On Thu, Nov 29, 2012 at 5:03 PM, Guillaume Cottenceau
<gc@mnc.ch> wrote:
Hello,
I am toying around with 9.2.1, trying to measure/determine how
index-only scans can improve our performance.
A small script which is attached to this mail, shows that as long
as the table has been VACUUM FULL'd, there is a unusual high
amount of heap fetches. It is strange that the visibilitymap_test
predicate fails in these situations, is the visibility map
somehow trashed in this situation? It should not, or at least the
documentation[1] should state it (my understanding is that vacuum
full does *more* than vacuum, but nothing less) (note to usual
anti vacuum full trollers: I know you hate vacuum full).
I don't find it very surprising given that VACUUM FULL is now implemented as a CLUSTER command which rewrites the entire heap, thus invalidating all the visibility map info whatsoever. The code paths that VACUUM FULL and LAZY VACUUM takes are now completely different.
Even with the old VACUUM FULL we would have seen some impact on heap fetches because it used to move tuples around and thus potentially resetting visibility map bits. But its definitely going to be worse with the new implementation.
Now can CLUSTER or VACUUM FULL recreate the visibility map with all bits set to visible, thats an entirely different question. I don't think it can, but then I haven't thought through this completely.
Thanks,
Pavan
