On Thu, Jul 20, 2017 at 11:47 PM, Ashutosh Bapat <ashutosh.ba...@enterprisedb.com> wrote: > On Thu, Jul 20, 2017 at 11:04 AM, Thomas Munro > <thomas.mu...@enterprisedb.com> wrote: >> On Fri, Jun 2, 2017 at 4:16 AM, Tom Lane <t...@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. >> > > The patch looks good to me. > > + /* > + * For semi-joins, if there is more than one distinct key in the RHS > + * relation then every non-null LHS row must find a match since it can > + * only be equal to one of them. > The word "match" confusing. Google's dictionary entry gives "be equal > to (something) in quality or strength." as its meaning. May be we want > to reword it as "... LHS row must find a joining row in RHS ..."?
Thanks! Yeah, here's a version with better comments. Does anyone know how to test a situation where the join is reversed according to get_join_variables, or "complicated cases where we can't tell for sure"? -- Thomas Munro http://www.enterprisedb.com
neqjoinsel-fix-v2.patch
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