Re: Why enable_hashjoin Completely disables HashJoin

Quan Zongliang <quanzongliang@yeah.net> writes:
> I found that the enable_hashjoin disables HashJoin completely.

Well, yeah.  It's what you asked for.

> Instead, it should add a disable cost to the cost calculation of 
> hashjoin.

Why?  The disable-cost stuff is a crude hack that we use when
turning off a particular plan type entirely might render us
unable to generate a valid plan.  Hash join is not in that
category.

> After disabling all three, the HashJoin path should still be chosen.

Why?

Personally, I'd get rid of disable_cost altogether if I could.
I'm not in a hurry to extend its use to more places.

            regards, tom lane