If a query has many OR-ed constructs which can use multiple indexes

  (key1=1 AND key2=10) OR
  (key1=2 AND key2=20) OR
  (key1=3 AND key2=30) OR
  ...

The range optimizer would construct and then discard a lot of potential
index_merge plans. This process
1. is CPU-intensive
2. can hit the @@optimizer_max_sel_args limitation after which all
   potential range or index_merge plans are discarded.

The fix is to apply a heuristic: if there is an OR clause with more than
100 branches, disallow construction of index_merge plans for the OR
branches.