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90063a7612
We've gotten enough push-back on that change to make it clear that it wasn't an especially good idea to do it like that. Revert plain EXPLAIN to its previous behavior, but keep the extra output in EXPLAIN ANALYZE. Per discussion. Internally, I set this up as a separate flag ExplainState.summary that controls printing of planning time and execution time. For now it's just copied from the ANALYZE option, but we could consider exposing it to users.
441 lines
15 KiB
Plaintext
441 lines
15 KiB
Plaintext
<!--
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doc/src/sgml/ref/explain.sgml
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PostgreSQL documentation
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-->
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<refentry id="SQL-EXPLAIN">
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<indexterm zone="sql-explain">
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<primary>EXPLAIN</primary>
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</indexterm>
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<indexterm zone="sql-explain">
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<primary>prepared statements</primary>
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<secondary>showing the query plan</secondary>
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</indexterm>
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<indexterm zone="sql-explain">
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<primary>cursor</primary>
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<secondary>showing the query plan</secondary>
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</indexterm>
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<refmeta>
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<refentrytitle>EXPLAIN</refentrytitle>
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<manvolnum>7</manvolnum>
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<refmiscinfo>SQL - Language Statements</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>EXPLAIN</refname>
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<refpurpose>show the execution plan of a statement</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<synopsis>
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EXPLAIN [ ( <replaceable class="parameter">option</replaceable> [, ...] ) ] <replaceable class="parameter">statement</replaceable>
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EXPLAIN [ ANALYZE ] [ VERBOSE ] <replaceable class="parameter">statement</replaceable>
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<phrase>where <replaceable class="parameter">option</replaceable> can be one of:</phrase>
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ANALYZE [ <replaceable class="parameter">boolean</replaceable> ]
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VERBOSE [ <replaceable class="parameter">boolean</replaceable> ]
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COSTS [ <replaceable class="parameter">boolean</replaceable> ]
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BUFFERS [ <replaceable class="parameter">boolean</replaceable> ]
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TIMING [ <replaceable class="parameter">boolean</replaceable> ]
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FORMAT { TEXT | XML | JSON | YAML }
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</synopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>
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This command displays the execution plan that the
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<productname>PostgreSQL</productname> planner generates for the
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supplied statement. The execution plan shows how the table(s)
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referenced by the statement will be scanned — by plain sequential scan,
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index scan, etc. — and if multiple tables are referenced, what join
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algorithms will be used to bring together the required rows from
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each input table.
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</para>
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<para>
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The most critical part of the display is the estimated statement execution
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cost, which is the planner's guess at how long it will take to run the
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statement (measured in cost units that are arbitrary, but conventionally
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mean disk page fetches). Actually two numbers
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are shown: the start-up cost before the first row can be returned, and
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the total cost to return all the rows. For most queries the total cost
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is what matters, but in contexts such as a subquery in <literal>EXISTS</literal>, the planner
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will choose the smallest start-up cost instead of the smallest total cost
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(since the executor will stop after getting one row, anyway).
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Also, if you limit the number of rows to return with a <literal>LIMIT</literal> clause,
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the planner makes an appropriate interpolation between the endpoint
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costs to estimate which plan is really the cheapest.
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</para>
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<para>
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The <literal>ANALYZE</literal> option causes the statement to be actually
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executed, not only planned. Then actual run time statistics are added to
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the display, including the total elapsed time expended within each plan
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node (in milliseconds) and the total number of rows it actually returned.
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This is useful for seeing whether the planner's estimates
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are close to reality.
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</para>
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<important>
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<para>
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Keep in mind that the statement is actually executed when
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the <literal>ANALYZE</literal> option is used. Although
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<command>EXPLAIN</command> will discard any output that a
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<command>SELECT</command> would return, other side effects of the
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statement will happen as usual. If you wish to use
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<command>EXPLAIN ANALYZE</command> on an
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<command>INSERT</command>, <command>UPDATE</command>,
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<command>DELETE</command>, <command>CREATE TABLE AS</command>,
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or <command>EXECUTE</command> statement
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without letting the command affect your data, use this approach:
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<programlisting>
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BEGIN;
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EXPLAIN ANALYZE ...;
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ROLLBACK;
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</programlisting>
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</para>
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</important>
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<para>
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Only the <literal>ANALYZE</literal> and <literal>VERBOSE</literal> options
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can be specified, and only in that order, without surrounding the option
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list in parentheses. Prior to <productname>PostgreSQL</productname> 9.0,
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the unparenthesized syntax was the only one supported. It is expected that
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all new options will be supported only in the parenthesized syntax.
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</para>
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</refsect1>
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<refsect1>
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<title>Parameters</title>
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<variablelist>
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<varlistentry>
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<term><literal>ANALYZE</literal></term>
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<listitem>
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<para>
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Carry out the command and show actual run times and other statistics.
