From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Andres Freund <andres(at)anarazel(dot)de> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Simon Riggs <simon(dot)riggs(at)enterprisedb(dot)com>, pgsql-hackers(at)lists(dot)postgresql(dot)org, Jakub Wartak <jakub(dot)wartak(at)enterprisedb(dot)com>, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Damage control for planner's get_actual_variable_endpoint() runaway |
Date: | 2022-11-21 22:15:53 |
Message-ID: | 3342023.1669068953@sss.pgh.pa.us |
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Andres Freund <andres(at)anarazel(dot)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
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