Re: Catching errors inside a LOOP is causing performance issues

I am adding an example that is runnable in order to demonstrate my point.

CREATE OR REPLACE FUNCTION initialVersion()
RETURNS VOID AS $$
declare
testDate DATE;
begin
for i in 1..999999 loop
begin
select now() into strict testDate where 1=0;
exception when others
then null;
end;
end loop;
end;
$$ Language plpgsql;

CREATE OR REPLACE FUNCTION secondVersion()
RETURNS VOID AS $$
declare
testDate DATE;
begin
for i in 1..999999 loop
select now() into testDate where 1=0;
if testDate is null then
continue;
end if;
end loop;
end;
$$ Language plpgsql;

select initialVersion(); -- 19.7 seconds

select secondVersion(); -- 5.2

As you can see there is a difference of almost 15 seconds.
In the example that I have provided initially, the difference is bigger because the SELECT FROM t1 runs against complex data and takes more time to execute that the simple SELECT provided in this second example.

Thanks,
Denisa

From: Denisa Cirstescu
Sent: Monday, September 25, 2017 7:12 PM
To: pgsql-general(at)postgresql(dot)org
Subject: Catching errors inside a LOOP is causing performance issues

Hi all,

I had a function with a performance issue:

totalCharge := 0;
FOR myRecord IN ... LOOP
......
IF severalConditionsAreMet THEN
BEGIN

SELECT t1.charge INTO STRICT recordCharge
FROM t1
WHERE t1.id = myRecord.id AND otherComplexConditionsHere;

totalCharge := totalCharge + recordCharge;

...........
EXCEPTION
WHEN OTHERS THEN
NULL;
END;
END IF;

END LOOP;

The function was being called 232 times (not counting the number of times the code from the FOR was accessed).
The IF from the FOR LOOP ended up being accessed 4466 times and was taking 561 seconds to complete all 4466 iterations.

For the particular data set that I had, the IF was always accessed, the SELECT from above never return data and the code was reaching the EXCEPTION branch each and every time.
I have changed the code to:

totalCharge := 0;
FOR myRecord IN ... LOOP
......
IF severalConditionsAreMet THEN

SELECT t1.charge INTO recordCharge
FROM t1
WHERE t1.id = myRecord.id AND otherComplexConditionsHere;

IF (recordCharge IS NULL) THEN
CONTINUE;
END IF;

totalCharge := totalCharge + recordCharge;

...........

END IF;

END LOOP;

Please note that for the table t1, the t1.charge column has a NOT NULL condition defined on it.
This time, the code from the IF takes 1-2 seconds to complete all 4466 iterations.

Basically, all I did was replace the

BEGIN
...
EXCEPTION
....
END;

With

IF conditionIsNotMet THEN
CONTINUE;
END IF;

Can someone please explain to me why this worked?
What happened behind the scenes?
I suspect that when you catch exceptions inside of a LOOP and the code ends up generating an exception, Postgres can't use cached plans to optimize that code so it ends up planning the code at each iteration and this causes performance issues.
Is my assumption correct?

Thanks a lot,
Denisa Cîrstescu