Обсуждение: Queries with timestamps

Hi all,

    I'm quite newbie in SQL and I have a performance problem. I have the
following table (with some extra fields) and without any index:

    CREATE TABLE STATISTICS
    (
        STATISTIC_ID             NUMERIC(10) NOT NULL DEFAULT
nextval('STATISTIC_ID_SEQ')
                                               CONSTRAINT pk_st_statistic_id
PRIMARY KEY,
        TIMESTAMP_IN          TIMESTAMP,
        VALUE                          NUMERIC(10)
    );

    The queries on this table are mainly related with the timestamp field,
e.g.:

        select * from statistics where time::date < current_date - interval
'1 month';

    As the number of rows grows the time needed to execute this query takes
longer.  What'd I should do improve the performance of this query?

Thank you very much

--
Arnau

Arnau,

>     As the number of rows grows the time needed to execute this query takes
> longer.  What'd I should do improve the performance of this query?

Tip #1)  add an index to the timestamp column
Tip #2) make sure that you VACUUM and ANALYZE regularly
Tip #3) You will get better performance if you pass the "current_date - 1
month" as a constant from the client instead of in the query.   This is a
known issue, expected to be fixed in 7.5.

--
-Josh Berkus
 Aglio Database Solutions
 San Francisco

On Wednesday 21 January 2004 19:06, Josh Berkus wrote:
> Arnau,
>
> >     As the number of rows grows the time needed to execute this query
> > takes longer.  What'd I should do improve the performance of this query?
>
> Tip #1)  add an index to the timestamp column
> Tip #2) make sure that you VACUUM and ANALYZE regularly
> Tip #3) You will get better performance if you pass the "current_date - 1
> month" as a constant from the client instead of in the query.   This is a
> known issue, expected to be fixed in 7.5.

(I think Tip 3 is already fixed in 7.3, or I misunderstand what Josh is
saying)

Note that this is timestamp-related and not "timestamp with time zone"
related. Most of the time you want the latter anyway. If you can use with
time zones and drop the cast you might well find the index is being used...

EXPLAIN ANALYSE SELECT * FROM log_core WHERE log_ts > CURRENT_DATE - '1
week'::interval;
                                                         QUERY PLAN

-----------------------------------------------------------------------------------------------------------------------------
 Index Scan using log_core_ts_idx on log_core  (cost=0.00..18.73 rows=239
width=117) (actual time=0.79..0.79 rows=0 loops=1)
   Index Cond: (log_ts > ((('now'::text)::date - '7
days'::interval))::timestamp with time zone)
 Total runtime: 1.03 msec
(3 rows)

It seems to help an accurate estimate of number-of-rows if you put an upper
and lower limit in.

--
  Richard Huxton
  Archonet Ltd

Richard,

> (I think Tip 3 is already fixed in 7.3, or I misunderstand what Josh is
> saying)

Yeah?  Certainly looks like it.   Apparently I can't keep track.

I'd swear that this issue reared its ugly head again shortly before the 7.4
release.

--
-Josh Berkus
 Aglio Database Solutions
 San Francisco