Re: Optimizer too eager to choose full table scans

See RANDOM_PAGE_COST under
http://developer.postgresql.org/docs/postgres/runtime-config.html#RUNTIME-CONFIG-OPTIMIZER

Greg Stark wrote:
> Here's the same query with slightly different paramers. The optimizer chooses
> to use a full table scan for the one set of parameters. Even though it's only
> 10% more records the query takes 4 times as long to execute, presumably
> because of the full table scan. (Yes, I ran these several times to reduce disk
> caching effects.)
>
> Is there a parameter to adjust to tilt the scales somewhat back in balance
> here? It seems to be overeager to use full table scans.
>
>
> slo=> explain analyze select count(*) from ad_dept where dept_id between 730 and 738;
> QUERY PLAN
> ----------------------------------------------------------------------------------------------------------------
> Aggregate (cost=5417.18..5417.18 rows=1 width=0) (actual time=875.74..875.74 rows=1 loops=1)
> -> Seq Scan on ad_dept (cost=0.00..5409.53 rows=3059 width=0) (actual time=0.18..869.45 rows=2767 loops=1)
> Filter: ((dept_id >= 730) AND (dept_id <= 738))
> Total runtime: 877.81 msec
> (4 rows)
>
> Time: 879.80 ms
> slo=> explain analyze select count(*) from ad_dept where dept_id between 731 and 738;
> QUERY PLAN
> -------------------------------------------------------------------------------------------------------------------------------------
> Aggregate (cost=5236.00..5236.00 rows=1 width=0) (actual time=232.25..232.25 rows=1 loops=1)
> -> Index Scan using ad_dept_dept on ad_dept (cost=0.00..5229.06 rows=2778 width=0) (actual time=0.40..214.76 rows=2477 loops=1)
> Index Cond: ((dept_id >= 731) AND (dept_id <= 738))
> Total runtime: 232.42 msec
> (4 rows)
>
> Time: 237.53 ms
>
>