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Help Center/ GaussDB(DWS)/ FAQs/ Database Usage/ How Do I Use a User-Defined Function to Rewrite the CRC32() Function?

How Do I Use a User-Defined Function to Rewrite the CRC32() Function?

Updated on 2023-11-21 GMT+08:00

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Currently, GaussDB(DWS) does not have a built-in CRC32() function. Instead, you can use the user-defined function of GaussDB(DWS) to rewrite the CRC32() function.

CRC32(expr)
Description: Calculates the cyclic redundancy. The input parameter expr is a string. If the parameter is NULL, NULL is returned. Otherwise, a 32-bit unsigned value is returned after redundancy calculation.

Example of rewriting the CRC32() function using the user-defined function statement of GaussDB(DWS):

CREATE OR REPLACE FUNCTION crc32(text_string text) RETURNS bigint AS $$
DECLARE
    val bigint;
    i int;
    j int;
    byte_length int;
    binary_string bytea;
BEGIN
    IF text_string is null THEN
        RETURN null;
    ELSIF text_string = '' THEN
        RETURN 0;
    END IF;

    i = 0;
    val = 4294967295;
    byte_length = bit_length(text_string) / 8;
    binary_string = decode(replace(text_string, E'\\', E'\\\\'), 'escape');
    LOOP
        val = (val # get_byte(binary_string, i))::bigint;
        i = i + 1;
        j = 0;
        LOOP
            val = ((val >> 1) # (3988292384 * (val & 1)))::bigint;
            j = j + 1;
            IF j >= 8 THEN
                EXIT;
            END IF;
        END LOOP;
        IF i >= byte_length THEN
            EXIT;
        END IF;
    END LOOP;
    RETURN (val # 4294967295);
END
$$ IMMUTABLE LANGUAGE plpgsql;

Verify the rewriting result.

select crc32(null),crc32(''),crc32('1');
 crc32 | crc32 |   crc32
-------+-------+------------
       |     0 | 2212294583
(1 row)

For details about how to use user-defined functions, see CREATE FUNCTION.

Parent topic: Database Usage

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