Re: [HACKERS] MERGE SQL Statement for PG11

On 26 March 2018 at 17:52, Robert Haas <robertmhaas@gmail.com> wrote:
> On Mon, Mar 26, 2018 at 12:16 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
>> On 26 March 2018 at 16:09, Robert Haas <robertmhaas@gmail.com> wrote:
>>> On Mon, Mar 26, 2018 at 5:53 AM, Simon Riggs <simon@2ndquadrant.com> wrote:
>>>> Since we now have MVCC catalog scans, all the name lookups are
>>>> performed using the same snapshot so in the above scenario the newly
>>>> created object would be invisible to the second name lookup.
>>>
>>> That's not true, because each lookup would be performed using a new
>>> snapshot -- not all under one snapshot.
>>
>> You're saying we take a separate snapshot for each table we lookup?
>> Sounds weird to me.
>
> I'm saying we take a separate snapshot for each and every catalog
> lookup, except when we know that no catalog changes can have occurred.
> See the commit message for 568d4138c646cd7cd8a837ac244ef2caf27c6bb8.
> If you do a lookup in pg_class and 3 lookups in pg_attribute each of
> the 4 can be done under a different snapshot, in the worst case.
> You're not the first person to believe that the MVCC catalog scan
> patch fixes that problem, but as the guy who wrote it, it definitely
> doesn't.  What that patch fixed was, prior to that patch, a catalog
> scan might find the WRONG NUMBER OF ROWS, like you might do a lookup
> against a unique index for an object that existed and, if the row was
> concurrently updated, you might find 0 rows or 2 rows instead of 1
> row.  IOW, it guaranteed that we used a consistent snapshot for each
> individual lookup, not a consistent snapshot for the whole course of a
> command.

That all makes sense, thanks for explaining.

I spent a few more minutes, going "but", "but" though I can now see
good reasons for everything to work this way.

>> So this error could happen in SELECT, UPDATE, DELETE or INSERT as well.
>>
>> Or you see this as something related specifically to MERGE, if so how?
>> Please explain what you see.
>
> As I said before, the problem occurs if the same command looks up the
> same table name in more than one place.  There is absolutely nothing
> to guarantee that we get the same answer every time.

> As far as I
> know, the proposed MERGE patch has that issue an existing DML commands
> don't; but someone else may have better information.

I will look deeper and report back.

-- 
Simon Riggs                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services