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Update discussion of EXPLAIN to reflect existence of ModifyTable nodes.

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Back-patch to 9.0, since this was changed then.
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Tom Lane 2011-03-01 11:36:09 -05:00
parent 15907c3623
commit 82a0e35222
1 changed files with 21 additions and 12 deletions
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doc/src/sgml/perform.sgml
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@ -430,18 +430,27 @@ WHERE t1.unique1 < 100 AND t1.unique2 = t2.unique2;
<para>
The <literal>Total runtime</literal> shown by <command>EXPLAIN
ANALYZE</command> includes executor start-up and shut-down time, as well
as time spent processing the result rows. It does not include parsing,
rewriting, or planning time. For a <command>SELECT</> query, the total
run time will normally be just a little larger than the total time
reported for the top-level plan node. For <command>INSERT</>,
<command>UPDATE</>, and <command>DELETE</> commands, the total run time
might be considerably larger, because it includes the time spent processing
the result rows. For these commands, the time for the top plan node is
essentially the time spent locating the old rows and/or computing
the new ones, but it doesn't include the time spent applying the changes.
Time spent firing triggers, if any, is also outside the top plan node,
and is shown separately for each trigger.
ANALYZE</command> includes executor start-up and shut-down time, but not
parsing, rewriting, or planning time. For <command>INSERT</>,
<command>UPDATE</>, and <command>DELETE</> commands, the time spent
applying the table changes is charged to a top-level Insert, Update,
or Delete plan node. (The plan nodes underneath this node represent
the work of locating the old rows and/or computing the new ones.)
Time spent firing triggers, if any, is charged to the Insert,
Update, or Delete node, and is also shown separately for each trigger.
Note, however, that deferred triggers will not be fired until end of
transaction and are thus not accounted for in the output of
<command>EXPLAIN ANALYZE</command>.
</para>
<para>
There are two significant ways in which runtimes measured by
<command>EXPLAIN ANALYZE</command> can deviate from normal execution of
the same query. First, since no output rows are delivered to the client,
network transmission costs and I/O formatting costs are not included.
Second, the overhead added by <command>EXPLAIN ANALYZE</command> can be
significant, especially on machines with slow <function>gettimeofday()</>
kernel calls.
</para>
<para>