Error when encountering an unsupported aggregate function

[sgrif]

When handling aggregate functions in cross-shard queries, we can only return a correct result for functions that we have explicit support for. No matter what the aggregate function is doing, we need to know how to combine the results from the separate shards.

Prior to this change, we would just silently do the wrong thing, treating this as any other non-aggregate expression and just returning a union of the rows from each query. We now explicitly check if an aggregate function with that name exists and error if we don't recognize it.

This is future proofed against new functions in later postgres versions, as well as user defined aggregates (which we can likely never support). This will produce a false positive if a function exists and is defined as aggregate for some argument types but not others. But frankly anyone doing that is asking for trouble.

This logic is objectively in the wrong place. What we are doing here is validation, not parsing. However as I'm familiarizing myself with these code paths and preparing a larger rearchitecture, I'm slowly hammering things into a shape that's easier to move around. I do not intend on leaving this logic here long term.

[blacksmith-sh]

Found 3 test failures on Blacksmith runners:

Failures

Test	View Logs
pg_tests/TestShardedTwoPc	View Logs
pg_tests/TestShardedTwoPc	View Logs
pg_tests/TestShardedTwoPc	View Logs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!