Re: Show estimated number of groups for IncrementalSort in EXPLAIN

On Thu, 11 Jun 2026 at 03:50, Ilia Evdokimov
<ilya(dot)evdokimov(at)tantorlabs(dot)com> wrote:
> In [0], a question was raised whether we should expose IncrementalSort group estimation in EXPLAIN, as we did for Memoize. Knowing the estimated number of groups helps understand why the planner chose IncrementalSort whether a bad estimate is to blame for a sub-optimal plan.
> [0]: https://www.postgresql.org/message-id/6642af90-561c-4f0c-9d5b-7e288e6e7f84%40gmail.com

I think it makes sense to have this information in EXPLAIN.

A couple of things about the patch:

1. I think the new est_input_groups field should be Cardinality rather
than double. Various other Path types and Plan nodes call this
"numGroups". I don't see any reason to deviate from that.
2. The header comment for cost_incremental_sort needs to be updated to
mention that p_input_groups may be passed as non-NULL to provide the
caller with the estimated number of sort groups.

David