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author | Tom Lane <tgl@sss.pgh.pa.us> | 2011-03-01 11:36:01 -0500 |
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committer | Tom Lane <tgl@sss.pgh.pa.us> | 2011-03-01 11:37:02 -0500 |
commit | 09b49a8439fde63826f93f55430868cf2585644b (patch) | |
tree | 686d84ff55ca2d7a3f85d7428d9d6439bd3acf8a | |
parent | 97c4ee94adf15f7a0a39cbb5549159e2aa0679d1 (diff) | |
download | postgresql-09b49a8439fde63826f93f55430868cf2585644b.tar.gz postgresql-09b49a8439fde63826f93f55430868cf2585644b.zip |
Update discussion of EXPLAIN to reflect existence of ModifyTable nodes.
Back-patch to 9.0, since this was changed then.
-rw-r--r-- | doc/src/sgml/perform.sgml | 33 |
1 files changed, 21 insertions, 12 deletions
diff --git a/doc/src/sgml/perform.sgml b/doc/src/sgml/perform.sgml index d4ac40df22c..72baad27f13 100644 --- a/doc/src/sgml/perform.sgml +++ b/doc/src/sgml/perform.sgml @@ -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> |