Consider the following LINQ:

var dog1 = "dog";
var dog2 = "dog";
var query = coll.AsQueryable().Where(x => dog1 == dog2);

This will render the following MQL because we know client-side at runtime that the expression evaluates to true.

[ { $match : { $expr : true } }]

We should optimize this to the equivalent and faster:

[ { $match : { } }]