84f9a35e39
When adjusting the estimate for the number of distinct values from a rel in a grouped query to take into account the selectivity of the rel's restrictions, use a formula that is less likely to produce under-estimates. The old formula simply multiplied the number of distinct values in the rel by the restriction selectivity, which would be correct if the restrictions were fully correlated with the grouping expressions, but can produce significant under-estimates in cases where they are not well correlated. The new formula is based on the random selection probability, and so assumes that the restrictions are not correlated with the grouping expressions. This is guaranteed to produce larger estimates, and of course risks over-estimating in cases where the restrictions are correlated, but that has less severe consequences than under-estimating, which might lead to a HashAgg that consumes an excessive amount of memory. This could possibly be improved upon in the future by identifying correlated restrictions and using a hybrid of the old and new formulae. Author: Tomas Vondra, with some hacking be me Reviewed-by: Mark Dilger, Alexander Korotkov, Dean Rasheed and Tom Lane Discussion: http://www.postgresql.org/message-id/flat/56CD0381.5060502@2ndquadrant.com |
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README
PostgreSQL tests ================ This directory contains a variety of test infrastructure as well as some of the tests in PostgreSQL. Not all tests are here -- in particular, there are more in individual contrib/ modules and in src/bin. Not all these tests get run by "make check". Check src/test/Makefile to see which tests get run automatically. examples/ Demonstration programs for libpq that double as regression tests via "make check" isolation/ Tests for concurrent behavior at the SQL level locale/ Sanity checks for locale data, encodings, etc mb/ Tests for multibyte encoding (UTF-8) support modules/ Extensions used only or mainly for test purposes, generally not suitable for installing in production databases perl/ Infrastructure for Perl-based TAP tests recovery/ Test suite for recovery and replication regress/ PostgreSQL's main regression test suite, pg_regress ssl/ Tests to exercise and verify SSL certificate handling thread/ A thread-safety-testing utility used by configure