Re: Optimize join selectivity estimation by not reading MCV stats for unique join attributes

Richard Guo <guofenglinux@gmail.com> writes:
> Yes, it is.  Using zero flag would short-cut get_attstatsslot() to just
> return whether the slot type exists without loading it.  Do you think we
> need to emphasize this use case in the comments for 'flags'?

Perhaps, it's not really obvious now.

> I wonder whether we need to also check statistic_proc_security_check()
> when determining if MCVs exists in both sides.

Yeah, I thought about hoisting the statistic_proc_security_check
tests up into get_mcv_stats.  I don't think it's a great idea
though.  Again, it'd complicate untangling this if we ever
generalize the use of MCVs in this function.  Also, I don't
think we should be micro-optimizing the case where the security
check doesn't pass --- if it doesn't, you're going to be hurting
from bad plans a lot more than you are from some wasted cycles
here.

            regards, tom lane