Improve performance of SHOW PARTITIONS

[electrum]

ShowQueriesRewrite calls getLayouts() with an alwaysTrue() constraint. This fetches all partition names for the table. It then rewrites to a query that queries an internal table which then does a second getLayouts() call with alwaysTrue(). In neither case are the query predicates passed in.

The first call is done to validate that exactly one layout exists and that the table has partition columns. We could keep the partition values, but the only feasible way to do that today would be to write to a VALUES node and that might not work well for a huge number of partitions.

MetadataQueryOptimizer has a similarly inefficient call to getLayouts() with alwaysTrue().

I think the API is the wrong shape for this. We could fix it by adding a separate ConnectorMetadata method to return the discrete predicates from a layout.

Additionally, we need to pass in a proper Constraint object, though this will require extracting the predicate logic from AddExchanges. Another potential issue is that the predicate function is extremely expensive with many object allocations and duplicated work on every invocation.

[stale]

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