Обсуждение: [MASSMAIL] Query Regarding frame options initialization in Window aggregate state

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[MASSMAIL] Query Regarding frame options initialization in Window aggregate state

От
Vallimaharajan G
Дата:
Hi Team,
     I am currently referring to the Postgres source code (psql (PostgreSQL) 14.3) and came across a particular section related to window aggregate initialization that has left me with a question. Specifically, I noticed a conditional case in the initialization of per aggregate (initialize_peraggregate in nodeWindowAgg.c) where the winstate frameOptions is being checked, but it appears that frameOptions is not set before this conditional case. I observed the same behaviour in PG version 16 as well.


/*
* Figure out whether we want to use the moving-aggregate implementation,
* and collect the right set of fields from the pg_attribute entry.
*
* It's possible that an aggregate would supply a safe moving-aggregate
* implementation and an unsafe normal one, in which case our hand is
* forced. Otherwise, if the frame head can't move, we don't need
* moving-aggregate code. Even if we'd like to use it, don't do so if the
* aggregate's arguments (and FILTER clause if any) contain any calls to
* volatile functions. Otherwise, the difference between restarting and
* not restarting the aggregation would be user-visible.
*/
if (!OidIsValid(aggform->aggminvtransfn))
use_ma_code = false; /* sine qua non */
else if (aggform->aggmfinalmodify == AGGMODIFY_READ_ONLY &&
aggform->aggfinalmodify != AGGMODIFY_READ_ONLY)
use_ma_code = true; /* decision forced by safety */
else if (winstate->frameOptions & FRAMEOPTION_START_UNBOUNDED_PRECEDING)
use_ma_code = false; /* non-moving frame head */
else if (contain_volatile_functions((Node *) wfunc))
use_ma_code = false; /* avoid possible behavioral change */
else
use_ma_code = true; /* yes, let's use it */
if (use_ma_code)
{
peraggstate->transfn_oid = transfn_oid = aggform->aggmtransfn;
peraggstate->invtransfn_oid = invtransfn_oid = aggform->aggminvtransfn;
peraggstate->finalfn_oid = finalfn_oid = aggform->aggmfinalfn;
finalextra = aggform->aggmfinalextra;
finalmodify = aggform->aggmfinalmodify;
aggtranstype = aggform->aggmtranstype;
initvalAttNo = Anum_pg_aggregate_aggminitval;
}
   
Frame options are being set to winstate after the initialization of peraggregate. (line 2504 in nodeWindowAgg.c)


/* copy frame options to state node for easy access */
winstate->frameOptions = frameOptions;

Could you kindly share the reason why this conditional case was added during the initialization of per aggregate, especially considering that winstate frameOptions is set after this initialization?

Thanks
Vallimaharajan G

Re: Query Regarding frame options initialization in Window aggregate state

От
Tom Lane
Дата:
Vallimaharajan G <vallimaharajan.gs@zohocorp.com> writes:
> I am currently referring to the Postgres source code (psql (PostgreSQL) 14.3) and came across a particular section
relatedto window aggregate initialization that has left me with a question. Specifically, I noticed a conditional case
inthe initialization of per aggregate (initialize_peraggregate in nodeWindowAgg.c) where the winstate frameOptions is
beingchecked, but it appears that frameOptions is not set before this conditional case. 

You are right, and that's a bug.  It's not of major consequence ---
it would just cause us to set up for moving-aggregate mode when
we won't ever actually move the frame head -- but it's at least a
little bit inefficient.

While I'm looking at it, there's a pretty obvious typo in the
adjacent comment:

- * and collect the right set of fields from the pg_attribute entry.
+ * and collect the right set of fields from the pg_aggregate entry.

            regards, tom lane