057012b205
If both sides of the operator have most-common-value statistics, eqjoinsel wants to check which MCVs have matches on the other side. Formerly it did this with a dumb compare-all-the-entries loop, which had O(N^2) behavior for long MCV lists. When that code was written, twenty-plus years ago, that seemed tolerable; but nowadays people frequently use much larger statistics targets, so that the O(N^2) behavior can hurt quite a bit. To add insult to injury, when asked for semijoin semantics, the entire comparison loop was done over, even though we frequently know that it will yield exactly the same results. To improve matters, switch to using a hash table to perform the matching. Testing suggests that depending on the data type, we may need up to about 100 MCVs on each side to amortize the extra costs of setting up the hash table and performing hash-value computations; so continue to use the old looping method when there are fewer MCVs than that. Also, refactor so that we don't repeat the matching work unless we really need to, which occurs only in the uncommon case where eqjoinsel_semi decides to truncate the set of inner MCVs it considers. The refactoring also got rid of the need to use the presented operator's commutator. Real-world operators that are using eqjoinsel should pretty much always have commutators, but at the very least this saves a few syscache lookups. Author: Ilia Evdokimov <ilya.evdokimov@tantorlabs.com> Co-authored-by: David Geier <geidav.pg@gmail.com> Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> Discussion: https://postgr.es/m/20ea8bf5-3569-4e46-92ef-ebb2666debf6@tantorlabs.com
PostgreSQL Database Management System
This directory contains the source code distribution of the PostgreSQL database management system.
PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings.
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