pgsql: Simplify create_merge_append_path for clarity

Simplify create_merge_append_path for clarity

We don't currently support parameterized MergeAppend paths: there's
little use for an ordered path on the inside of a nestloop. Given
this, we can simplify create_merge_append_path by directly setting
param_info to NULL instead of calling get_appendrel_parampathinfo. We
can also simplify the Assert for child paths a little bit.

This change won't make any measurable difference in performance; it's
just for clarity's sake.

Author: Richard Guo
Reviewed-by: Alena Rybakina, Paul A Jungwirth
Discussion: /messages/by-id/CAMbWs4_n1bgH2nACMuGsXZct3KH6PBFS0tPdQsXdstRfyxTunQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/f47b33a19115f432ad80777db0d1350d23bb6cf5

Modified Files
--------------
src/backend/optimizer/util/pathnode.c | 15 ++++++++++-----
1 file changed, 10 insertions(+), 5 deletions(-)