Filing this as a followup to the discussion about MDEV-21958.

The idea is to do the following: catch this pattern in the WHERE/ON condition:

colX IN (constA_1, ..., constA_N1) OR

colX IN (constB_1, ..., const_BN2) OR 

...

and rewrite it as

colX IN (constA_1, ..., constA_N1, constB_1, ..., const_BN2) OR ...

That is, join the IN-predicates together into one.

Do the same for sibling NOT IN (... ) predicates which are part of an AND.

The idea is that single IN-list is processed efficiently by the range optimizer, while multiple IN-lists are not. This will work around the problem.

The change needs to be performed (TODO: don't recall: late in Prepare stage, or the first in the execution stage). Execution stage is better because this is a rewrite. Prepare is better because we can make destructive modifications to the IN-lists.