On Fri, 13 Dec 2002, Josh Berkus wrote:
> First, as expected, a regular aggregate is slow:
> So we use the workaround standard for PostgreSQL:
>
> ... which is fast, but returns NULL, since nulls sort to the bottom! So we
> add IS NOT NULL:
>
> jwnet=> explain analyze select date_resolved from case_clients where
> date_resolved is not null order by date_resolved desc limit 1;
> NOTICE: QUERY PLAN:
>
> Limit (cost=0.00..4.06 rows=1 width=4) (actual time=219.63..219.64 rows=1
> loops=1)
> -> Index Scan Backward using idx_caseclients_resolved on case_clients
> (cost=0.00..163420.59 rows=40272 width=4) (actual time=219.62..219.62 rows=2
> loops=1)
> Total runtime: 219.76 msec
>
> Aieee! Almost as slow as the aggregate!
I'd suggest trying a partial index on date_resolved where date_resolve is
not null. In my simple tests on about 200,000 rows of ints where 50% are
null that sort of index cut the runtime on my machine from 407.66 msec to
0.15 msec.
