FIXME (WIP): Allow SPE plans to show "Partitions selected: 1 (out of 5)"

[pobbatihari]

""GPDB_12_MERGE_FIXME: we used to show something along the lines of "Partitions selected: 1 (out of 5)" under the partition selector. By eleminating the (static) partition selector during translation, we only get the survivor count, and lose the size of the universe temporarily. However, if we manage to shift the static pruning information sufficiently adjacent to (or better, into) a DXL Dynamic Table Scan, we should be able to get that information back.""

Through this PR I am trying to pass the total partition information through ORCA and into the plan node used in explain.

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[dgkimura]

I took a quick stab at the approach to leverage NumberLeafPartitions and existing CTableDescriptor::PartitionCount(). Commit here [1]. Feel free to take a look and use/change/disregard what you see fit. It may be worth noting that no changes were needed in CExpressionPreprocessor::PrunePartitions().

[1] https://github.com/dgkimura/gpdb/commit/930a2cd9cbb7ec9937dc44279341b7e5cd1b241c

[pobbatihari]

PartitionCount()

thanks @dgkimura, I will look into it.