From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | "Thalis A(dot) Kalfigopoulos" <thalis(at)cs(dot)pitt(dot)edu> |
Cc: | pgsql-general(at)postgresql(dot)org |
Subject: | Re: INNER JOIN ON vs ','+WHERE |
Date: | 2001-06-25 18:40:39 |
Message-ID: | 16882.993494439@sss.pgh.pa.us |
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Lists: | pgsql-general |
"Thalis A. Kalfigopoulos" <thalis(at)cs(dot)pitt(dot)edu> writes:
> Changing the type of "join" syntax affects the plan-generation time
> and the execution-time. Would it be logical to EXPLAIN the query once
> using the 'FROM a,b,c WHERE...' syntax and then assuming that it
> returns the optimal execution plan, use the join order of this plan to
> rewrite the query in a 'a INNER JOIN b ON ... INNER JOIN c ON...'
> fashion so as to save the plan generation time from then on?
Yup, that's more or less what the documentation is trying to suggest,
down at the bottom.
> Is the plan generation time significant compared to actual execution
> time when we are talking about large tables even if there's lots of
> them? The question actually is: is there any more time involved to
> plan generation other than what it takes when I do an EXPLAIN on the
> query? After that, all time is consumed to performing the actual
> joins, correct?
EXPLAIN runs the same planner as actual execution does. If EXPLAIN
doesn't take long enough to bother you, then you don't need to worry
about planning time.
regards, tom lane
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