Re: Why does a simple query not use an obvious index?

On Fri, 2004-08-27 at 11:12, Jack Kerkhof wrote:
> The query:
>
>     select count(*) from billing where timestamp > now()-60
>
> should obviously use the index
>
>     CREATE INDEX billing_timestamp_idx ON billing USING btree
> ("timestamp" timestamp_ops);
>
> on a table with 1400000 rows.
>
> But it uses a Seq Scan. If I set enable_seqscan=no, it indicates a
> queryplan could not be calculated.

Have you tried this:

marlowe=> select now()-60;
ERROR:  operator does not exist: timestamp with time zone - integer
HINT:  No operator matches the given name and argument type(s). You may
need to add explicit type casts.

you likely need:

smarlowe=> select now()-'60 seconds'::interval;
           ?column?
-------------------------------
 2004-08-29 12:25:38.249564-06

inside there.

Also, count(*) is likely to always generate a seq scan due to the way
aggregates are implemented currently in pgsql.  you might want to try:

select somefield from sometable where timestampfield > now()-'60
seconds'::interval

and count the number of returned rows.  If there's a lot, it won't be
any faster, if there's a few, it should be a win.