Fixed-Point Data Types

Table of Contents

NUMERIC
DECIMAL

Fixed-point data types ensure the predictability of multiplication and division operations, making them the choice for storing monetary values. Firebird implements two fixed-point data types: NUMERIC and DECIMAL. According to the standard, both types limit the stored number to the declared scale (the number of digits after the decimal point).

Different treatments limit precision for each type: precision for NUMERIC columns is exactly “as declared”, while DECIMAL columns accepts numbers whose precision is at least equal to what was declared.

For instance, NUMERIC(4, 2) defines a number consisting altogether of four digits, including two digits after the decimal point; that is, it can have up to two digits before the point and no more than two digits after the point. If the number 3.1415 is written to a column with this data type definition, the value of 3.14 will be saved in the NUMERIC(4, 2) column.

The form of declaration for fixed-point data, for instance, NUMERIC(p, s), is common to both types. It is important to realise that the s argument in this template is scale, rather than “a count of digits after the decimal point”. Understanding the mechanism for storing and retrieving fixed-point data should help to visualise why: for storage, the number is multiplied by 10^s (10 to the power of s), converting it to an integer; when read, the integer is converted back.

The method of storing fixed-point data in the DBMS depends on several factors: declared precision, database dialect, declaration type.

Table 3.2. Method of Physical Storage for Real Numbers

Scale	Data type	Dialect 1	Dialect 3
1 - 4	NUMERIC	SMALLINT	SMALLINT
1 - 4	DECIMAL	INTEGER	INTEGER
5 - 9	NUMERIC OR DECIMAL	INTEGER	INTEGER
10 - 18	NUMERIC OR DECIMAL	DOUBLE PRECISION	BIGINT

NUMERIC

Data Declaration Format:

       NUMERIC(p, s)

Storage Examples: Further to the explanation above, the DBMS will store NUMERIC data according the declared precision (p) and scale (s). Some more examples are:

       NUMERIC(4) stored as      SMALLINT (exact data)
       NUMERIC(4,2)              SMALLINT (data * 10²)
       NUMERIC(10,4) (Dialect 1) DOUBLE PRECISION
                     (Dialect 3) BIGINT (data * 10⁴)

	Caution
Always keep in mind that the storage format depends on the precision. For instance, you define the column type as NUMERIC(2,2) presuming that its range of values will be -0.99...0.99. However, the actual range of values for the column will be -327.68..327.67, which is due to storing the NUMERIC(2,2) data type in the SMALLINT format. In storage, the NUMERIC(4,2), NUMERIC(3,2) and NUMERIC(2,2) data types are the same, in fact. It means that if you really want to store data in a column with the NUMERIC(2,2) data type and limit the range to -0.99...0.99, you will have to create a constraint for it.

Caution

Always keep in mind that the storage format depends on the precision. For instance, you define the column type as NUMERIC(2,2) presuming that its range of values will be -0.99...0.99. However, the actual range of values for the column will be -327.68..327.67, which is due to storing the NUMERIC(2,2) data type in the SMALLINT format. In storage, the NUMERIC(4,2), NUMERIC(3,2) and NUMERIC(2,2) data types are the same, in fact. It means that if you really want to store data in a column with the NUMERIC(2,2) data type and limit the range to -0.99...0.99, you will have to create a constraint for it.

DECIMAL

Data Declaration Format:

       DECIMAL(p, s)

Storage Examples: The storage format in the database for DECIMAL is very similar to NUMERIC, with some differences that are easier to observe with the help of some more examples:

       DECIMAL(4) stored as      INTEGER (exact data)
       DECIMAL(4,2)              INTEGER (data * 10²)
       DECIMAL(10,4) (Dialect 1) DOUBLE PRECISION
                     (Dialect 3) BIGINT (data * 10⁴)