Rails core - Feb 2013 - Using a sorted set for collecting association ids when eager loading

Consider the following gist: https://gist.github.com/nmk/5002233

Eager loading :reporter and :assignee for the bug object results in two 
very similar queries:

User Load (0.2ms) SELECT "users".* FROM "users" WHERE
"users"."id" IN (1, 2)
User Load (0.1ms) SELECT "users".* FROM "users" WHERE
"users"."id" IN (2, 1)

This prevents AR from using its query cache and the DB from using its own
caching if any were applicable.

If, instead of using arrays to collect the reporter_ids and assignee_ids, we
were using sorted sets the above eager
loading would have resulted in the same query being issued twice:

User Load (0.2ms) SELECT "users".* FROM "users" WHERE
"users"."id" IN (1, 2)

thus reusing the AR cache and (if not caught at the AR level) possibly getting
help from the caching layer in the DB.

Is there an explicit reason AR is using arrays here? Are we looking for lists of
association objects instead of sets of association objects?

Surely this would only help in a small number of cases and the performance
impact of using sets instead of arrays might become an issue.

I am willing to take a crack at this, unless someone chimes in with reasonable
arguments against it.

Best,
Nickolay

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