2017-03-24 [7b504eb28] Implement multivariate n-distinct coefficients 2017-04-05 [2686ee1b7] Collect and use multi-column dependency stats 2017-05-12 [bc085205c] Change CREATE STATISTICS syntax
The existing notes say: |Add multi-column optimizer statistics to compute the correlation ratio and number of distinct values (Tomas Vondra, David Rowley, Álvaro Herrera) |New commands are CREATE STATISTICS, ALTER STATISTICS, and DROP STATISTICS. |This feature is helpful in estimating query memory usage and when combining the statistics from individual columns. "correlation ratio" is referring to stxkind=d (dependencies), right ? That's very unclear. "helpful in estimating query memory usage": I guess it means that this allows the planner to correctly account for large vs small number of GROUP BY values, but it sounds more like it's going to help a user to estimate memory use. "when combining the statistics from individual columns." this is referring to stxkind=d, handling correlated/redundant clauses, but it'd be hard for a user to know that. Also, maybe it should say "combining stats from columns OF THE SAME TABLE". So I propose: |Allow creation of multi-column statistics objects, for computing the |dependencies between columns and number of distinct values of combinations of columns |(Tomas Vondra, |David Rowley, Álvaro Herrera) |The new commands are CREATE STATISTICS, ALTER STATISTICS, and DROP STATISTICS. |Improved statistics allow the planner to generate better query plans with more accurate |estimates of the row count and memory usage when grouping by multiple |columns, and more accurate estimates of the row count if WHERE clauses apply |to multiple columns and values of some columns are correlated with values of |other columns.
