Re: What exactly is drop-index-concurrently-1.spec trying to test?

On 2013-12-31 17:14:11 -0500, Tom Lane wrote:
> Peter pointed out in
> http://www.postgresql.org/message-id/527C0FE9.7000600@gmx.net
> that Kyotaro-san's patch to treat unique indexes as satisfying any sort
> condition that they are a prefix of broke the drop-index-concurrently-1
> isolation test.  The latest iterations of the patch respond to that by
> changing the expected output.  However, that seems rather wrongheaded,
> because AFAICT the intent of that part of the test is to demonstrate that
> a seqscan has particular behavior; so if the planner starts generating an
> indexscan instead, the test no longer proves anything of the kind.
> 
> What I'm wondering though is what's the point of testing that a concurrent
> DROP INDEX doesn't affect a seqscan?  That seems kinda silly, so it's
> tempting to address the patch's problem by just removing the steps
> involving the getrow_seq query, rather than hacking it up enough so we'd
> still get a seqscan plan.

The point is to show that an index scan returns the same rows a
sequential scan would, even though the index is in the process of being
dropped and has been updated *after* the DROP started. That was broken
at some point.
Now, you could argue that that would also be shown without the
sequential scan, but I think that would make understanding the faulty
output harder.

> I'd have thought the test would be designed to allow
> the DROP to complete and then re-test that the results of the prepared
> query are still sane, but it does no such thing.

We could add a permutation like this, but ISTM that it would just test
plan invalidation?

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
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