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Re: BUG #14469: Wrong cost estimates for merge append plan with partitions.

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Alexey Ermakov <alexey(dot)ermakov(at)postgresql-consulting(dot)com>
Cc: maxim(dot)boguk(at)gmail(dot)com, pgsql-bugs(at)postgresql(dot)org
Subject: Re: BUG #14469: Wrong cost estimates for merge append plan with partitions.
Date: 2016-12-17 17:48:22
Message-ID: 26142.1481996902@sss.pgh.pa.us (view raw, whole thread or download thread mbox)
Thread:
Lists: pgsql-bugs
Alexey Ermakov <alexey(dot)ermakov(at)postgresql-consulting(dot)com> writes:
> I think we should change that estimation and in case when child node 
> expected to return only one row (and really 0 rows) we should use it's 
> total_cost as base for estimation of startup_cost.

This would make sense as part of the overall rethinking of startup_cost
discussed in
https://www.postgresql.org/message-id/flat/31065.1481742760%40sss.pgh.pa.us
but I'm disinclined to do it in isolation.  Redefining startup_cost for
just one path type seems likely to create more problems than it solves.

			regards, tom lane


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