Re: A qsort template

On Sun, Apr 3, 2022 at 11:11 AM Andres Freund <andres@anarazel.de> wrote:
> On 2022-04-03 09:45:13 +1200, Thomas Munro wrote:
> > I think we just need to decide up front if we're in a situation that
> > can't provide datum1/isnull1 (in this case because it's an expression
> > index), and skip the optimised paths.  Here's an experimental patch...
> > still looking into whether there are more cases like this...

I didn't find anything else.

Maybe it'd be better if we explicitly declared whether datum1 is used
in each tuplesort mode's 'begin' function, right next to the code that
installs the set of routines that are in control of that?  Trying that
in this version.  Is it clearer what's going on like this?

> That's a lot of redundant checks. How about putting all the checks for
> optimized paths into one if (state->sortKeys && !state->disabl1e_datum1)?

OK, sure.

> I'm a bit worried that none of the !ubsan tests failed on this...

In accordance with whoever-it-was-that-said-that's law about things
that aren't tested, this are turned out to be broken already[1].  Once
we fix that we should have a new test in the three that might also
eventually have failed under this UB, given enough chaos.

[1]
https://www.postgresql.org/message-id/CA%2BhUKG%2BbA%2BbmwD36_oDxAoLrCwZjVtST2fqe%3Db4%3DqZcmU7u89A%40mail.gmail.com