mirror/postgresql
2
0
Fork 0
mirror of https://git.postgresql.org/git/postgresql.git synced 2025-01-06 15:24:56 +08:00
Code Issues Packages Projects Releases Wiki Activity

Update EXPLAIN documentation to reflect the fact that the

Browse Source 
planner now produces two cost numbers instead of one.
This commit is contained in:
Tom Lane 2000-02-15 23:37:49 +00:00
parent 47dde30222
commit 9c80cceb02
1 changed files with 37 additions and 12 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

49
doc/src/sgml/ref/explain.sgml
View File

@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/explain.sgml,v 1.8 1999/07/22 15:09:12 thomas Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/ref/explain.sgml,v 1.9 2000/02/15 23:37:49 tgl Exp $
Postgres documentation
-->
@ -15,7 +15,7 @@ Postgres documentation
EXPLAIN
</refname>
<refpurpose>
Shows statement execution details
Shows statement execution plan
</refpurpose>
</refnamediv>
@ -102,12 +102,32 @@ EXPLAIN
</title>
<para>
This command outputs details about the supplied query.
The default output is the computed query cost.
The cost value is only meaningful to the optimizer in comparing
various query plans.
VERBOSE displays the full query plan and cost to your screen,
and pretty-prints the plan to the postmaster log file.
This command displays the execution plan that the Postgres planner
generates for the supplied query. The execution plan shows how
the table(s) referenced by the query 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 tuples from each input table.
</para>
<para>
The most critical part of the display is the estimated query execution
cost, which is the planner's guess at how long it will take to run the
query (measured in units of disk page fetches). Actually two numbers
are shown: the startup time before the first tuple can be returned, and
the total time to return all the tuples. For most queries the total time
is what matters, but in contexts such as an EXISTS sub-query the planner
will choose the smallest startup time instead of the smallest total time
(since the executor will stop after getting one tuple, anyway).
Also, if you limit the number of tuples to return with a LIMIT clause,
the planner makes an appropriate interpolation between the endpoint
costs to estimate which plan is really the cheapest.
</para>
<para>
The VERBOSE option emits the full internal representation of the plan tree,
rather than just a summary (and sends it to the postmaster log file, too).
Usually this option is only useful for debugging Postgres.
</para>
<refsect2 id="R2-SQL-EXPLAIN-3">
@ -143,7 +163,7 @@ EXPLAIN SELECT * FROM foo;
<computeroutput>
NOTICE: QUERY PLAN:
Seq Scan on foo (cost=5.22 rows=128 width=4)
Seq Scan on foo (cost=0.00..2.28 rows=128 width=4)
EXPLAIN
</computeroutput>
@ -160,7 +180,7 @@ EXPLAIN SELECT * FROM foo WHERE i = 4;
<computeroutput>
NOTICE: QUERY PLAN:
Index Scan using fi on foo (cost=2.05 rows=1 width=4)
Index Scan using fi on foo (cost=0.00..0.42 rows=1 width=4)
EXPLAIN
</computeroutput>
@ -178,11 +198,16 @@ EXPLAIN SELECT sum(i) FROM foo WHERE i = 4;
<computeroutput>
NOTICE: QUERY PLAN:
Aggregate (cost=2.05 rows=1 width=4)
-> Index Scan using fi on foo (cost=2.05 rows=1 width=4)
Aggregate (cost=0.42..0.42 rows=1 width=4)
-> Index Scan using fi on foo (cost=0.00..0.42 rows=1 width=4)
</computeroutput>
</programlisting>
</para>
<para>
Note that the specific numbers shown, and even the selected query
strategy, may vary between Postgres releases due to planner improvements.
</para>
</refsect1>
<refsect1 id="R1-SQL-EXPLAIN-3">