<!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 8.0.11 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="2007-02-02T03:57:22"></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" >PostgreSQL 8.0.11 Documentation</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="sql-execute.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="sql-execute.html" >Fast Backward</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="top" ><A HREF="sql-fetch.html" >Fast Forward</A ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="top" ><A 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="AEN43048" ></A ><H2 >Name</H2 >EXPLAIN -- show the execution plan of a statement</DIV ><A NAME="AEN43051" ></A ><A NAME="AEN43053" ></A ><A NAME="AEN43056" ></A ><DIV CLASS="REFSYNOPSISDIV" ><A NAME="AEN43059" ></A ><H2 >Synopsis</H2 ><PRE CLASS="SYNOPSIS" >EXPLAIN [ ANALYZE ] [ VERBOSE ] <TT CLASS="REPLACEABLE" ><I >statement</I ></TT ></PRE ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN43062" ></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 <TT CLASS="LITERAL" >ANALYZE</TT > 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="AEN43082" ></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.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" >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="AEN43108" ></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="performance-tips.html#USING-EXPLAIN" >Section 13.1</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 <TT CLASS="COMMAND" >ANALYZE</TT > 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 may be chosen. </P ><P > Prior to <SPAN CLASS="PRODUCTNAME" >PostgreSQL</SPAN > 7.3, the plan was emitted in the form of a <TT CLASS="LITERAL" >NOTICE</TT > message. Now it appears as a query result (formatted like a table with a single text column). </P ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN43120" ></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 > And 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, may 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="AEN43138" ></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="AEN43142" ></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 >