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This parameter defaults to <literal>FALSE</literal>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>VERBOSE</literal></term>
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<listitem>
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<para>
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Display additional information regarding the plan. Specifically, include
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the output column list for each node in the plan tree, schema-qualify
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table and function names, always label variables in expressions with
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their range table alias, and always print the name of each trigger for
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which statistics are displayed. This parameter defaults to
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<literal>FALSE</literal>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>COSTS</literal></term>
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<listitem>
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<para>
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Include information on the estimated startup and total cost of each
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plan node, as well as the estimated number of rows and the estimated
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width of each row.
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This parameter defaults to <literal>TRUE</literal>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>BUFFERS</literal></term>
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<listitem>
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<para>
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Include information on buffer usage. Specifically, include the number of
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shared blocks hit, read, dirtied, and written, the number of local blocks
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hit, read, dirtied, and written, and the number of temp blocks read and
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written.
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A <emphasis>hit</> means that a read was avoided because the block was
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found already in cache when needed.
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Shared blocks contain data from regular tables and indexes;
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local blocks contain data from temporary tables and indexes;
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while temp blocks contain short-term working data used in sorts, hashes,
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Materialize plan nodes, and similar cases.
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The number of blocks <emphasis>dirtied</> indicates the number of
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previously unmodified blocks that were changed by this query; while the
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number of blocks <emphasis>written</> indicates the number of
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previously-dirtied blocks evicted from cache by this backend during
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query processing.
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The number of blocks shown for an
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upper-level node includes those used by all its child nodes. In text
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format, only non-zero values are printed. This parameter may only be
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used when <literal>ANALYZE</literal> is also enabled. It defaults to
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<literal>FALSE</literal>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>TIMING</literal></term>
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<listitem>
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<para>
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Include actual startup time and time spent in each node in the output.
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The overhead of repeatedly reading the system clock can slow down the
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query significantly on some systems, so it may be useful to set this
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parameter to <literal>FALSE</literal> when only actual row counts, and
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not exact times, are needed. Run time of the entire statement is
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always measured, even when node-level timing is turned off with this
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option.
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This parameter may only be used when <literal>ANALYZE</literal> is also
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enabled. It defaults to <literal>TRUE</literal>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>FORMAT</literal></term>
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<listitem>
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<para>
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Specify the output format, which can be TEXT, XML, JSON, or YAML.
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Non-text output contains the same information as the text output
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format, but is easier for programs to parse. This parameter defaults to
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<literal>TEXT</literal>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">boolean</replaceable></term>
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<listitem>
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<para>
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Specifies whether the selected option should be turned on or off.
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You can write <literal>TRUE</literal>, <literal>ON</>, or
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<literal>1</literal> to enable the option, and <literal>FALSE</literal>,
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<literal>OFF</>, or <literal>0</literal> to disable it. The
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<replaceable class="parameter">boolean</replaceable> value can also
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be omitted, in which case <literal>TRUE</literal> is assumed.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">statement</replaceable></term>
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<listitem>
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<para>
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Any <command>SELECT</>, <command>INSERT</>, <command>UPDATE</>,
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<command>DELETE</>, <command>VALUES</>, <command>EXECUTE</>,
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<command>DECLARE</>, <command>CREATE TABLE AS</>, or
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<command>CREATE MATERIALIZED VIEW AS</> statement, whose execution
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plan you wish to see.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Outputs</title>
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<para>
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The command's result is a textual description of the plan selected
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for the <replaceable class="parameter">statement</replaceable>,
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optionally annotated with execution statistics.
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<xref linkend="using-explain"> describes the information provided.
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</para>
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</refsect1>
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<refsect1>
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<title>Notes</title>
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<para>
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In order to allow the <productname>PostgreSQL</productname> query
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planner to make reasonably informed decisions when optimizing
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queries, the <link
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linkend="catalog-pg-statistic"><structname>pg_statistic</structname></link>
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data should be up-to-date for all tables used in the query. Normally
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the <link linkend="autovacuum">autovacuum daemon</link> will take care
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of that automatically. But if a table has recently had substantial
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changes in its contents, you might need to do a manual
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<xref linkend="sql-analyze"> rather than wait for autovacuum to catch up
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with the changes.
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</para>
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<para>
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In order to measure the run-time cost of each node in the execution
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plan, the current implementation of <command>EXPLAIN
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ANALYZE</command> adds profiling overhead to query execution.
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As a result, running <command>EXPLAIN ANALYZE</command>
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on a query can sometimes take significantly longer than executing
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the query normally. The amount of overhead depends on the nature of
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the query, as well as the platform being used. The worst case occurs
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for plan nodes that in themselves require very little time per
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execution, and on machines that have relatively slow operating
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system calls for obtaining the time of day.
