Re: Multiple Query IDs for a rewritten parse tree

> On Mon, Jan 31, 2022 at 02:59:17PM +0500, Andrey V. Lepikhov wrote:
> On 1/28/22 9:51 PM, Dmitry Dolgov wrote:
> > > On Fri, Jan 21, 2022 at 11:33:22AM +0500, Andrey V. Lepikhov wrote:
> > > Registration of an queryId generator implemented by analogy with extensible
> > > methods machinery.
> >
> > Why not more like suggested with stakind and slots in some data
> > structure? All of those generators have to be iterated anyway, so not
> > sure if a hash table makes sense.
> Maybe. But it is not obvious. We don't really know, how many extensions
> could set an queryId.
> For example, adaptive planning extensions definitely wants to set an unique
> id (for example, simplistic counter) to trace specific {query,plan} across
> all executions (remember plancache too). And they would register a personal
> generator for such purpose.

I don't see how the number of extensions justify a hash table? I mean,
all of the generators will still most likely be executed at once (not on
demand) and store the result somewhere.

In any way I would like to remind, that this functionality wants to be
on the pretty much hot path, which means strong evidences of no
significant performance overhead are needed. And sure, there could be
multiple queryId consumers, but in the vast majority of cases only one
queryId will be generated.

> > > 2. We need a custom queryId, that is based on a generated queryId (according
> > > to the logic of pg_stat_statements).
> >
> > Could you clarify?
> pg_stat_statements uses origin queryId and changes it for a reason
> (sometimes zeroed it, sometimes not).

IIRC it does that only for utility statements. I still fail to see the
problem, why would a custom extension not register a new generator if it
needs something different?

> > > 5. We should add an EXPLAIN hook, to allow an extension to print this custom
> > > queryId.
> >
> > Why? It would make sense if custom generation code will be generating
> > some complex structure, but the queryId itself is still a hash.
> >
> Extension can print not only queryId, but an explanation of a kind, maybe
> additional logic.
> Moreover why an extension can't show some useful monitoring data, collected
> during an query execution, in verbose mode?

A good recommendation which pops up every now and then in hackers, is to
concentrace on what is immediately useful, leaving "maybe useful" things
for later. I have a strong feeling this is the case here.