Peter Geoghegan <p...@bowt.ie> writes:
> On Thu, Jun 1, 2017 at 8:40 AM, Tom Lane <t...@sss.pgh.pa.us> wrote:
>> The thing that would actually have a chance of improving matters for Q20
>> would be if we could see our way to looking through the aggregation
>> subquery and applying the foreign key constraint for lineitem. That
>> seems like a research project though; it's surely not happening for v10.
> Do you mean teaching the optimizer to do something like this?:
Uh, no. I don't think we want to add any run-time checks. The point in
this example is that we'd get a better rowcount estimate if we noticed
that the FK constraint could be considered while estimating the size of
the partsupp-to-aggregated-subquery join.
> Apparently selectivity estimation isn't particularly challenging with
> the TPC-H queries.
Maybe not with the rest of them, but we're certainly having an issue there
with Q20.
regards, tom lane
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