Math operations with lag()

I have a questdb 8.3.0 and i want to graph a typical snmp counter containing network incoming traffic (ifHCInOctets).

If I run this query:

SELECT 
    timestamp,
    ifDescr,
    ifHCInOctets,
    (LAG(ifHCInOctets,1)) OVER (PARTITION BY ifDescr ORDER BY timestamp) as previous
FROM snmpInterface
WHERE ifDescr = 'ether8' AND timestamp > 1745792550000000;

I get the current value and the previous one:

However, I want to get the substracted value (ifHCInOctets - LAG(...)) and I can't get it :(

this query works with InfluxDB 3.0:

SELECT ("ifHCInOctets" - LAG("ifHCInOctets", 1) OVER (PARTITION BY "ifDescr" ORDER BY time
  )) AS in, ("ifHCOutOctets" - LAG("ifHCOutOctets", 1) OVER (PARTITION BY "ifDescr" ORDER BY time
  )) AS out, time FROM "snmpInterface" WHERE "ifDescr" == 'ether8' AND "time" >= $__timeFrom AND "time" <= $__timeTo ORDER BY "time" ASC 

is it possible to have the same query with QuestDB ? How? thanks.

It should work using a subquery around the query with the window function:

SELECT timestamp, ifDescr, ifHCInOctets - previous AS difference FROM (
    SELECT 
        timestamp,
        ifDescr,
        ifHCInOctets,
        (LAG(ifHCInOctets,1)) OVER (PARTITION BY ifDescr ORDER BY timestamp) as previous
    FROM snmpInterface
    WHERE ifDescr = 'ether8' AND timestamp > 1745792550000000
);