Giorgio Valoti wrote:
On 07/ago/08, at 10:35, Richard Huxton wrote:
Giorgio Valoti wrote:
Hi, I have a timestamptz field that I want to use with a query, but I
don’t need the full timestamp resolution, so I’ve created a
day_trunc(timestamptz) immutable function which I’ll use with the
query and with a new index:
logs=> create index test_idx on blackbox (day_trunc(ts));
However, the query plan doesn’t use the index:
Does it use it ever? e.g. with
SELECT * FROM blackbox WHERE day_trunk(ts) = '...'
It’s used:
[snip]
OK - so the index is working.
If you disable seq-scans before running the query, does it use it then?
SET enable_seqscan = off;
> logs=>explain select count(*) from blackbox group by day_trunc(ts) order
> by day_trunc(ts);
> QUERY PLAN
>
------------------------------------------------------------------------------------------
>
> GroupAggregate (cost=98431.58..119773.92 rows=74226 width=8)
In particular:
1. Is the estimated cost more or less than 119773.92?
2. How does that match the actual time taken?
--
Richard Huxton
Archonet Ltd
