How will MongoDB evaluate this query:
db.testCol.find(
{
"$or" : [ {a:1, b:12}, {b:9, c:15}, {c:10, d:"foo"} ]
});
When scanning values in a document if first OR statement is TRUE will the other statements be also be evaluated?
Logically if the MongoDB is optimized other values in OR statement should not be evaluated, but I don’t know how MongoDB is implemented.
UPDATE:
I updated my query because it was wrong and it didn’t explain correctly what I was trying to accomplish. I need to find a set of documents that have different properties and if an exact combination of these properties is found the document must be returned.
The SQL equivalent of my query would be:
SELECT * FROM testCol
WHERE (a = 1 AND b = 12) OR (b = 9 AND c = 15) OR (c = 10 AND d = 'foo');
MongoDB will execute each clause of the $or operation as a seperate query and remove duplicates as a post processing pass. As such each clause can use a seperate index which is often very useful.
In other words, it will NOT look at 1 document, see which of the OR clauses apply and do an early-out if the first clause is a match. Rather it does a full dataset query per clause and de-dupe after the fact. This may seem less than efficient but in practice it’s almost always faster since the first approach would only be able to hit at most one index for all clauses which is rarely efficient.