Does MongoDB simplify query logic?

Question

Are the following logical equivalent queries handled by the server differently?

{"Name":"1"}

{$and:[{"Name":"1"},{$or:[{"Name":"1"},{"Tag":"a"}]}]}

Since the second query include the "Tag" field, does it affect index usage?

Answer 1

If you want to experiment and see what mongo is doing for each query you can use an explainable object in the mongo shell.

• You can create it with a cursor (https://docs.mongodb.org/manual/reference/method/cursor.explain/):

  db.example.find({a:17, b:55}).sort({b:-1}).explain()
  

• Or you can create an explainable object and execute queries with it (https://docs.mongodb.org/v3.0/reference/explain-results/#explain-output):

  var exp = db.example.explain()
  exp.help() // see what you can do with it
  exp.find({a:17, b:55}).sort({b:-1}) // execute query over the object
  

I cannot answer your question since you do not provide information about the indexes defined in your database, but using this you can see it by yourself in the "queryPlanner" section. If it's using an index it shows "IXSCAN" at "stage" field.