Not all databases support counting indexes. This is why counts we’re a bit issue at Parse: mongo didn’t support them and if the count is for an unindexed query,

Not all databases support counting indexes. This is why counts we’re a bit issue at Parse: mongo didn’t support them and if the count is for an unindexed query, it could be a full table scan on a multi-tenant system. Just as bad, mongo as of 3.0 (last I worked with it heavily) did not ever yield during an index scan so counting large numbers on an indexed query could cause massive problems too. I recommend the atomic increment operator for counts that both Parse and Firestore offer for this problem.