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</para>
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</refsect1>
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<refsect1>
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<title>Examples</title>
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<para>
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To show the plan for a simple query on a table with a single
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<type>integer</type> column and 10000 rows:
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<programlisting>
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EXPLAIN SELECT * FROM foo;
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QUERY PLAN
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---------------------------------------------------------
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Seq Scan on foo (cost=0.00..155.00 rows=10000 width=4)
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(1 row)
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</programlisting>
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</para>
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<para>
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Here is the same query, with JSON output formatting:
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<programlisting>
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EXPLAIN (FORMAT JSON) SELECT * FROM foo;
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QUERY PLAN
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--------------------------------
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[ +
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{ +
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"Plan": { +
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"Node Type": "Seq Scan",+
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"Relation Name": "foo", +
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"Alias": "foo", +
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"Startup Cost": 0.00, +
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"Total Cost": 155.00, +
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"Plan Rows": 10000, +
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"Plan Width": 4 +
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} +
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} +
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]
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(1 row)
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</programlisting>
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</para>
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<para>
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If there is an index and we use a query with an indexable
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<literal>WHERE</literal> condition, <command>EXPLAIN</command>
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might show a different plan:
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<programlisting>
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EXPLAIN SELECT * FROM foo WHERE i = 4;
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QUERY PLAN
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--------------------------------------------------------------
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Index Scan using fi on foo (cost=0.00..5.98 rows=1 width=4)
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Index Cond: (i = 4)
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(2 rows)
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</programlisting>
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</para>
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<para>
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Here is the same query, but in YAML format:
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<programlisting>
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EXPLAIN (FORMAT YAML) SELECT * FROM foo WHERE i='4';
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QUERY PLAN
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-------------------------------
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- Plan: +
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Node Type: "Index Scan" +
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Scan Direction: "Forward"+
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Index Name: "fi" +
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Relation Name: "foo" +
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Alias: "foo" +
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Startup Cost: 0.00 +
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Total Cost: 5.98 +
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Plan Rows: 1 +
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Plan Width: 4 +
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Index Cond: "(i = 4)"
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(1 row)
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</programlisting>
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XML format is left as an exercise for the reader.
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</para>
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<para>
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Here is the same plan with cost estimates suppressed:
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<programlisting>
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EXPLAIN (COSTS FALSE) SELECT * FROM foo WHERE i = 4;
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QUERY PLAN
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----------------------------
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Index Scan using fi on foo
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Index Cond: (i = 4)
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(2 rows)
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</programlisting>
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</para>
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<para>
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Here is an example of a query plan for a query using an aggregate
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function:
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<programlisting>
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EXPLAIN SELECT sum(i) FROM foo WHERE i < 10;
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QUERY PLAN
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---------------------------------------------------------------------
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Aggregate (cost=23.93..23.93 rows=1 width=4)
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-> Index Scan using fi on foo (cost=0.00..23.92 rows=6 width=4)
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Index Cond: (i < 10)
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(3 rows)
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</programlisting>
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</para>
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<para>
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Here is an example of using <command>EXPLAIN EXECUTE</command> to
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display the execution plan for a prepared query:
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<programlisting>
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PREPARE query(int, int) AS SELECT sum(bar) FROM test
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WHERE id > $1 AND id < $2
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GROUP BY foo;
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EXPLAIN ANALYZE EXECUTE query(100, 200);
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QUERY PLAN
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------------------------------------------------------------------------------------------------------------------------
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HashAggregate (cost=9.54..9.54 rows=1 width=8) (actual time=0.156..0.161 rows=11 loops=1)
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Group Key: foo
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-> Index Scan using test_pkey on test (cost=0.29..9.29 rows=50 width=8) (actual time=0.039..0.091 rows=99 loops=1)
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Index Cond: ((id > $1) AND (id < $2))
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Planning time: 0.197 ms
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Execution time: 0.225 ms
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(6 rows)
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</programlisting>
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</para>
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<para>
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Of course, the specific numbers shown here depend on the actual
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contents of the tables involved. Also note that the numbers, and
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even the selected query strategy, might vary between
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<productname>PostgreSQL</productname> releases due to planner
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improvements. In addition, the <command>ANALYZE</command> command
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uses random sampling to estimate data statistics; therefore, it is
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possible for cost estimates to change after a fresh run of
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<command>ANALYZE</command>, even if the actual distribution of data
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in the table has not changed.
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</para>
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</refsect1>
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<refsect1>
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<title>Compatibility</title>
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<para>
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There is no <command>EXPLAIN</command> statement defined in the SQL standard.
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</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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|
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<simplelist type="inline">
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<member><xref linkend="sql-analyze"></member>
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</simplelist>
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</refsect1>
|
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</refentry>
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