Querying on text values being casted into dates in postgresql - sql

Question

I was fiddling around with one of our databases earlier today and I was curious on how to do something in psql. Let's say I have a query like the following (with value1 being a text type in mytable):
SELECT * FROM mytable WHERE value1::date < '2013-10-24'::date;
This works fine if all the rows contain cast-able date strings. The second it finds a string that CAN NOT be casted into a date, an error is thrown like the following:
ERROR: invalid input syntax for type date: "C"
This makes sense and should happen. But is there a way to modify the above query so that if we come to a row where value1 would trigger this error that it would just move on, and skip over that row? I'm asking more out of curiosity than an actual need for an answer, and digging around on the web hasn't produced much (although, that could be do to the keywords I'm using, of course.)

Answer 1

You can use like-pattern or regex to pre-filter only value1s looking like dates:
SELECT * FROM mytable WHERE
value1 like '____-__-__'
and value1::date < '2013-10-24'::date;
SELECT * FROM mytable WHERE
value1 similar to '[1-2][0-9]{3}-[0-1][0-9]-[0-3][0-9]'
and value1::date < '2013-10-24'::date;
Here it is in SQLfiddle - http://sqlfiddle.com/#!1/06916/6

Answer 2

Technically we can't assume left-to-right evaluation order of the WHERE clauses, which means that in such a clause:
WHERE value1 ~ '^\d{4}-\d{2}-\d{2}$' AND value1::date < '2013-10-24'::date
the planner may decide to evaluate value1::date first and the execution will error out before testing the regexp. Should it estimate that the cast plus comparison is faster than the regexp test, it's a perfectly reasonable choice to make.
I don't think the current PostgreSQL code is sophisticated enough to do that specific rearrangement, but this problem is covered by the doc in Expression Evaluation Rules, and it recommends to use CASE to conditionally avoid the evaluation of problematic expressions.
Following this advice, the query would be like:
SELECT * FROM mytable WHERE
CASE WHEN value1 ~ '^\d{4}-\d{2}-\d{2}$'
THEN value1::date < '2013-10-24'::date
ELSE false
END;
Also if the content format seems to match a date but which happens to be invalid (e.g. 2013-01-32), the query will still fail. If this is a concern, you should encapsulate the cast in a function that traps the error:
create function cast_date(text) returns date as $$
begin
return $1::date;
exception when others then return null;
end; $$ language plpgsql;
and replace the test with cast_date(value1) < '2013-10-24'::date

Answer 3

May be this will work:
SELECT * FROM mytable WHERE value1 ~ '^\d{4}-\d{2}-\d{2}$' AND
value1::date < '2013-10-24'::date
regular expression will check if value1 is in needed format. and if it's not then cast to date shouldn't happen.