multi column index order for order by

Question

I have created the following multi column index

CREATE INDEX ON message_log (website,  in_use, insert_timestamp);

However when I perform the following query, it does not seem to be using the query as it's taking a very long time to complete.

select id from message_log where website = 'forgezilla' AND in_use = false ORDER BY insert_timestamp NULLS FIRST;

Can the multi column index be used for order by? if so did I create the ordering wrong when creating the index?

Answer 1

No, your given index cannot be used for the ORDER BY. You can see this by doing set enable_sort=off and then explaining the query, you get a sort anyway because that is the only way PostgreSQL can get the job done.

But if you create the correct index:

CREATE INDEX ON message_log (website,  in_use, insert_timestamp NULLS FIRST);

then it will be used for the ORDER BY.