Approximate count(distinct) queries use hll as their intermediate data type. We have a mismatch between hll target entry and sort clause's sortop oid, so we can't push an order by on the hll data type to worker nodes.

When enabled, count(distinct) approximation uses hll as the intermediate data type. We currently have a mismatch between hll target entry and sort clause's sortop oid, so we can't push an order by on the hll data type to the worker node. One fix is changing eqop and sortop oids of sort clause when we change the underlying data type.

Start by tracing the Citus planner path for approximate count(distinct) queries and how hll target entries are matched to sort clauses. Reproduce the mismatch with an order by on the approximate result, then verify that the order by can be pushed to worker nodes with consistent eqop and sortop OIDs.

Written by the indexing model from the issue text.