<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML
><HEAD
><TITLE
>EXPLAIN</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
REV="MADE"
HREF="mailto:pgsql-docs@postgresql.org"><LINK
REL="HOME"
TITLE="PostgreSQL 8.5alpha3 Documentation"
HREF="index.html"><LINK
REL="UP"
TITLE="SQL Commands"
HREF="sql-commands.html"><LINK
REL="PREVIOUS"
TITLE="EXECUTE"
HREF="sql-execute.html"><LINK
REL="NEXT"
TITLE="FETCH"
HREF="sql-fetch.html"><LINK
REL="STYLESHEET"
TYPE="text/css"
HREF="stylesheet.css"><META
HTTP-EQUIV="Content-Type"
CONTENT="text/html; charset=ISO-8859-1"><META
NAME="creation"
CONTENT="2010-04-15T09:31:23"></HEAD
><BODY
CLASS="REFENTRY"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="5"
ALIGN="center"
VALIGN="bottom"
><A
HREF="index.html"
>PostgreSQL 8.5alpha3 Documentation</A
></TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
TITLE="EXECUTE"
HREF="sql-execute.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
TITLE="EXECUTE"
HREF="sql-execute.html"
>Fast Backward</A
></TD
><TD
WIDTH="60%"
ALIGN="center"
VALIGN="bottom"
></TD
><TD
WIDTH="10%"
ALIGN="right"
VALIGN="top"
><A
TITLE="FETCH"
HREF="sql-fetch.html"
>Fast Forward</A
></TD
><TD
WIDTH="10%"
ALIGN="right"
VALIGN="top"
><A
TITLE="FETCH"
HREF="sql-fetch.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><H1
><A
NAME="SQL-EXPLAIN"
></A
>EXPLAIN</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN64219"
></A
><H2
>Name</H2
>EXPLAIN -- show the execution plan of a statement</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN64230"
></A
><H2
>Synopsis</H2
><PRE
CLASS="SYNOPSIS"
>EXPLAIN [ ( { ANALYZE <TT
CLASS="REPLACEABLE"
><I
>boolean</I
></TT
> | VERBOSE <TT
CLASS="REPLACEABLE"
><I
>boolean</I
></TT
> | COSTS <TT
CLASS="REPLACEABLE"
><I
>boolean</I
></TT
> | BUFFERS <TT
CLASS="REPLACEABLE"
><I
>boolean</I
></TT
> | FORMAT { TEXT | XML | JSON | YAML } } [, ...] ) ] <TT
CLASS="REPLACEABLE"
><I
>statement</I
></TT
>
EXPLAIN [ ANALYZE ] [ VERBOSE ] <TT
CLASS="REPLACEABLE"
><I
>statement</I
></TT
></PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN64238"
></A
><H2
>Description</H2
><P
> This command displays the execution plan that the
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> planner generates for the
supplied statement. The execution plan shows how the table(s)
referenced by the statement will be scanned — by plain sequential scan,
index scan, etc. — and if multiple tables are referenced, what join
algorithms will be used to bring together the required rows from
each input table.
</P
><P
> The most critical part of the display is the estimated statement execution
cost, which is the planner's guess at how long it will take to run the
statement (measured in units of disk page fetches). Actually two numbers
are shown: the start-up time before the first row can be returned, and
the total time to return all the rows. For most queries the total time
is what matters, but in contexts such as a subquery in <TT
CLASS="LITERAL"
>EXISTS</TT
>, the planner
will choose the smallest start-up time instead of the smallest total time
(since the executor will stop after getting one row, anyway).
Also, if you limit the number of rows to return with a <TT
CLASS="LITERAL"
>LIMIT</TT
> clause,
the planner makes an appropriate interpolation between the endpoint
costs to estimate which plan is really the cheapest.
</P
><P
> The <TT
CLASS="LITERAL"
>ANALYZE</TT
> option causes the statement to be actually executed, not only
planned. The total elapsed time expended within each plan node (in
milliseconds) and total number of rows it actually returned are added to
the display. This is useful for seeing whether the planner's estimates
are close to reality.
