Hi,
Hi, all.
Some of my clients use JOIN's with three - four clauses. Quite
frequently, I see complaints on unreasonable switch of JOIN algorithm to
Merge Join instead of Hash Join. Quick research have shown one weak
place - estimation of an average bucket size in final_cost_hashjoin (see
q2.sql in attachment) with very conservative strategy.
Unlike estimation of groups, here we use smallest ndistinct value across
all buckets instead of multiplying them (or trying to make multivariate
analysis).
It works fine for the case of one clause. But if we have many clauses,
and if each has high value of ndistinct, we will overestimate average
size of a bucket and, as a result, prefer to use Merge Join. As the
example in attachment shows, it leads to worse plan than possible,
sometimes drastically worse.
I assume, this is done with fear of functional dependencies between hash
clause components. But as for me, here we should go the same way, as
estimation of groups.
I can reproduce the visitation you want to improve and verify the patch
can do it expectedly. I think this is a right thing to do.
The attached patch shows a sketch of the solution.
I understand that this is a sketch of the solution, but the below changes still
make me confused.
+ if (innerbucketsize > virtualbuckets)
+ innerbucketsize = 1.0 / virtualbuckets;
innerbucketsize is a fraction of rows in all the rows, so it is between 0.0 and 1.0.
and virtualbuckets is the number of buckets in total (when considered the mutli
batchs), how is it possible for 'innerbucketsize > virtualbuckets' ? Am