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Re: Index scan startup time

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
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 (view raw or flat)
Thread:
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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