</P
><DIV
CLASS="IMPORTANT"
><BLOCKQUOTE
CLASS="IMPORTANT"
><P
><B
>Important: </B
> Keep in mind that the statement is actually executed when
the <TT
CLASS="LITERAL"
>ANALYZE</TT
> option is used. Although
<TT
CLASS="COMMAND"
>EXPLAIN</TT
> will discard any output that a
<TT
CLASS="COMMAND"
>SELECT</TT
> would return, other side effects of the
statement will happen as usual. If you wish to use
<TT
CLASS="COMMAND"
>EXPLAIN ANALYZE</TT
> on an
<TT
CLASS="COMMAND"
>INSERT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
>,
<TT
CLASS="COMMAND"
>DELETE</TT
>, <TT
CLASS="COMMAND"
>CREATE TABLE AS</TT
>,
or <TT
CLASS="COMMAND"
>EXECUTE</TT
> statement
without letting the command affect your data, use this approach:
</P><PRE
CLASS="PROGRAMLISTING"
>BEGIN;
EXPLAIN ANALYZE ...;
ROLLBACK;</PRE
><P>
</P
></BLOCKQUOTE
></DIV
><P
> Only the <TT
CLASS="LITERAL"
>ANALYZE</TT
> and <TT
CLASS="LITERAL"
>VERBOSE</TT
> options
can be specified, and only in that order, without surrounding the option
list in parentheses. Prior to <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 8.5,
the unparenthesized syntax was the only one supported. It is expected that
all new options will be supported only in the parenthesized syntax.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN64263"
></A
><H2
>Parameters</H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="LITERAL"
>ANALYZE</TT
></DT
><DD
><P
> Carry out the command and show the actual run times. This
parameter defaults to <TT
CLASS="LITERAL"
>FALSE</TT
>.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>VERBOSE</TT
></DT
><DD
><P
> Display additional information regarding the plan. Specifically, include
the output column list for each node in the plan tree, schema-qualify
table and function names, always label variables in expressions with
their range table alias, and always print the name of each trigger for
which statistics are displayed. This parameter defaults to
<TT
CLASS="LITERAL"
>FALSE</TT
>.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>COSTS</TT
></DT
><DD
><P
> Include information on the estimated startup and total cost of each
plan node, as well as the estimated number of rows and the estimated
width of each row. This parameter defaults to <TT
CLASS="LITERAL"
>TRUE</TT
>.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>BUFFERS</TT
></DT
><DD
><P
> Include information on buffer usage. Specifically, include the number of
shared blocks hits, reads, and writes, the number of local blocks hits,
reads, and writes, and the number of temp blocks reads and writes.
Shared blocks, local blocks, and temp blocks contain tables and indexes,
temporary tables and temporary indexes, and disk blocks used in sort and
materialized plans, respectively. The number of blocks shown for an
upper-level node includes those used by all its child nodes. In text
format, only non-zero values are printed. This parameter may only be
used with <TT
CLASS="LITERAL"
>ANALYZE</TT
> parameter. It defaults to
<TT
CLASS="LITERAL"
>FALSE</TT
>.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>FORMAT</TT
></DT
><DD
><P
> Specify the output format, which can be TEXT, XML, JSON, or YAML.
Non-text output contains the same information as the text output
format, but is easier for programs to parse. This parameter defaults to
<TT
CLASS="LITERAL"
>TEXT</TT
>.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>boolean</I
></TT
></DT
><DD
><P
> Specifies whether the selected option should be turned on or off.
You can write <TT
CLASS="LITERAL"
>TRUE</TT
>, <TT
CLASS="LITERAL"
>ON</TT
>, or
<TT
CLASS="LITERAL"
>1</TT
> to enable the option, and <TT
CLASS="LITERAL"
>FALSE</TT
>,
<TT
CLASS="LITERAL"
>OFF</TT
>, or <TT
CLASS="LITERAL"
>0</TT
> to disable it. The
<TT
CLASS="REPLACEABLE"
><I
>boolean</I
></TT
> value can also
be omitted, in which case <TT
CLASS="LITERAL"
>TRUE</TT
> is assumed.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>statement</I
></TT
></DT
><DD
><P
> Any <TT
CLASS="COMMAND"
>SELECT</TT
>, <TT
CLASS="COMMAND"
>INSERT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
>,
<TT
CLASS="COMMAND"
>DELETE</TT
>, <TT
CLASS="COMMAND"
>VALUES</TT
>, <TT
CLASS="COMMAND"
>EXECUTE</TT
>,
<TT
CLASS="COMMAND"
>DECLARE</TT
>, or <TT
CLASS="COMMAND"
>CREATE TABLE AS</TT
>
statement, whose execution plan you wish to see.
</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN64323"
></A
><H2
>Notes</H2
><P
> There is only sparse documentation on the optimizer's use of cost
information in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>. Refer to
<A
HREF="using-explain.html"
>Section 14.1</A
> for more information.
