Re: size of Decimal(18,4)

[Rick James]

Which version of MySQL? The underlying implementation changed a few years ago.

Currently, I would say 9 bytes, empirically derived:

mysql> create table f401075 (
a DECIMAL(18,4) NOT NULL,
b DECIMAL(18,4) NOT NULL,
c DECIMAL(18,4) NOT NULL
) ENGINE=MyISAM;
mysql> insert into f401075 values (1,2,3);

mysql> show table status like 'f401075'\G
*************************** 1. row ***************************
Name: f401075
Engine: MyISAM
Version: 10
Row_format: Fixed
Rows: 1
Avg_row_length: 28
...

1 byte overhead + 3 fields * 9 bytes = 28 bytes.

Possibly the 9 bytes is
3 bytes for 5 digits +
4 bytes for 9 digits +
2 bytes for 4 digits (after decimal point).