pgsql: Clarify the 'rows' parameter in create_append_path

Clarify the 'rows' parameter in create_append_path

This is extracted from a larger patch to improve the UNION planner.
While working on that, I found myself having to check what the 'rows'
parameter is for.  It's not obvious that passing a negative number is the
way to have the rows estimate calculated and to find that out you need
to read code in create_append_path() and in cost_append().

Discussion: https://postgr.es/m/CAApHDvpb_63XQodmxKUF8vb9M7CxyUyT4sWvEgqeQU-GB7QFoQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/87027cb55bf7b33416880f7f5f5e67f4202d5881

Modified Files
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src/backend/optimizer/util/pathnode.c | 4 ++++
1 file changed, 4 insertions(+)