On Sat, 2023-11-25 at 09:19 -0800, Paul A Jungwirth wrote: > Here is a patch to improve rowcount estimates for > `UNNEST(some_array_column)`. Today we hard code this to 10, but we > have statistics about array size, so it's easy to use them. > > I've seen plans where this would make a difference. If the array has > only 1 or 2 elements, then overestimating the rowcount by 10 leads to > unnecessary seqscans downstream. I can see how an underestimate would > cause issues too.
The idea sounds good to me. I didn't test or scrutinize the code, but I noticed that you use EXPLAIN in the regression tests. I think that makes the tests vulnerable to changes in the parameters or in the block size. Perhaps you can write a function that runs EXPLAIN and extracts just the row count. That should be stable enough. Yours, Laurenz Albe
