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>EXPLAIN</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN57603"
></A
><H2
>Name</H2
>EXPLAIN -- show the execution plan of a statement</DIV
><A
NAME="AEN57606"
></A
><A
NAME="AEN57608"
></A
><A
NAME="AEN57611"
></A
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN57614"
></A
><H2
>Synopsis</H2
><PRE
CLASS="SYNOPSIS"
>EXPLAIN [ ANALYZE ] [ VERBOSE ] <TT
CLASS="REPLACEABLE"
><I
>statement</I
></TT
></PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN57617"
></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
>, 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
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN57637"
></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.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>VERBOSE</TT
></DT
><DD
><P
> Show the full internal representation of the plan tree, rather
than just a summary. Usually this option is only useful for
specialized debugging purposes. The
<TT
CLASS="LITERAL"
>VERBOSE</TT
> output is either pretty-printed or
not, depending on the setting of the <A
HREF="runtime-config-client.html#GUC-EXPLAIN-PRETTY-PRINT"
>explain_pretty_print</A
> configuration parameter.
</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
>, or
<TT
CLASS="COMMAND"
>DECLARE</TT
> statement, whose execution plan you wish to see.
</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN57664"
></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
> Genetic query optimization (<ACRONYM
CLASS="ACRONYM"
>GEQO</ACRONYM
>) randomly tests
execution plans. Therefore, when the number of join relations
exceeds <A
HREF="runtime-config-query.html#GUC-GEQO-THRESHOLD"
>geqo_threshold</A
> causing genetic query
optimization to be used, the execution plan is likely to change
each time the statement is executed.
</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="AEN57679"
></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 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="AEN57697"
></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="AEN57701"
></A
><H2
>See Also</H2
><A
HREF="sql-analyze.html"
><I
>ANALYZE</I
></A
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