Re: Tricky bugs in concurrent index build

stark <stark@enterprisedb.com> writes:
> Tom Lane <tgl@sss.pgh.pa.us> writes:
>> [ thinks for a bit... ]  At least, it seems hopeless if we use
>> SnapshotNow.  Does it help if we use a real snapshot?  I'm thinking
>> pass 1 inserts exactly those tuples that are good according to a
>> snap taken at its beginning, and then pass 2 considers only tuples
>> that are good according to a snap taken at *its* beginning.  But
>> having consumed no caffeine yet this morning, I'm not sure I can
>> spot any flaws that might exist in this idea.

> What about tuples that are inserted and committed in the window between the
> two phases. Ie, they're RECENTLY_DEAD but not in phase2's snapshot.

We'd put them in the index but skip uniqueness check.

> Or do you mean we use SatisfiesVacuum to determine what to insert but
> SatisfiesSnapshot to determine whether to check uniqueness?

Right.  The problems seem to all stem from the risk of trying to
unique-check more than one version of a tuple, and using a snap would
stop that.  We need to think through all the cases though and be sure
they all work.
        regards, tom lane