On 11.11.2011 17:47, Tom Lane wrote:
Robert Haas<robertmh...@gmail.com> writes:
On Fri, Nov 11, 2011 at 9:59 AM, Tom Lane<t...@sss.pgh.pa.us> wrote:
It occurs to me to wonder how this scenario will interact with the
recent changes to let VACUUM skip pages. AFAIR there is not a way for a
manual VACUUM to set the anti-wraparound mode, is there?
I tweaked Simon's original patch to address exactly this scenario;
VACUUM FREEZE prevents page-skipping behavior.
That doesn't address my concern. (1) The manual does not say you must
use VACUUM FREEZE for this, nor do the HINT messages. (2) You probably
wouldn't want to use VACUUM FREEZE, as that could force a great deal
more I/O than might be necessary to fix the problem.
set vacuum_freeze_table_age=0; VACUUM;
will do the trick.
(3) In disaster
recovery scenarios, the last thing we want is to be imposing extra
conditions on what an already-stressed DBA has to do to fix things;
especially extra conditions that are different from the way it's worked
for the last ten years.
True.
And there's also (4) if someone is doing a
manual VACUUM, they might well wish the table to be completely vacuumed,
not just sort of.
I think we'd be better advised to restrict the page-skipping behavior
to autovacuums, period, and remove the connection to FREEZE.
Yeah, I think you're right. We can accomplish that by setting the
default vacuum_freeze_age to 0. That way it's still possible to get the
page-skipping behavior in manual VACUUMs by setting it to non-zero, but
you get a full scan by default.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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