> On 7 Mar 2023, at 10:51, David Rowley <dgrowle...@gmail.com> wrote: > > On Sun, 5 Mar 2023 at 13:21, Lukas Fittl <lu...@fittl.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.
Lukas: do you have an updated patch for this commitfest to address David's comments? -- Daniel Gustafsson
