Re: -HEAD planner issue wrt hash_joins on dbt3 ?

Stefan Kaltenbrunner <stefan(at)kaltenbrunner(dot)cc> writes:
> Tom Lane wrote:
>> Apparently we've made the planner a bit too optimistic about the savings
>> that can be expected from repeated indexscans occurring on the inside of
>> a join.

> effective_cache_size was set to 10GB(my fault for copying over the conf
> from a 16GB box) during the run - lowering it just a few megabytes(!) or
> to a more realistic 6GB results in the following MUCH better plan:
> http://www.kaltenbrunner.cc/files/dbt3_explain_analyze2.txt

Interesting. It used to be that effective_cache_size wasn't all that
critical... what I think this report is showing is that with the 8.2
changes to try to account for caching effects in repeated indexscans,
we've turned that into a pretty significant parameter.

It'd be nice not to have to depend on the DBA to give us a good number
for this setting. But I don't know of any portable ways to find out
how much RAM is in the box, let alone what fraction of it we should
assume is available per-query.

regards, tom lane