Convert hex in text representation to decimal number

Question

I am trying to convert hex to decimal using PostgreSQL 9.1

with this query:

SELECT to_number('DEADBEEF', 'FMXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX'); 

I get the following error:

ERROR:  invalid input syntax for type numeric: " " 

What am I doing wrong?

Answer 1

Ways without dynamic SQL

There is no cast from hex numbers in text representation to a numeric type, but we can use bit(n) as waypoint. There are undocumented casts from bit strings (bit(n)) to integer types (int2, int4, int8) - the internal representation is binary compatible. Quoting Tom Lane:

This is relying on some undocumented behavior of the bit-type input converter, but I see no reason to expect that would break. A possibly bigger issue is that it requires PG >= 8.3 since there wasn't a text to bit cast before that.

integer for max. 8 hex digits

Up to 8 hex digits can be converted to bit(32) and then coerced to integer (standard 4-byte integer):

SELECT ('x' || lpad(hex, 8, '0'))::bit(32)::int AS int_val FROM  (    VALUES       ('1'::text)     , ('f')     , ('100')     , ('7fffffff')     , ('80000000')     -- overflow into negative number     , ('deadbeef')     , ('ffffffff')     , ('ffffffff123')  -- too long    ) AS t(hex);

   int_val ------------           1          15         256  2147483647 -2147483648  -559038737          -1 

Postgres uses a signed integer type, so hex numbers above '7fffffff' overflow into negative integer numbers. This is still a valid, unique representation but the meaning is different. If that matters, switch to bigint; see below.

For more than 8 hex digits the least significant characters (excess to the right) get truncated.

4 bits in a bit string encode 1 hex digit. Hex numbers of known length can be cast to the respective bit(n) directly. Alternatively, pad hex numbers of unknown length with leading zeros (0) as demonstrated and cast to bit(32). Example with 7 hex digits and int or 8 digits and bigint:

SELECT ('x'|| 'deafbee')::bit(28)::int      , ('x'|| 'deadbeef')::bit(32)::bigint; 

  int4     | int8 -----------+------------  233503726 | 3735928559 

bigint for max. 16 hex digits

Up to 16 hex digits can be converted to bit(64) and then coerced to bigint (int8, 8-byte integer) - overflowing into negative numbers in the upper half again:

SELECT ('x' || lpad(hex, 16, '0'))::bit(64)::bigint AS int8_val FROM  (    VALUES       ('ff'::text)     , ('7fffffff')     , ('80000000')     , ('deadbeef')     , ('7fffffffffffffff')     , ('8000000000000000')     -- overflow into negative number     , ('ffffffffffffffff')     , ('ffffffffffffffff123')  -- too long    ) t(hex);

       int8_val ---------------------                  255           2147483647           2147483648           3735928559  9223372036854775807 -9223372036854775808                   -1                   -1 

uuid for max. 32 hex digits

The Postgres uuid data type is not a numeric type. But it's the most efficient type in standard Postgres to store up to 32 hex digits, only occupying 16 bytes of storage. There is a direct cast from text to uuid (no need for bit(n) as waypoint), but exactly 32 hex digits are required.

SELECT lpad(hex, 32, '0')::uuid AS uuid_val FROM  (    VALUES ('ff'::text)         , ('deadbeef')         , ('ffffffffffffffff')         , ('ffffffffffffffffffffffffffffffff')         , ('ffffffffffffffffffffffffffffffff123') -- too long    ) t(hex);

              uuid_val --------------------------------------  00000000-0000-0000-0000-0000000000ff  00000000-0000-0000-0000-0000deadbeef  00000000-0000-0000-ffff-ffffffffffff  ffffffff-ffff-ffff-ffff-ffffffffffff  ffffffff-ffff-ffff-ffff-ffffffffffff 

As you can see, standard output is a string of hex digits with typical separators for UUID.

md5 hash

This is particularly useful to store md5 hashes:

SELECT md5('Store hash for long string, maybe for index?')::uuid AS md5_hash; 

           md5_hash --------------------------------------  02e10e94-e895-616e-8e23-bb7f8025da42 

See:

• What is the optimal data type for an MD5 field?