> In this particular case the inaccurate estimate doesn't matter too much,
> I think, although it might be encouraging the system to select hash
> aggregation since it thinks the hashtable will be pretty small. If the
> estimate were getting used to plan higher-up plan steps then it could
> be a bigger problem.
That's my problem; this is a subselect feeding in to a larger query. That
wrong estimate causes the planner to select a nested-loop at the next step
up. At 83,000 rows, the word is "ouch!"
At any rate, I discovered this while dissecting a giant & slow query.
Hence, while disabling nested-loop joins avoids this particular pitfall,
it's not good for the bigger picture. I think I'm going to end up
splitting this larger query into smaller parts and reassemble the pieces
in the application so I can push some smarts past other subselect
boundaries. For my purposes, that should skirt the issue of union+group
estimates not being calculated.
As always, thanks for the fast answers!
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