12.10.10 14:44, Craig Ringer ÐÐÐÐÑÐÐ(ÐÐ):
in the case where you are doing a count(*) where query and the where is
on an indexed column, could the search just look at the index + the
visibility mapping rather than doing an sequential search through the
table?
Nope, because the visibility map, which is IIRC only one bit per page,
doesn't record how many tuples there are on the page, or enough
information about them to determine how many of them are visible to
the current transaction*.
I'd say it can tell you that your may not recheck given tuple, can't it?
You still have to count all index tuples and recheck the ones that are
uncertain. Does it work in this way? This can help a lot for wide tuples
in table, but with narrow index and mostly read-only data.
Best regards, Vitalii Tymchyshyn
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