Q:

If I create a Postgres index what's the difference between ASC and DESC?

Does it mean ASC is better for queries such as
PRICE > 40
and DESC is better for queries such as
PRICE < 40?

A:

The where condition might not be affected, but the ORDER BY is definitely affected 
PostgreSQL Index Ordering Documentation

A:

PostgreSQL (or any other DB engine, for that matter) will read your index in either way fine. You'll get an index scan or a reverse index scan.
The problem is when you've got a multi-column scan. In this case:
index on (foo, bar)

will work for foo asc, bar asc, as well as foo desc, bar desc. On some databases this will not work on foo desc, bar asc (reverse scans foo, ignores bar), or foo asc, bar desc (scans foo, ignores bar).

