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Thread: Freeing transient memory in aggregate functions

Freeing transient memory in aggregate functions

From
Matt Magoffin
Date:
I have a question about trying to keep memory from growing too much in a C aggregate function with pass-by-reference types. I am trying to keep track of a last-seen value in my aggregate state, so I have code roughly doing this:

Datum current;
MemoryContext aggContext;
AggCheckCallContext(fcinfo, &aggContext);
old = MemoryContextSwitchTo(aggContext);

if (!PG_ARGISNULL(0)) {
  current = PG_GETARG_DATUM(0);
  state->last = datumCopy(&current, typbyval, typlen);
}
MemoryContextSwitchTo(old);

I’m essentially doing a datumCopy() on every non-null input value. I was wondering if there is a way to free the previously copied datum, since I don’t really need it anymore? Something like

if (!PG_ARGISNULL(0)) {
  current = PG_GETARG_DATUM(0);
  if (state->last != NULL) {
    pfree(state->last);
  }
  state->last = datumCopy(&current, typbyval, typlen);
}

I wasn’t sure it was allowed to call pfree() like this. My actual function is dealing with array input values, and for large sets of inputs I didn’t want to grow memory use as large as the entire data set being aggregated.

Kind regards,
Matt

Re: Freeing transient memory in aggregate functions

From
Tom Lane
Date:
Matt Magoffin <postgresql.org@msqr.us> writes:
> I’m essentially doing a datumCopy() on every non-null input value.

If you're doing that in the aggContext, you definitely need to free
the prior one(s).

> I was wondering if there is a way to free the previously copied datum, since I don’t really need it anymore?
Somethinglike 

>   if (state->last != NULL) {
>     pfree(state->last);
>   }

Not quite like that.  Look into nodeAgg.c, which solves a similar problem
for the transvalues themselves with code like

            /* forget the old value, if any */
            if (!oldIsNull && !pertrans->inputtypeByVal)
                pfree(DatumGetPointer(oldVal));

            regards, tom lane