6cb86143e8
Formerly the planner had a hard-wired rule of thumb for guessing the amount of space consumed by an aggregate function's transition state data. This estimate is critical to deciding whether it's OK to use hash aggregation, and in many situations the built-in estimate isn't very good. This patch adds a column to pg_aggregate wherein a per-aggregate estimate can be provided, overriding the planner's default, and infrastructure for setting the column via CREATE AGGREGATE. It may be that additional smarts will be required in future, perhaps even a per-aggregate estimation function. But this is already a step forward. This is extracted from a larger patch to improve the performance of numeric and int8 aggregates. I (tgl) thought it was worth reviewing and committing this infrastructure separately. In this commit, all built-in aggregates are given aggtransspace = 0, so no behavior should change. Hadi Moshayedi, reviewed by Pavel Stehule and Tomas Vondra
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