2cb49c609b
For UNION, EXCEPT and INTERSECT, we were not very good at estimating the PathTarget.width for the set operation. Since the targetlist of the set operation is made up of Vars with varno==0, this would result in get_expr_width() applying a default estimate based on the Var's type rather than taking width estimates from any relation's statistics. Here we attempt to improve the situation by looking at the width estimates for the set operation child paths and calculating the average width of the relevant child paths weighted over the estimated number of rows. For UNION and INTERSECT, the relevant paths to look at are *all* child paths. For EXCEPT, since we don't return rows from the right-hand child (only possibly remove left-hand rows matching those), we use only the left-hand child for width estimates. This also adjusts the hashed-UNION Path's PathTarget to use the same PathTarget as its Append subpath. Both PathTargets will be the same and are void of any resjunk columns, per generate_append_tlist(). Making the AggPath use the same PathTarget saves having to adjust the "width" of the AggPath's PathTarget too. This was reported as a bug by sunw.fnst, but it's not something we ever claimed to do properly. Plus, if we were to adjust this in back branches, plans could change as the estimated input sizes to Sorts and Hash Aggregates could go up or down. Plan choices aren't something we want to destabilize in stable versions. Reported-by: sunw.fnst <936739278@qq.com> Author: David Rowley <drowleyml@gmail.com> Discussion: https://postgr.es/m/tencent_34CF8017AB81944A4C08DD089D410AB6C306@qq.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.
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/.