difference between the various character types?
- 4.13.0) How do I create a serial/auto-incrementing field?
- 4.13.1) How do I get the value of a SERIAL insert?
- 4.13.2) Doesn't currval() lead to a race condition with other users?
- 4.13.3) Why aren't my sequence numbers reused on transaction abort?
+ 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?
- 4.14) What is an OID? What is a TID?
- 4.15) What is the meaning of some of the terms used in PostgreSQL?
- 4.16) 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()"?
- 4.17) How do I tell what PostgreSQL version I am running?
- 4.18) 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"?
- 4.19) How do I create a column that will default to the current time?
- 4.20) Why are my subqueries using IN so slow?
- 4.21) How do I perform an outer join?
- 4.22) How do I perform queries using multiple databases?
- 4.23) How do I return multiple rows or columns from a function?
- 4.24) 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) Why are my subqueries using IN so slow?
+ 4.19) How do I perform an outer join?
+ 4.20) How do I perform queries using multiple databases?
+ 4.21) How do I return multiple rows or columns from a function?
+ 4.22) Why can't I reliably create/drop temporary tables in PL/PgSQL
functions?
- 4.25) What encryption options are available?
+ 4.23) What encryption options are available?
Extending PostgreSQL
See the EXPLAIN manual page.
- 4.8) 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.
-
- 4.9) 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.
-
- 4.10) 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?
functional index, it will be used:
CREATE INDEX tabindex ON tab (lower(col));
- 4.11) 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.
- 4.12) What is the difference between the various character types?
+ 4.10) What is the difference between the various character types?
Type Internal Name Notes
--------------------------------------------------
particularly values that include NULL bytes. All the types described
here have similar performance characteristics.
- 4.13.1) 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:
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.
- 4.13.2) 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 4.13.1, 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')");
billion. In Perl, using DBI with the DBD::Pg module, the oid value is
made available via $sth->{pg_oid_status} after $sth->execute().
- 4.13.3) 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.
- 4.13.4) 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.
- 4.14) What is an OID? What is a TID?
+ 4.12) What is an OID? What is a TID?
Every row that is created in PostgreSQL gets a unique OID unless
created WITHOUT OIDS. OIDs are autotomatically assigned unique 4-byte
values. TIDs change after rows are modified or reloaded. They are used
by index entries to point to physical rows.
- 4.15) 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:
http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary
/glossary.html
- 4.16) 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
problem with the SQL client because the backend is returning too much
data, try it before starting the client.
- 4.17) 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();
- 4.18) 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
If you are using a client interface like ODBC you may need to set
auto-commit off.
- 4.19) 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 );
- 4.20) Why are my subqueries using IN so slow?
+ 4.18) 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
In version 7.4 and later, IN actually uses the same sophisticated join
techniques as normal queries, and is prefered to using EXISTS.
- 4.21) How do I perform an outer join?
+ 4.19) How do I perform an outer join?
PostgreSQL supports outer joins using the SQL standard syntax. Here
are two examples:
WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
ORDER BY col1
- 4.22) How do I perform queries using multiple databases?
+ 4.20) 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
course, a client can make simultaneous connections to different
databases and merge the results on the client side.
- 4.23) How do I return multiple rows or columns from a function?
+ 4.21) 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.
- 4.24) Why can't I reliably create/drop temporary tables in PL/PgSQL
+ 4.22) Why can't I reliably create/drop temporary tables in PL/PgSQL
functions?
PL/PgSQL caches function contents, and an unfortunate side effect is
table access in PL/PgSQL. This will cause the query to be reparsed
every time.
- 4.25) What encryption options are available?
+ 4.23) What encryption options are available?
