Re: Damage control for planner's get_actual_variable_endpoint() runaway

Andres Freund <andres@anarazel.de> writes:
> On 2022-11-21 16:17:56 -0500, Robert Haas wrote:
>> But ... what if they're not? Could the index contain a large number of
>> pages containing just 1 tuple each, or no tuples at all? If so, maybe
>> we can read ten bazillion index pages trying to find each heap tuple
>> and still end up in trouble.

> ISTM that if you have an index in such a poor condition that a single
> value lookup reads thousands of pages inside the index, planner
> estimates taking long is going to be the smallest of your worries...

Yeah, that sort of situation is going to make any operation on the
index slow, not only get_actual_variable_endpoint().

I think we should content ourselves with improving the demonstrated
case, which is where we're forced to do a lot of heap fetches due
to lots of not-all-visible tuples.  Whether we can spend a lot of
time scanning the index without ever finding a tuple at all seems
hypothetical.  Without more evidence of a real problem, I do not
wish to inject warts as horrid as this one into the index AM API.

            regards, tom lane