From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Peter Eisentraut <peter_e(at)gmx(dot)net> |
Cc: | Michael Stone <mstone+postgres(at)mathom(dot)us>, pgsql-performance(at)postgresql(dot)org |
Subject: | Re: Index scan startup time |
Date: | 2006-03-30 15:24:23 |
Message-ID: | 14952.1143732263@sss.pgh.pa.us |
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Lists: | pgsql-performance |
Peter Eisentraut <peter_e(at)gmx(dot)net> writes:
> So from when to when is the startup time (the "x" in "x..y") actually
> measured? When does the clock start ticking and when does it stop?
> That is what's confusing me.
The planner thinks of the startup time (the first estimated-cost number)
as the time before the output scan can start, eg, time to do the sort in
a sort node. EXPLAIN ANALYZE however reports the actual time until the
first output row is delivered. When you've got a filter applied to the
node result, as in this case, there can be a material difference between
the two definitions, because of the time spent scanning rows that don't
get past the filter.
regards, tom lane
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