On 6/6/16 3:57 PM, Peter Geoghegan wrote:
On Mon, Jun 6, 2016 at 11:35 AM, Andres Freund <and...@anarazel.de> wrote:
We need a read-only utility which checks that the system is in a correct
and valid state.  There are a few of those which have been built for
different pieces, I believe, and we really should have one for the
visibility map, but I don't think it makes sense to imply in any way
that VACUUM can or should be used for that.


Meh. This is vacuum behaviour that *has existed* up to this point. You
essentially removed it. Sure, I'm all for adding a verification
tool. But that's just pie in the skie at this point.  We have a complex,
data loss threatening feature, which just about nobody can verify at
this point. That's crazy.

FWIW, I agree with the general sentiment. Building a stress-testing
suite would have been a good idea. In general, testability is a design
goal that I'd be willing to give up other things for.

Related to that, I suspect it would be helpful if it was possible to test boundary cases in this kind of critical code by separating the logic from the underlying implementation. It becomes very hard to verify the system does the right thing in some of these scenarios, because it's so difficult to put the system into that state to begin with. Stuff that depends on burning through a large number of XIDs is an example of that. (To be clear, I'm talking about unit-test kind of stuff here, not validating an existing system.)
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Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
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