Re: optimizing selects on time-series data in Pg

On Fri, Aug 01, 2003 at 11:28:05AM -0700, Stephan Szabo wrote:
> Does it lower the realtime after a set enable_seqscan=off; (also an
> explain analyze would let us see if there's a type difference that causes
> it to not want to use the index for the date comparison).

hm, that's interesting, it doesn't want to use the combined index if I turn
off seqscan and in any case it isn't any better doing an index scan with just
the virtualhost:

zp1139=> set enable_seqscan=off; SET VARIABLE
zp1139=> explain analyze select count(*) from zp_log where virtualhost =
'www.bloki.com' and timestamp > (now() - interval '2 days');
NOTICE: QUERY PLAN:

Aggregate (cost=902991.72..902991.72 rows=1 width=0) (actual
time=18876.95..18876.95 rows=1 loops=1)
-> Index Scan using zp_log_vhost on zp_log (cost=0.00..902794.58 rows=78856
width=0) (actual time=45.12..18783.10 rows=23241 loops=1)
Total runtime: 18878.03 msec

EXPLAIN
zp1139=> set enable_seqscan=on; SET VARIABLE
zp1139=> explain analyze select count(*) from zp_log where virtualhost =
'www.bloki.com' and timestamp > (now() - interval '2 days');
NOTICE: QUERY PLAN:

Aggregate (cost=59157.66..59157.66 rows=1 width=0) (actual
time=38736.04..38736.04 rows=1 loops=1)
-> Seq Scan on zp_log (cost=0.00..58960.52 rows=78856 width=0) (actual
time=0.19..38611.08 rows=23203 loops=1)
Total runtime: 38737.25 msec

EXPLAIN
zp1139=> \d zp_log_vhost_timestamp
Index "zp_log_vhost_timestamp"
Column | Type
-------------+--------------------------
virtualhost | text
timestamp | timestamp with time zone
btree