* contrib/pgcrypto contains many encryption functions for use in SQL
queries.
alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
- <P>Last updated: Sat Jan 29 23:06:02 EST 2005</P>
+ <P>Last updated: Sat Jan 29 23:15:42 EST 2005</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
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="#4.8">4.8</A>) What is an R-tree index?<BR>
- <A href="#4.9">4.9</A>) What is the Genetic Query Optimizer?<BR>
- <A href="#4.10">4.10</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="#4.11">4.11</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="#4.12">4.12</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="#4.13.0">4.13.0</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="#4.13.1">4.13.1</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="#4.13.2">4.13.2</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="#4.13.3">4.13.3</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="#4.14">4.14</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="#4.15">4.15</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="#4.16">4.16</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="#4.17">4.17</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="#4.18">4.18</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="#4.19">4.19</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="#4.20">4.20</A>) Why are my subqueries using
+ <A href="#4.18">4.18</A>) Why are my subqueries using
<CODE><SMALL>IN</SMALL></CODE> so slow?<BR>
- <A href="#4.21">4.21</A>) How do I perform an outer join?<BR>
- <A href="#4.22">4.22</A>) How do I perform queries using multiple
+ <A href="#4.19">4.19</A>) How do I perform an outer join?<BR>
+ <A href="#4.20">4.20</A>) How do I perform queries using multiple
databases?<BR>
- <A href="#4.23">4.23</A>) How do I return multiple rows or columns
+ <A href="#4.21">4.21</A>) How do I return multiple rows or columns
from a function?<BR>
- <A href="#4.24">4.24</A>) Why can't I reliably create/drop
+ <A href="#4.22">4.22</A>) Why can't I reliably create/drop
temporary tables in PL/PgSQL functions?<BR>
- <A href="#4.25">4.25</A>) What encryption options are available?<BR>
+ <A href="#4.23">4.23</A>) What encryption options are available?<BR>
<H2 align="center">Extending PostgreSQL</H2>
<P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
- <H4><A name="4.8">4.8</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="4.9">4.9</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="4.10">4.10</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>
CREATE INDEX tabindex ON tab (lower(col));
</PRE>
- <H4><A name="4.11">4.11</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="4.12">4.12</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
particularly values that include <SMALL>NULL</SMALL> bytes. All the
types described here have similar performance characteristics.</P>
- <H4><A name="4.13.1">4.13.1</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
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="4.13.2">4.13.2</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="#4.13.1">4.13.1</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')");
new_id = execute("SELECT currval('person_id_seq')");
</PRE>
- <P>Finally, you could use the <A href="#4.14"><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.
available via <I>$sth->{pg_oid_status}</I> after
<I>$sth->execute()</I>.</P>
- <H4><A name="4.13.3">4.13.3</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="4.13.4">4.13.4</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>
completes. This causes gaps in numbering from aborted
transactions.</P>
- <H4><A name="4.14">4.14</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>Every row that is created in PostgreSQL gets a unique
are modified or reloaded. They are used by index entries to point
to physical rows.</P>
- <H4><A name="4.15">4.15</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
<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="4.16">4.16</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,
backend is returning too much data, try it before starting the
client.
- <H4><A name="4.17">4.17</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="4.18">4.18</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>
<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="4.19">4.19</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>
</CODE>
</PRE>
- <H4><A name="4.20">4.20</A>) Why are my subqueries using
+ <H4><A name="4.18">4.18</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
sophisticated join techniques as normal queries, and is prefered
to using <CODE>EXISTS</CODE>.
- <H4><A name="4.21">4.21</A>) How do I perform an outer join?</H4>
+ <H4><A name="4.19">4.19</A>) How do I perform an outer join?</H4>
<P>PostgreSQL supports outer joins using the SQL standard syntax.
Here are two examples:</P>
ORDER BY col1
</PRE>
- <H4><A name="4.22">4.22</A>) How do I perform queries using
+ <H4><A name="4.20">4.20</A>) How do I perform queries using
multiple databases?</H4>
<P>There is no way to query a database other than the current one.
connections to different databases and merge the results on the
client side.</P>
- <H4><A name="4.23">4.23</A>) How do I return multiple rows or
+ <H4><A name="4.21">4.21</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
<a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
http://techdocs.postgresql.org/guides/SetReturningFunctions</a>.
- <H4><A name="4.24">4.24</A>) Why can't I reliably create/drop
+ <H4><A name="4.22">4.22</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
<SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
will cause the query to be reparsed every time.</P>
- <H4><A name="4.25">4.25</A>) What encryption options are available?
+ <H4><A name="4.23">4.23</A>) What encryption options are available?
</H4>
<UL>
<LI><I>contrib/pgcrypto</I> contains many encryption functions for
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