Improve planner's width estimates for set operations

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

0 files changed, 0 insertions, 0 deletions