Re: [HACKERS] <> join selectivity estimate question

On Fri, Jun 2, 2017 at 4:16 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> I don't think it does really.  The thing about a <> semijoin is that it
> will succeed unless *every* join key value from the inner query is equal
> to the outer key value (or is null).  That's something we should consider
> to be of very low probability typically, so that the <> selectivity should
> be estimated as nearly 1.0.  If the regular equality selectivity
> approaches 1.0, or when there are expected to be very few rows out of the
> inner query, then maybe the <> estimate should start to drop off from 1.0,
> but it surely doesn't move linearly with the equality selectivity.

Ok, here I go like a bull in a china shop: please find attached a
draft patch.  Is this getting warmer?

In the comment for JOIN_SEMI I mentioned a couple of refinements I
thought of but my intuition was that we don't go for such sensitive
and discontinuous treatment of stats; so I made the simplifying
assumption that RHS always has more than 1 distinct value in it.

Anti-join <> returns all the nulls from the LHS, and then it only
returns other LHS rows if there is exactly one distinct non-null value
in RHS and it happens to be that one.  But if we make the same
assumption I described above, namely that there are always at least 2
distinct values on the RHS, then the join selectivity is just
nullfrac.

-- 
Thomas Munro
http://www.enterprisedb.com

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