til/postgres/create-an-index-across-two-columns.md

Create An Index Across Two Columns

Most commonly when we create an index, it is targeted at a single column of a table. Sometimes an expensive query that works with two different columns would be better off with an index that combines those two columns. This is called a composite index.

Let's consider this query:

select * from events
  where user_id = 123
  order by created_at desc
  limit 1;

Though this query will use the index on created_at to do an Index Scan, it will still have to do a bunch of expensive filtering of user_id values after the fact.

What this query needs to be efficient is a composite index on user_id and created_at. We can create one like so:

create index events_user_id_created_at_idx
  on events (user_id, created_at);

Instead of doing a bunch of post-index filtering on user_id values, that expensive query will factor user_id into its Index Scan and complete much quicker.

See the Postgres docs on multicolumn indexes for more details.