As a result of SERVER-97662 (https://github.com/mongodb/mongo/commit/d2cff6b001c838531921917653c72528cfe01ca2#diff-50b7dd2ce8ee719a74d5badf8d2dd01c45342c9fc4e43d27d5e97e5bd3faa75d note: because the patch is long the link doesn't work thanks github we're talking about the file `index_tag.h`) , 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 taggedMatchExpressionHash 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