How do I search for a range of integers in PostgreSQL?

Question

I have an integer column and I need to search for rows in which the said column starts with 19

In MySQL I would use SELECT ... WHERE id LIKE \'19%\'

Answer 1

For all positive integers >= 10, you can do this with no TEXT/VARCHAR necessary:

SELECT * FROM "table"
WHERE $1 = id / (10 ^ (floor(log(id)-1)))::integer

You can index this, as well:

CREATE INDEX idx_table_2_digits 
    ON "table" ( 10 / (id ^ (floor(log(id)-1)))::integer);

The calculation basically works like this:

1. Find the largest power of 10 that is less than a given ID (let's call it X).
2. Drop the magnitude of X by one (since we want two digits)
3. Raise 10 to the power of X and cast to the result to INTEGER type (to force last step to be a SQL DIV instead of floating-point division).
4. DIV the original ID by the 10^x to get the first two digits.

This algorithm could even be generalized into a function, f(x,y), where x is the id and y is the number of leading digits to return:

# python-esque pseudocode:

def integer_leading_digits (id, digits):
    if id < (10 * (digits - 1)):
        return -1        # error condition
    return id DIV int(10 ^ (floor(log(id)-(digits-1))))