On 6/11/17 7:54 PM, Peter Geoghegan wrote:
On Sun, Jun 11, 2017 at 10:36 AM, Tom Lane <t...@sss.pgh.pa.us> wrote:
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.

Sorry for not considering the context of the thread more carefully.
Robert said something about selectivity estimation and TPC-H to me,
which I decide to research; I then rediscovered this thread.

Clearly Q20 is designed to reward systems that do better with moving
predicates into subqueries, as opposed to systems with better
selectivity estimation.

I do strongly recommend reading this paper analyzing choke points of individual TPC-H queries:


It's slightly orthogonal to the issue at hand (poor estimate in Q20 causing choice of inefficient plan), it's a great paper to read. I thought I've already posted a link to the this paper sometime in the past, but I don't see it in the archives.


Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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