Speed-up build of MCV lists with many distinct values
When building multi-column MCV lists, we compute base frequency for each
item, i.e. a product of per-column frequencies for values from the item.
As a value may be in multiple groups, the code was scanning the whole
array of groups while adding items to the MCV list. This works fine as
long as the number of distinct groups is small, but it's easy to trigger
trigger O(N^2) behavior, especially after increasing statistics target.
This commit precomputes frequencies for values in all columns, so that
when computing the base frequency it's enough to make a simple bsearch
lookup in the array.
Backpatch to 12, where multi-column MCV lists were introduced.
Discussion: https://postgr.es/m/20190618205920.qtlzcu73whfpfqne@development
Branch
------
master
Details
-------
https://git.postgresql.org/pg/commitdiff/e365a581c246a8e18f38cc530013391329dcdb02
Modified Files
--------------
src/backend/statistics/mcv.c | 130 ++++++++++++++++++++++++++++++++++++++++---
1 file changed, 122 insertions(+), 8 deletions(-)