RE: Query ID Calculation Fix for DISTINCT / ORDER BY and LIMIT / OFFSET

Hello!

>> Variant B is not acceptable IMO as it adds a whole bunch of 
>> null-terminators unnecessarily. For example, in a simple "select 1", 
>> (expr == NULL) is true 19 times, so that is an extra 19 bytes.

> Variant B is not acceptable here.

Could we improve Variant B?

I was thinking about adding a count of NULL pointers encountered by the query
jumble walker since the last scalar or non-null field visited.
This way, we can traverse the query as follows:
====
QueryA->subNodeOne = &(Value X);
/* JumbleState->Count = 0;
   QueryA_ID = hash_any_extended(&(Value X), sizeof(Value X), QueryA_ID) */
QueryA->subNodeTwo = NULL;
/* JumbleState->Count = 1; */
QueryA->subNodeThee = NULL;
/* JumbleState->Count = 2; */
QueryA->subNodeFour = NULL;
/* JumbleState->Count = 3; */
QueryA->scalar = Value Z;
/* QueryA_ID = hash_any_extended(&(JumbleState->Count),
    sizeof(JumbleState->Count), QueryA_ID)
   JumbleState->Count = 0;
   QueryA_ID = hash_any_extended(&(Value Y), sizeof(Value Y), QueryA_ID) */
====

Variant A improvement
---------------------

I have attached the updated Variant A patch with the following changes:
- Implemented a pg_stat_statements regression test (see similar test results
  on REL_17_3 in Query-ID-collision-at_pg_stat_statements-1ea6e890b22.txt).
- Added extra description about the Query ID collision
  in src/include/nodes/parsenodes.h.