Re: Optimization with dates

I have a similar table (~16M rows) with an indexed timestamp, and have
had similar problems.  I have found that even when I am using a
constant timestamp like in this query.

SELECT * FROM caseweights1 WHERE dt > '2001-11-01';

I start getting sequential scans with 7.1.3 long before they are
faster than index based queries.  I believe that there is a constant
that can be fiddled to modify this behavior, and it seems like I have
also read that this constant has been modified in the new 7.2 release.
However, for queries that you *know* will be faster using the index
you can always issue:

set enable_seqscan to off;

before running your query.  This will force PostgreSQL to use the
index even in queries like this one:

SELECT * FROM caseweights1 WHERE dt > (SELECT now() - '30 days'::interval);

Jason

Jean-Christophe Boggio <cat@thefreecat.org> writes:

> Hello,
> 
> I really have a problem dealing with dates :
> 
> I have a big table (~10M rows) like :
> 
> create table gains (
>        dategain    timestamp,
>        otherfields
> );
> 
> There's an index on dategain and I want to use it to get the last
> 30 days records :
> 
> explain select count(*) from gains where dategain>=now()-30;
> 
> Aggregate  (cost=256546.78..256546.78 rows=1 width=0)
>   ->  Seq Scan on gains  (cost=0.00..250627.68 rows=2367640 width=0)
> 
> whereas :
>   
> explain select count(*) from gains where dategain>='now';
> 
> Aggregate  (cost=27338.47..27338.47 rows=1 width=0)
>   ->  Index Scan using ix_gains_dategain on gains  (cost=0.00..27320.71 rows=7103 width=0)
> 
> I have tried :
>   where dategain>=(select now()-30);
>   where dategain+30>='now'
>   where date(dategain)>=date('now')-30
>   and many other, syntactically absurd :-)
> 
> Anyone can help me use this index ?
> 
> TIA,
>   
> --
> Jean-Christophe Boggio                       
> cat@thefreecat.org                           -o)
> Independant Consultant and Developer         /\\
> Delphi, Linux, Perl, PostgreSQL, Debian     _\_V
> 
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 4: Don't 'kill -9' the postmaster