Re: Single table forcing sequential scans on query plans

Cristian Gafton <gafton(at)rpath(dot)com> writes:
> On Sun, 16 Mar 2008, Tom Lane wrote:
>> While thinking about your report I was considering having VACUUM and
>> ANALYZE always set relpages to at least 1. Then seeing relpages=0
>> would indeed indicate a never-analyzed table, whereas relpages=1
>> when physical table size is zero could be taken to indicate that
>> we should trust the table to be really empty. I'm not sure though
>> whether this sort of convention would confuse any existing code.

> If having a discrepancy between relpages and table size is a concern,
> could relpages be a negative value to mark a non-analyzed table?

No, the value is really a uint32, though we don't declare it that way
for lack of having any such SQL type :-(. (uint32)-1 is just as legal
a value as 1, though perhaps a lot less likely. Anyway, client code
looking at the column is probably more likely to get confused by a
negative value for relpages than by a value that doesn't match
underlying reality (which it can't easily see anyway).

>> Could you confirm that your problem cases are actually caused by this
>> effect and not something else?

> Yes, confirmed. The runaway queries all are joining against an empty
> temporary table.

Good, just wanted to be sure. If there are not objections, I'll
put in the at-least-1 hack.

regards, tom lane