Re: Index scan startup time

From: Tom Lane
Subject: Re: Index scan startup time
Date: ,
Msg-id: 14952.1143732263@sss.pgh.pa.us
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In response to: Re: Index scan startup time  (Peter Eisentraut)
List: pgsql-performance

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Index scan startup time  (Peter Eisentraut, )
 Re: Index scan startup time  ("Steinar H. Gunderson", )
  Re: Index scan startup time  (Peter Eisentraut, )
   Re: Index scan startup time  ("Steinar H. Gunderson", )
    Re: Index scan startup time  (Michael Stone, )
     Re: Index scan startup time  ("Steinar H. Gunderson", )
      Re: Index scan startup time  (Michael Stone, )
       Re: Index scan startup time  (Tom Lane, )
        Re: Index scan startup time  (Peter Eisentraut, )
         Re: Index scan startup time  (Tom Lane, )
    Re: Index scan startup time  (Peter Eisentraut, )
     Re: Index scan startup time  ("Steinar H. Gunderson", )
 Re: Index scan startup time  (Michael Stone, )
  Re: Index scan startup time  (Peter Eisentraut, )
   Re: Index scan startup time  (Markus Schaber, )
 Re: Index scan startup time  (Simon Riggs, )

Peter Eisentraut <> 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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