| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Julien Rouhaud <julien(dot)rouhaud(at)dalibo(dot)com>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Stefan Huehner <stefan(at)huehner(dot)org>, PostgreSQL mailing lists <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: pg9.6 segfault using simple query (related to use fk for join estimates) |
| Date: | 2016-06-06 17:44:51 |
| Message-ID: | 26676.1465235091@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> writes:
> When it comes to improving multiple (multi-column) foreign keys, I think
> it may get way more complicated that it might seem. What if the foreign
> keys overlap, for example? Or what if the keys go in opposite directions
> (cycle). And so on ...
I think you can group all FKs referencing the same table and discard
all their matched join clauses in favor of a single 1/N estimate
(and when I say "discard", that means you don't match those clauses
against later FKs, which should take care of the reciprocal-FK issue).
This is clearly correct if no nulls are involved. We need to make some
estimate of how much to de-rate that figure for nulls, but I don't see
that it's any harder than what's already required for a single multicol
FK.
regards, tom lane
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