PostgreSQL create index on cast from string to date - postgresql

Question

I'm trying to create an index on the cast of a varchar column to date. I'm doing something like this:
CREATE INDEX date_index ON table_name (CAST(varchar_column AS DATE));
I'm getting the error: functions in index expression must be marked IMMUTABLE But I don't get why, the cast to date doesn't depends on the timezone or something like that (which makes a cast to timestamp with time zone give this error).
Any help?

Answer 1

Your first error was to store a date as a varchar column. You should not do that.
The proper fix for your problem is to convert the column to a real date column.
Now I'm pretty sure the answer to that statement is "I didn't design the database and I cannot change it", so here is a workaround:
CAST and to_char() are not immutable because they can return different values for the same input value depending on the current session's settings.
If you know you have a consistent format of all values in the table (which - if you had - would mean you can convert the column to a real date column) then you can create your own function that converts a varchar to a date and is marked as immutable.
create or replace function fix_bad_datatype(the_date varchar)
returns date
language sql
immutable
as
$body$
select to_date(the_date, 'yyyy-mm-dd');
$body$
ROWS 1
/
With that definition you can create an index on the expression:
CREATE INDEX date_index ON table_name (fix_bad_datatype(varchar_column));
But you have to use exactly that function call in your query so that Postgres uses it:
select *
from foo
where fix_bad_datatype(varchar_column) < current_date;
Note that this approach will fail badly if you have just one "illegal" value in your varchar column. The only sensible solution is to store dates as dates,

Answer 2

Please provide the database version, table ddl, and some example data.
Would making your own immutable function do what you want, like this? Also look into creating a new cast in the docs and see if that does anything for you.
create table emp2 (emp2_id integer, hire_date VARCHAR(100));
insert into emp2(hire_date)
select now();
select cast(hire_date as DATE)
from emp2
CREATE FUNCTION my_date_cast(VARCHAR) RETURNS DATE
AS 'select cast($1 as DATE)'
LANGUAGE SQL
IMMUTABLE
RETURNS NULL ON NULL INPUT;
CREATE INDEX idx_emp2_hire_date ON emp2 (my_date_cast(hire_date));