As a result of SERVER-97662, MatchExpression s in the classic plan cache have their index choices cached as positional values (the n th index in the index list), but this positional value can affect the QuerySolution 's hash because the position is not stable.

We should instead represent index choices in a more stable manner to avoid otherwise-identical MatchExpression s from hashing to different values solely because the index position is different.

For example: https://github.com/10gen/mongo/pull/33328/files#r1988069769

index_x_1: {x: 1}
index_y_1: {y: 1}
query: find({x: 5, y:1})

Cached plan:
$and || Selected Index #1 pos 0 combine 1
x $eq 5
y $eq 1 || Selected Index #1 pos 0 combine 1

Plan retrieved from cache:
$and || Selected Index #2 pos 0 combine 1
x $eq 5
y $eq 1 || Selected Index #2 pos 0 combine 1