Kevin Grittner wrote:
> I rewrote the query to use IN predicates rather than EXISTS predicates,
> and the cost estimates look like this:
>
> EXISTS, no index: 1.6 billion
> EXISTS, with index: 0.023 billion
> IN, no index: 13.7 billion
> IN, with index: 10.6 billion
>
> At least for the two EXISTS cases, the estimates were roughly accurate.
> These plans were run against the data after the fix, but analyze has
> not been run since then, so the estimates should be comparable with the
> earlier post.
>
> I'm not used to using the IN construct this way, so maybe someone can
> spot something horribly stupid in how I tried to use it.
I will have a look at your queries tomorrow. Some general advice (rdbms
agnostic) on when to use IN and when to use EXISTS taken from "SQL
performance tuning":
- if the inner table has few rows and the outer has many then IN is
preferred
- if however you have a restrictive expression on the outer query you
should preferr EXISTS
- use NOT EXISTS instead of NOT IN (break out early)
regards,
Lukas
