MergeAppend could consider sorting cheapest child path

Hi.

Now when planner finds suitable pathkeys in
generate_orderedappend_paths(), it uses them, even if explicit sort of
the cheapest child path could be more efficient.

We encountered this issue on partitioned table with two indexes, where
one is suitable for sorting, and another is good for selecting data.
MergeAppend was generated
with subpaths doing index scan on suitably ordered index and filtering a
lot of data.
The suggested fix allows MergeAppend to consider sorting on cheapest
childrel total path as an alternative.

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Best regards,
Alexander Pyhalov,
Postgres Professional