explain.7

PostgreSQL 8.2中增加了很多企业用户所需要的功能和性能上的提高,其开发团队说,该版本将加速更多企业向该数据库移植.核心开发成员之一Bruce Momjian表示,在新版PostgreSQL

.\\" auto-generated by docbook2man-spec $Revision: 1.1.1.1 $
.TH "EXPLAIN" "" "2008-01-03" "SQL - Language Statements" "SQL Commands"
.SH NAME
EXPLAIN \- show the execution plan of a statement

.SH SYNOPSIS
.sp
.nf
EXPLAIN [ ANALYZE ] [ VERBOSE ] \fIstatement\fR
.sp
.fi
.SH "DESCRIPTION"
.PP
This command displays the execution plan that the
PostgreSQL planner generates for the
supplied statement. The execution plan shows how the table(s)
referenced by the statement will be scanned \(em by plain sequential scan,
index scan, etc. \(em and if multiple tables are referenced, what join
algorithms will be used to bring together the required rows from
each input table.
.PP
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 EXISTS, 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 LIMIT clause,
the planner makes an appropriate interpolation between the endpoint
costs to estimate which plan is really the cheapest.
.PP
The ANALYZE 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.
.sp
.RS
.B "Important:"
Keep in mind that the statement is actually executed when
ANALYZE is used. Although
\fBEXPLAIN\fR will discard any output that a
\fBSELECT\fR would return, other side effects of the
statement will happen as usual. If you wish to use
\fBEXPLAIN ANALYZE\fR on an
\fBINSERT\fR, \fBUPDATE\fR,
\fBDELETE\fR, or \fBEXECUTE\fR statement
without letting the command affect your data, use this approach:
.sp
.nf
BEGIN;
EXPLAIN ANALYZE ...;
ROLLBACK;
.sp
.fi
.RE
.sp
.SH "PARAMETERS"
.TP
\fBANALYZE\fR
Carry out the command and show the actual run times.
.TP
\fBVERBOSE\fR
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
VERBOSE output is either pretty-printed or
not, depending on the setting of the explain_pretty_print configuration parameter.
.TP
\fB\fIstatement\fB\fR
Any \fBSELECT\fR, \fBINSERT\fR, \fBUPDATE\fR,
\fBDELETE\fR, \fBVALUES\fR, \fBEXECUTE\fR, or
\fBDECLARE\fR statement, whose execution plan you wish to see.
.SH "NOTES"
.PP
There is only sparse documentation on the optimizer's use of cost
information in PostgreSQL. Refer to
in the documentation for more information.
.PP
In order to allow the PostgreSQL query
planner to make reasonably informed decisions when optimizing
queries, the \fBANALYZE\fR 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 \fBANALYZE\fR 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.
.PP
Genetic query optimization (GEQO) randomly 
tests execution plans. Therefore, when the number of tables exceeds
geqo_threshold causing genetic query optimization to be 
used, the execution plan is likely to change each time the statement
is executed.
.SH "EXAMPLES"
.PP
To show the plan for a simple query on a table with a single
\fBinteger\fR column and 10000 rows:
.sp
.nf
EXPLAIN SELECT * FROM foo;

                       QUERY PLAN
---------------------------------------------------------
 Seq Scan on foo  (cost=0.00..155.00 rows=10000 width=4)
(1 row)
.sp
.fi
.PP
If there is an index and we use a query with an indexable
WHERE condition, \fBEXPLAIN\fR
might show a different plan:
.sp
.nf
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)
.sp
.fi
.PP
And here is an example of a query plan for a query
using an aggregate function:
.sp
.nf
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)
.sp
.fi
.PP
Here is an example of using \fBEXPLAIN EXECUTE\fR to
display the execution plan for a prepared query:
.sp
.nf
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)
.sp
.fi
.PP
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
PostgreSQL releases due to planner
improvements. In addition, the \fBANALYZE\fR command
uses random sampling to estimate data statistics; therefore, it is
possible for cost estimates to change after a fresh run of
\fBANALYZE\fR, even if the actual distribution of data
in the table has not changed.
.SH "COMPATIBILITY"
.PP
There is no \fBEXPLAIN\fR statement defined in the SQL standard.
.SH "SEE ALSO"
ANALYZE [\fBanalyze\fR(7)]