On Wed, Jan 7, 2026 at 8:39 PM Jacob Jackson <[email protected]> wrote: > On Wed, Jan 7, 2026 at 5:22 AM Neil Chen <[email protected]> wrote: > > If we want the query planner to generate an execution plan as if a column > > were unique, would setting n_distinct = -1 in the table statistics achieve > > the same effect? > > Setting n_distinct is less clear in describing the data (it isn't tied > to the table schema itself and can be ambiguous in whether values are > actually totally unique or just close enough, which can complicate > things), and, because it only impacts statistics estimations, it still > doesn't help in queries where uniqueness is required. Postgres will > still add a node to ensure uniqueness in that case. There are also > some other limitations with n_distinct (e.g. extended stats are > required for multi-column pairs, which further complicates > documentation and adds complexity/overhead). >
Just thinking out loud here, but I wonder if you might be able to modify pg_hint_plan to have a "NoUnique" (or "NoOpUnique") hint which causes the planner to either skip any unique nodes, or have them function as just a passthrough. I'm not entirely sure that's doable actually; we don't currently have hints for other types of aggregation nodes, and at the moment I don't think we have any hints that can produce wrong answers, and this one feels like it could... Robert Treat https://xzilla.net
