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

On Sun, 5 Mar 2023 at 13:21, Lukas Fittl <lukas(at)fittl(dot)com> wrote:
> Alternatively (or in addition) we could consider showing the "ndistinct" value that is calculated in cost_memoize_rescan - since that's the most significant contributor to the cache hit ratio (and you can influence that directly by improving the ndistinct statistics).

I think the ndistinct estimate plus the est_entries together would be
useful. I think showing just the hit ratio number might often just
raise too many questions about how that's calculated. To calculate the
hit ratio we need to estimate the number of entries that can be kept
in the cache at once and also the number of input rows and the number
of distinct values. We can see the input rows by looking at the outer
side of the join in EXPLAIN, but we've no idea about the ndistinct or
how many items the planner thought could be kept in the cache at once.

The plan node already has est_entries, so it should just be a matter
of storing the ndistinct estimate in the Path and putting it into the
Plan node so the executor has access to it during EXPLAIN.

David