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
