[philliphaydon]

PostgreSQL can use multiple indexes. So if you have millions of rows, and 3 indexes covering the table, on 3 different columns. PostgreSQL can use any & all of those indexes to filter the data.

Selecting only the columns you need just prevents the database from having to go back to the clustered index to retrieve the remaining information that wasn't included on the index.

Selecting * from a table has no impact on the indexes used. But it can be expensive to need to go back to the clustered index to get the additional information.