be69a5ff1f
A UNION (not UNION ALL) removes duplicates, so its output has no more rows than its input. The planner did not account for this: it set the set-op relation's row count to the total size of the appended input, as though dedup removed nothing. That inflated estimate then propagated to every node above the UNION, leading to poor plan choices such as a hash join with a full table scan where an index nested loop would have been cheaper. This patch estimates the number of distinct output rows as the sum of the per-child distinct-group estimates instead. This relies on the fact that: distinct(A union B) <= distinct(A) + distinct(B) that is, the union cannot have more distinct rows than its children do in total. And because each child's distinct-group estimate never exceeds that child's row-count estimate, this sum is never larger than the old estimate, so it only tightens the previous over-estimate. Author: Richard Guo <guofenglinux@gmail.com> Reviewed-by: David Rowley <dgrowleyml@gmail.com> Reviewed-by: Chengpeng Yan <chengpeng_yan@outlook.com> Discussion: https://postgr.es/m/CAMbWs48Fu1nhGXPa60oc+adj7ge4dn0nHhqngqKvOVVQP61duA@mail.gmail.com |
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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.
Copyright and license information can be found in the file COPYRIGHT.
General documentation about this version of PostgreSQL can be found at https://www.postgresql.org/docs/devel/. In particular, information about building PostgreSQL from the source code can be found at https://www.postgresql.org/docs/devel/installation.html.
The latest version of this software, and related software, may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.