POSTGRES - evaluate (sub)expression error as NULL - sql

Question

I have sensor data in a Postgres table measurements with columns id, timestamp, s0, s1, s2, ...
Besides, there is an index on columns (id, timestamp). I want to allow for dynamic math expressions (in the example below: sin(s3)*0.1000/s5) for calculation of derived values.
SELECT
timestamp,
trunc((sin(s3) * 0.1000/s5)::numeric, 3) AS "calculated"
FROM measurements
WHERE id = 42
ORDER BY timestamp DESC
LIMIT 10000;
Obviously, this is prone to a "division by zero" error which will make the query fail. Is there a way to catch this error and return e.g. NULL for the calculated value where the error would occur?
Inspired by
Postgres return null values on function error/failure when casting
Store a formula in a table and use the formula in a function
I already tried defining a postgres function eval_numeric(sensors int[], formula text) that parses the formula and returns NULL on exception. The third row of the SQL statement above now reads
trunc(eval_numeric(ARRAY[s3,s5],'sin(var1)*0.1/var2'), 3) AS "calculated"
This gives the desired behavior but execution time as reported by EXPLAIN ANALYZE increases by a factor of 20 (~20ms -> ~400ms). Any other ideas?
UPDATE
The dynamic expression to be evaluated stems from a web application user. So the formula above is only an example (might require checking for negative argument to square root). I'd rather have a generic error checking possibility and would prefer not having logic in the math expression. This would be easier for the end user and I could validate the allowed math e.g. with a math parser thereby preventing SQL injection.

Answer 1

Can you change the expression to this?
SELECT timestamp,
trunc((sin(s3) * 0.1000/nullif(s5, 0))::numeric, 3) AS "calculated",
FROM measurements
WHERE id = 42
ORDER BY timestamp DESC
LIMIT 10000;
This is the simplest way to accomplish what you want to do.

Answer 2

You could also use a CASE expression:
SELECT
timestamp,
CASE WHEN s5 != 0
THEN trunc((sin(s3) * 0.1000/s5)::numeric, 3)
ELSE NULL AS "calculated",
FROM measurements
WHERE id = 42
ORDER BY timestamp DESC
LIMIT 10000;
This has the potential benefit that you may replace the value with anything you want, including NULL.
Another option, if you don't care about rows which would have triggered a divide by zero, would be to just add the check on s5 to the WHERE clause and filter off those rows before the division happens.