</P
><P
> In order to allow the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> query
planner to make reasonably informed decisions when optimizing
queries, the <A
HREF="sql-analyze.html"
><I
>ANALYZE</I
></A
>
statement should be run to record statistics about the distribution
of data within the table. If you have not done this (or if the
statistical distribution of the data in the table has changed
significantly since the last time <TT
CLASS="COMMAND"
>ANALYZE</TT
> was
run), the estimated costs are unlikely to conform to the real
properties of the query, and consequently an inferior query plan
might be chosen.
</P
><P
> In order to measure the run-time cost of each node in the execution
plan, the current implementation of <TT
CLASS="COMMAND"
>EXPLAIN
ANALYZE</TT
> can add considerable profiling overhead to query
execution. As a result, running <TT
CLASS="COMMAND"
>EXPLAIN ANALYZE</TT
>
on a query can sometimes take significantly longer than executing
the query normally. The amount of overhead depends on the nature of
the query.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN64335"
></A
><H2
>Examples</H2
><P
> To show the plan for a simple query on a table with a single
<TT
CLASS="TYPE"
>integer</TT
> column and 10000 rows:
</P><PRE
CLASS="PROGRAMLISTING"
>EXPLAIN SELECT * FROM foo;
QUERY PLAN
---------------------------------------------------------
Seq Scan on foo (cost=0.00..155.00 rows=10000 width=4)
(1 row)</PRE
><P>
</P
><P
> If there is an index and we use a query with an indexable
<TT
CLASS="LITERAL"
>WHERE</TT
> condition, <TT
CLASS="COMMAND"
>EXPLAIN</TT
>
might show a different plan:
</P><PRE
CLASS="PROGRAMLISTING"
>EXPLAIN SELECT * FROM foo WHERE i = 4;
QUERY PLAN
--------------------------------------------------------------
Index Scan using fi on foo (cost=0.00..5.98 rows=1 width=4)
Index Cond: (i = 4)
(2 rows)</PRE
><P>
</P
><P
> Here is the same plan with costs suppressed:
</P><PRE
CLASS="PROGRAMLISTING"
>EXPLAIN (COSTS FALSE) SELECT * FROM foo WHERE i = 4;
QUERY PLAN
----------------------------
Index Scan using fi on foo
Index Cond: (i = 4)
(2 rows)</PRE
><P>
</P
><P
> Here is an example of a query plan for a query using an aggregate
function:
</P><PRE
CLASS="PROGRAMLISTING"
>EXPLAIN SELECT sum(i) FROM foo WHERE i < 10;
QUERY PLAN
---------------------------------------------------------------------
Aggregate (cost=23.93..23.93 rows=1 width=4)
-> Index Scan using fi on foo (cost=0.00..23.92 rows=6 width=4)
Index Cond: (i < 10)
(3 rows)</PRE
><P>
</P
><P
> Here is an example of using <TT
CLASS="COMMAND"
>EXPLAIN EXECUTE</TT
> to
display the execution plan for a prepared query:
</P><PRE
CLASS="PROGRAMLISTING"
>PREPARE query(int, int) AS SELECT sum(bar) FROM test
WHERE id > $1 AND id < $2
GROUP BY foo;
EXPLAIN ANALYZE EXECUTE query(100, 200);
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
HashAggregate (cost=39.53..39.53 rows=1 width=8) (actual time=0.661..0.672 rows=7 loops=1)
-> Index Scan using test_pkey on test (cost=0.00..32.97 rows=1311 width=8) (actual time=0.050..0.395 rows=99 loops=1)
Index Cond: ((id > $1) AND (id < $2))
Total runtime: 0.851 ms
(4 rows)</PRE
><P>
</P
><P
> Of course, the specific numbers shown here depend on the actual
contents of the tables involved. Also note that the numbers, and
even the selected query strategy, might vary between
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> releases due to planner
improvements. In addition, the <TT
CLASS="COMMAND"
>ANALYZE</TT
> command
uses random sampling to estimate data statistics; therefore, it is
possible for cost estimates to change after a fresh run of
<TT
CLASS="COMMAND"
>ANALYZE</TT
>, even if the actual distribution of data
in the table has not changed.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN64355"
></A
><H2
>Compatibility</H2
><P
> There is no <TT
CLASS="COMMAND"
>EXPLAIN</TT
> statement defined in the SQL standard.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN64359"
></A
><H2
>See Also</H2
><A
HREF="sql-analyze.html"
><I
>ANALYZE</I
></A
></DIV
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="sql-execute.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="sql-fetch.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>EXECUTE</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="sql-commands.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>FETCH</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>
