Zeugswetter Andreas OSB SD wrote:
Heikki wrote:
It seems that the worst case for this patch is a scan on a table that
doesn't fit in shared_buffers, but is fully cached in the OS cache. In
that case, the posix_fadvise calls would be a certain waste of time.
I think this is a misunderstanding, the fadvise is not issued to read
the
whole table and is not issued for table scans at all (and if it were it
would
only advise for the next N pages).
So it has nothing to do with table size. The fadvise calls need to be
(and are)
limited by what can be used in the near future, and not for the whole
statement.
Right, I was sloppy. Instead of table size, what matters is the amount
of data the scan needs to access. The point remains that if the data is
already in OS cache, the posix_fadvise calls are a waste of time,
regardless of how many pages ahead you advise.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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