Should an index be used on a created_at timestamp column if you know you
will be using "ORDER BY created_at ASC|DESC" from time to time?
Yes.
Thanks. This is stated explicitly in 8.3 docs (as opposed to 8.2)
http://www.postgresql.org/docs/8.3/static/indexes-ordering.html
And you should use EXPLAIN.
You mean Postgres wouldn't *always* use created_at's index with such
access patterns on a big table (even if one exists):
select * from foo order by created_at desc;
?
Mind you the distribution of created_at values are going to be as
different as the time is (i.e. almost as many different values as there
are tables in the row).
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