Why isn't Postgres using the index?

Question

I have a table with an integer column called account_id. I have an index on that column.

But seems Postgres doesn\'t want to use my index:

Answer 1

Because of:

Seq Scan on invoices  (...) (actual ... rows=118027 <— this
   Filter: (account_id = 1)
   Rows Removed by Filter: 51462                    <— vs this
 Total runtime: 39.917 ms

You're selecting so many rows that it's cheaper to read the entire table.

Related earlier questions and answers from today for further reading:

• Why doesn't Postgresql use index for IN query?

• Postgres using wrong index when querying a view of indexed expressions?

(See also Craig's longer answer on the second one for additional notes on indexes subtleties.)