Re: Query Regarding frame options initialization in Window aggregate state

[Tom Lane]

Vallimaharajan G  writes:
> 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.

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

Query Regarding frame options initialization in Window aggregate state

[Vallimaharajan G]

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