On Fri, Mar 22, 2019 at 11:56 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> In cases where, say, the first child requires no sort but also doesn't
> emit very many rows, while the second child requires an expensive sort,
> the planner will have a ridiculously optimistic opinion of the cost of
> fetching slightly more rows than are available from the first child.
> This might lead it to wrongly choose a merge join over a hash for example.
I think this is very much a valid point, especially in view of the
fact that we already choose supposedly fast-start plans too often. I
don't know whether it's a death sentence for this patch, but it should
at least make us stop and think hard.
--
Robert Haas
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