Question

I have a Rails app on a Postgres database which relies heavily on queries like this:

SELECT DISTINCT client_id FROM orders WHERE orders.total>100

I need, essentially, the ids of all the clients who have orders which meet a certain condition. I only need the id, so I figured this is way faster than using joins.

Would I benefit from adding an index to the column “total”? I don’t mind insert speed, I just need the query to run extremely fast.

Answer 1

I would expect the following multicolumn index to be fastest:

CREATE INDEX orders_foo_idx ON orders (total DESC, client_id);

PostgreSQL 9.2 could benefit even more. With it’s “index-only tuples” feature, it could serve the query without hitting the table under favorable circumstances: no writes since the last VACUUM.

DESC or ASC hardly matters in this case. A B-tree index can be searched in both directions almost equally efficient.