Thread

Re: Add estimated hit ratio to Memoize in EXPLAIN to explain cost adjustment

Ilia Evdokimov <ilya.evdokimov@tantorlabs.com> — 2025-04-02T09:35:33Z

On 02.04.2025 00:06, David Rowley wrote:
> I tried to move things along to address Tom's concern about not
> wanting to show this in EXPLAIN's standard output. I suggested in [1]
> that we could use EXPLAIN ANALYZE, but nobody commented on that.
> EXPLAIN ANALYZE is much more verbose than EXPLAIN already, and if we
> put these in EXPLAIN ANALYZE then the viewer can more easily compare
> planned vs actual. I had mentioned that Hash Join does something like
> this for buckets.
>
> David
>
> [1]https://postgr.es/m/CAApHDvqPkWmz1Lq23c9CD+mAW=hgPrD289tC30f3H1f6Ng59+g@mail.gmail.com


Apologies for not addressing your earlier suggestion properly. After 
reconsidering, I agree that showing ndistinct and est_entries in EXPLAIN 
ANALYZE when there are actual cache misses is the best approach. This is 
typically when users notice performance regressions and start 
investigating the cause, so surfacing planner expectations at that point 
can be the most useful. I attached v6 patch with changes.

--
Best regards,
Ilia Evdokimov,
Tantor Labs LLC.