MySql FLOAT datatype and problems with more then 7 digit scale

Question

We are using MySql 5.0 on Ubuntu 9.04. The full version is: 5.0.75-0ubuntu10

I created a test database. and a test table in it. I see the following output from an insert statement:

mysql> CREATE TABLE test (floaty FLOAT(8,2)) engine=InnoDb;
Query OK, 0 rows affected (0.02 sec)

mysql> insert into test value(858147.11);
Query OK, 1 row affected (0.01 sec)

mysql> SELECT * FROM test;
+-----------+
| floaty    |
+-----------+
| 858147.12 | 
+-----------+
1 row in set (0.00 sec)

There seems to be a problem with the scale/precision set up in mySql...or did I miss anything?

UPDATE:

Found a boundary for one of the numbers we were inserting, here is the code:

mysql> CREATE TABLE test (floaty FLOAT(8,2)) engine=InnoDb;
Query OK, 0 rows affected (0.03 sec)

mysql> insert into test value(131071.01);
Query OK, 1 row affected (0.01 sec)

mysql> insert into test value(131072.01);
Query OK, 1 row affected (0.00 sec)

mysql> SELECT * FROM test;
+-----------+
| floaty    |
+-----------+
| 131071.01 | 
| 131072.02 | 
+-----------+
2 rows in set (0.00 sec)

mysql>

Answer 1

MySQL docs mention "MySQL performs rounding when storing values" and I suspect this is the issue here. I duplicated your issue but changed the storage type to be DOUBLE:

CREATE TABLE test (val, DOUBLE);

and the retrieved value matched the test value you provided.

My suggestion, for what it's worth, is use DOUBLE or maybe DECIMAL. I tried the same original test with:

CREATE TABLE test (val, DECIMAL(8,2));

and it retrieved the value I gave it: 858147.11.

Answer 2

Face Palm!!!!

Floats are 32 bit numbers stored as mantissa and exponents. I am not 100% sure how MySql will split the storage but taking Java as an example they would use 24 bits for a signed mantissa and 8 bits for an exponent (scientific notation). This means that the maximum value a FLOAT can have is +8388608*10^127 and the minimum is -8388608*10^127. This means only 7 significant digits, and my FLOAT definition used 8.

We are going to switch all of these 8,2 to DOUBLE from FLOAT.