Postgres Date checking - is_date fn shortcircuit - postgresql

Question

I have a set of records in a table, with some records having invalid date. I wanted to ignore those invalid records and do a check with rest of the records. I framed a query like below but I don't find it working.
select * from tbl_name i
where is_date(i.dob) and i.dob::date > CURRENT_DATE;
I got to know that sql doesn't short circuit so it also consider invalid record and end up in date/time out of range. Please help me alter this query in a way i could eliminate invalid dates and do date comparison on valid dates only.

Answer 1

There is no guarantee for short-circuiting in Postgres. Neither in a "plain" WHERE clause, nor when using a derived table (from (select ...) where ...). One way to force the evaluation in two steps would be a materialized common table expressions:
with data as materialized (
select *
from tbl_name i
where is_date(i.dob)
)
select *
from data
where dob::date > CURRENT_DATE;
The materialized keyword prevents the optimizer from pushing the condition of the outer query into the CTE.
Obviously this assumes that is_date() will never return false positives

Answer 2

Using CASE in the WHERE to differentiate between a valid date and an invalid one and run the > comparison for valid date otherwise return FALSE.
create or replace function is_date(s varchar) returns boolean as $$
begin
if s is null then
return false;
end if;
perform s::date;
return true;
exception when others then
return false;
end;
$$ language plpgsql;
create table date_str (id integer, dt_str varchar);
insert into date_str values (1, '2022-11-02'), (2, '1234'), (3, '2022-12-03');
insert into date_str values (4, 'xyz'), (5, '2022-01-01'), (6, '2023-02-02');
select * from date_str;
id | dt_str
----+------------
1 | 2022-11-02
2 | 1234
3 | 2022-12-03
4 | xyz
5 | 2022-01-01
6 | 2023-02-02
select current_date;
current_date
--------------
11/02/2022
SELECT
*
FROM
date_str
WHERE
CASE WHEN is_date (dt_str) = 't' THEN
dt_str::date > CURRENT_DATE
ELSE
FALSE
END;
id | dt_str
----+------------
3 | 2022-12-03
6 | 2023-02-02