On 06/18/2016 06:52 PM, Tom Lane wrote:
Tomas Vondra <tomas.von...@2ndquadrant.com> writes:
A few more comments, about re-reading the patch more thoroughly. I
wouldn't say any of those qualify as bugs, but rather as discussion
about some of the design choices:

1) NULL handling

I'd argue that we should do something about this, although I agree it's
non-trivial to estimate - at least until we get some sort of correlation
stats (e.g. my patch already provides most of the pieces, I believe).

I concur, actually, but I feel that it's out of scope for this
particular patch, which is only trying to replace the functionality
that was committed previously. If you want to come up with a patch on
top of this that adds some accounting for NULLs, I'd be willing to
consider it as a post-beta2 improvement.

Sure, fair enough. By post-beta2 you mean for 9.7, or still for 9.6?

But I'd argue that in the case of multi-column foreign keys we can do
better even without it - my experience is that in such cases either all
values are NULL or none of them, and a single NULL value breaks the FK
of course. So I think max(null_frac) would work.

Yeah, I was thinking along the same lines: max of the per-column null
fractions is probably an OK estimate.


3) ForeignKeyOptInfo->rinfos as a List
Can we actually get a list of matching RestrictInfos for a single
foreign key? I've been unable to construct such query.

I think you'd actually have to write redundant outer join quals,
along the lines of
      select ... a left join b on (a.x = b.y and a.x = b.y)
I don't believe we take the trouble to eliminate such duplicates
unless they get absorbed by an EC, which outer-join quals would
not be.  (Haven't tried this, though, as I don't have the patch
installed right now.)

OK. Let's look into this post-beta2 then.

The beta2 deadline is just about upon us; I feel that if we're going
to get this into this release at all, we need to push it today so
that we get a full buildfarm cycle on it before the wrap.

I plan to spend an hour or two adjusting the qual match logic as
discussed above, and re-reading the whole patch another time for
sanity.  If I've not heard objections by the time I'm done,
I will push it.


If I could wish one more thing - could you briefly explain why you rewrote the patch the way you did? I'd like to learn from this and while I think I kinda understand most of the changes, I'm not really sure I got it right.


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

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