Re:Using Nested Selects for Performance in Rails
by kenn on Friday July 10, 2015 @06:01AM
(#9863086)
It would be slow on MySQL, though.
If you EXPLAIN that query, it's a DEPENDENT SUBQUERY which has to fetch N*M records. It would be mitigated if the columns are property indexed, but two separate queries (N+M) just as eager loading would be faster and more predictable than nested scans in terms of IOPS.
If you EXPLAIN that query, it's a DEPENDENT SUBQUERY which has to fetch N*M records. It would be mitigated if the columns are property indexed, but two separate queries (N+M) just as eager loading would be faster and more predictable than nested scans in terms of IOPS.