Convert value from string representation in base N to numeric - postgresql

Question

On my database i keep a numbers in specific notation as varchar values. Is there any way to typecast this values into decimals with selected notation?
What i basically looking here should look like this:
SELECT to_integer_with_notation('d', '20')
int4
---------
13
One more example:
SELECT to_integer_with_notation('d3', '23')
int4
---------
302

Answer 1

Unfortunately, there is no built-in function for that in PostgreSQL, but can be written fairly easy:
CREATE OR REPLACE FUNCTION number_from_base(num TEXT, base INTEGER)
RETURNS NUMERIC
LANGUAGE sql
IMMUTABLE
STRICT
AS $function$
SELECT sum(exp * cn)
FROM (
SELECT base::NUMERIC ^ (row_number() OVER () - 1) exp,
CASE
WHEN ch BETWEEN '0' AND '9' THEN ascii(ch) - ascii('0')
WHEN ch BETWEEN 'a' AND 'z' THEN 10 + ascii(ch) - ascii('a')
END cn
FROM regexp_split_to_table(reverse(lower(num)), '') ch(ch)
) sub
$function$;
Note: I used numeric as a return type, as int4 is not enough in many cases (with longer string input).
Edit: Here is a sample reverse function, which can convert a bigint to its text representation within a custom base:
CREATE OR REPLACE FUNCTION number_to_base(num BIGINT, base INTEGER)
RETURNS TEXT
LANGUAGE sql
IMMUTABLE
STRICT
AS $function$
WITH RECURSIVE n(i, n, r) AS (
SELECT -1, num, 0
UNION ALL
SELECT i + 1, n / base, (n % base)::INT
FROM n
WHERE n > 0
)
SELECT string_agg(ch, '')
FROM (
SELECT CASE
WHEN r BETWEEN 0 AND 9 THEN r::TEXT
WHEN r BETWEEN 10 AND 35 THEN chr(ascii('a') + r - 10)
ELSE '%'
END ch
FROM n
WHERE i >= 0
ORDER BY i DESC
) ch
$function$;
Example usage:
SELECT number_to_base(1248, 36);
-- +----------------+
-- | number_to_base |
-- +----------------+
-- | yo |
-- +----------------+