Currently the MatchExpression $or->$in rewrite is only capable of collapsing a single field with multiple equality disjunctions into a $in. For example,

{ $or: [
     { a: 10 },
     { b: 11 },
     { c: 12 },
     { d: 13 },
     { a: 14 }, 
     { b: 15 },
     { c: 16 },
     { d: 17 },
     { a: 18 },
     { b: 19 }
  ]
} 

is optimized to

{
  $or: [
    { b: 11 },
    { b: 15 },
    { b: 19 },
    { c: 12 },
    { c: 16 },
    { d: 13 },
    { d: 17 },
    { a: { $in : [ 10, 14, 18 ] } }
  ]
}

This leads to an SBE plan with a large number of unnecessary comparisons. SBE is able to evaluate $in predicates more efficiently by using a hashset, and the interval builder is able to generate index bounds more easily on an $in list.

These expressions can be further reduce by allowing the $or->$in rewrite to handle multiple fields.

There are two high-value workloads (MatchExpressionWidePredicate and MatchExpressionWidePredicateWithDeepFieldpaths) which could be improved by this optimization.