aggregate functions on massive number of rows

I have an aggregate function setup (attached) that I'm calling over a
massive amount of data and am running into:

ERROR: cannot have more than 2^32-1 commands in a transaction
CONTEXT: PL/pgSQL function "float8_jitter_add" line 16 at if

error. Since I'm able to do count() and avg() over the same values
without this aggregate function, it's theoretically possible.

Something was making me think that it was the extract() that was doing
it (I used to have 'select extract(milliseconds from v_rtt_in) into
v_rtt' and something leaded me to believe the error was with that), but
I don't recall what that is, so it looks like it's just the if like it
says.

Since I'm not really doing anything transaction-oriented in
float8_jitter_add, is there a way to get around this? Or is this a
limitation in pl/pgsql?

thanks in advance,
-Todd

---<snip>---
create or replace function float8_jitter_add(float8[], interval)
returns float8[] as '
declare
v_old_state ALIAS FOR $1;
v_rtt_in ALIAS FOR $2;
v_state float8[];
v_rtt float8;
BEGIN
v_state := v_old_state;
v_rtt := extract(milliseconds from v_rtt_in);

IF v_old_state is NULL THEN
v_state := ''{0,0,0,0}'';
v_state[1] = 0;
v_state[2] = 0;
v_state[3] = v_rtt;
v_state[4] = 1;
ELSIF v_rtt IS NOT NULL THEN
if v_old_state[4] = 1 THEN
v_state[1] := v_old_state[2] + (v_old_state[3] - v_rtt);
v_state[2] := v_old_state[2] + 1;
END IF;
v_state[3] := v_rtt;
v_state[4] := 1;
ELSE
v_state[4] := 0;
END IF;
return v_state;
END;
' language 'plpgsql';

create or replace function float8_jitter_sum(float8[])
returns float8 as '
declare
v_state ALIAS FOR $1;
v_avg float8;
BEGIN
v_avg := NULL;
if v_state[1] != 0 THEN
v_avg := v_state[1] / v_state[2];
END IF;
return v_avg;
END;
' language 'plpgsql';

create aggregate jitter (
basetype = interval,
sfunc = float8_jitter_add,
stype = float8[],
finalfunc = float8_jitter_sum
);