Thread

Re: explain analyze rows=%.0f

Ilia Evdokimov <ilya.evdokimov@tantorlabs.com> — 2025-03-31T20:59:48Z

On 31.03.2025 22:09, Robert Haas wrote:
> Oh, right. I've never really understood why we round off to integers,
> but the fact that we don't allow row counts < 1 feels like something
> pretty important. My intuition is that it probably helps a lot more
> than it hurts, too.


We definitely shouldn’t remove the row counts < 1 check, since there are 
many places in the planner where we divide by rows. This mechanism was 
added specifically to prevent division by zero. Also, allowing rows 
estimates below 1 can sometimes make the planner overly optimistic, 
leading it to prefer cheaper-looking plans that may not perform well in 
practice. For example, choosing a Nested Loop instead of a more 
appropriate Hash Join.

Allowing fractional rows > 1 might help improve planner accuracy in some 
cases, but this needs further study to fully understand the impact.

--
Best regards,
Ilia Evdokimov,
Tantor Labs LLC.