Re: index on timestamp performance

--On Wednesday, January 29, 2003 08:51:30 -0800 Stephan Szabo
<sszabo(at)megazone23(dot)bigpanda(dot)com> wrote:

> On Wed, 29 Jan 2003, Eric Cholet wrote:
>
>> I have this schema:
>>
>>
>> motid | integer | not null
>> objid | integer | not null
>> date | timestamp without time zone | not null
>> Indexes: dico_frs_motid_date btree (motid, date)
>> dico_frs_objid btree (objid)
>>
>> The performance I'm getting from the index that contains
>> 'date' is much slower than when using the objid index
>> (different queries of course). This is a 10 million row
>> table. Am I right to assume that postgres needs to do
>> more work because it has to convert the dates to some
>> internal (integer?) format?
>
> What does explain (analyze if possible) show for the two queries?

I can't really run two equivalent queries that will each use
a different index. Here's a query that uses the index with 'date'
(output wrapped manually)

=> explain analyze select objid from dico_frs where motid=1247
and date <= '2003-01-29 17:55:17' and date >= '2002-10-29 17:55:17'
order by date desc limit 11;

Limit (cost=4752.14..4752.17 rows=11 width=12) (actual time=63.20..63.37
rows=11 loops=1)
-> Sort (cost=4752.14..4755.11 rows=1187 width=12) (actual
time=63.17..63.23 rows=12 loops=1)
Sort Key: date
-> Index Scan using dico_frs_motid_date on dico_frs
(cost=0.00..4691.50 rows=1187 width=12) (actual time=0.08..41.88 rows=2924
loops=1)
Index Cond: ((motid = 1247) AND (date <= '2003-01-29
17:55:17'::timestamp without time zone) AND (date >= '2002-10-29
17:55:17'::timestamp without time zone))
Total runtime: 63.93 msec
(6 rows)

> It could just be a difference in plans or estimates.

Right, but still I'd like to know whether the timestamp datatype in the
index
results in more work than an integer datatype.

Thanks,
--
Eric Cholet