Re: Help Me Understand Why I'm Getting a Bad Query Plan

Bryan Murphy <bmurphy1976(at)gmail(dot)com> writes:
> I tried that already, but I decided to try again in case I messed up
> something last time. Here's what I ran. As you can see, it still
> chooses to do a sequential scan. Am I changing the stats for those
> columns correctly?

I think what you should be doing is messing with the cost parameters
... and not in the direction you tried before. I gather from
effective_cache_size = 12GB
that you have plenty of RAM on this machine. If the tables involved
are less than 1GB then it's likely that you are operating in a fully
cached condition, and the default cost parameters are not set up for
that. You want to be charging a lot less for page accesses relative to
CPU effort. Try reducing both seq_page_cost and random_page_cost to 0.5
or even 0.1. You'll need to watch your other queries to make sure
nothing gets radically worse though ...

regards, tom lane