Re: [HACKERS] MERGE SQL Statement for PG11

Hi,

On 02/07/2018 10:24 AM, Pavan Deolasee wrote:
>
> ...
>
> Here is v15 of the patch.
>

I've been looking at this version of the patch, mostly to educate myself
before attempting to write the "status summary".

One bit that I don't quite understand is GetXactWALBytes(). It pretty
much just returns XactLastRecEnd and is used in ExecMerge like this:

int64 startWAL = GetXactWALBytes();
bool qual = ExecQual(action->whenqual, econtext);

/*
* SQL Standard says that WHEN AND conditions must not
* write to the database, so check we haven't written
* any WAL during the test. Very sensible that is, since
* we can end up evaluating some tests multiple times if
* we have concurrent activity and complex WHEN clauses.
*
* XXX If we had some clear form of functional labelling
* we could use that, if we trusted it.
*/
if (startWAL < GetXactWALBytes())
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("cannot write to database ...")));

I think this actually fails to enforce the rule, because some writes may
not produce WAL (think of unlogged tables). I also suspect it may be
incorrect "in the opposite direction" because a query may not do any
changes and yet it may produce WAL (e.g. due to wal_hint_bits=true).

So we may need to think of a different way to enforce this ...

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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