Question

How i should index two timestamp columns (ie starts_at and ends_at), which would be used almost always together in queries (like starts_at >= ? AND ends_at <= ?)? Create composite index for both columns or index each one separatly?

Btw, i’m using Postgres, if it matters 🙂

Answer 1

If these two columns are true timestamps with a low number of rows containing precisely the same timestamp, then there really is no use in a composite key, since the range scan would not be able to pick a single value for the first. This can be different if the fields are dates, as frequently dates can match where timestamps don’t. There is a small benefit to it but my experience is that it’s not enormous.

If you are using a sufficiently large and dense datase (if a single index scan will need to scan a large portion of the index) the query planner can use multiple indexes, combining them with a bitmap.

http://www.postgresql.org/docs/current/static/indexes-bitmap-scans.html

As always with this type of question, it’s worth running tests on your dataset.