VACUUM touching file but not updating relation

Hi,

I just noticed that the VACUUM process touches a lot of relations
(affects mtime) but for one file I looked at, it didn't change. This
doesn't always happen, and many relations aren't touched at all.

I had the following relation:

-rw------- 1 thom staff 40960 13 Oct 16:06 11946

Ran MD5 over the file:

MD5 (11946) = d6626f930f1fb6d77c3907d3279fe693

Then VACUUM ANALYSE'd all databases in full.

This relation was supposedly affected:

-rw------- 1 thom staff 40960 14 Oct 11:27 11946

But then I ran MD5 back over it:

MD5 (11946) = d6626f930f1fb6d77c3907d3279fe693

This is the same as before. What is it doing? Does this happen
often? And I can't find out what this particular OID relates to
either.

I'm using 9.2devel btw.

--
Thom Brown
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