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Approximate Floating-Point Data Types

Approximate floating-point values are stored in an IEEE 754 binary format that comprises sign, exponent and mantissa.Precision is dynamic, corresponding to the physical storage format of the value, which is exactly 4 bytes for the FLOAT type and 8 bytes for DOUBLE PRECISION.

Considering the peculiarities of storing floating-point numbers in a database, these data types are not recommended for storing monetary data.For the same reasons, columns with floating-point data are not recommended for use as keys or to have uniqueness constraints applied to them.

For testing data in columns with floating-point data types, expressions should check using a range, for instance, BETWEEN, rather than searching for exact matches.

When using these data types in expressions, extreme care is advised regarding the rounding of evaluation results.

FLOAT

Data Type Declaration Format
FLOAT [(bin_prec)]
Table 1. FLOAT Type Parameters
Parameter Description

bin_prec

Precision in binary digits, default is 24

1 - 24: 32-bit single precision25 - 53: 64-bit double precision

The FLOAT data type defaults to a 32-bit single precision floating-point type with an approximate precision of 7 decimal digits after the decimal point (24 binary digits).To ensure the safety of storage, rely on 6 decimal digits of precision.

The syntax FLOAT(bin_prec) behaves as follows:

  • 1 <= _bin_prec <= 23: 32-bit single precision (synonym for FLOAT)

  • 25 <= _bin_prec <= 53: 64-bit double precision (synonym for DOUBLE PRECISION)

The behaviour of FLOAT (without explicit precision) behaves as the SQL standard type [fblangref50-datatypes-real].

Note
Compatibility Notes

  • Firebird 3.0 and earlier supported FLOAT(dec_prec) where dec_prec was the approximate precision in decimal digits, with 0 <= dec_prec <= 7 mapped to 32-bit single precision and P > 7 mapped to 64-bit double precision.This syntax was never documented.

  • For bin_prec in FLOAT(bin_prec), the values 1 <= bin_prec <= 24 are all treated as bin_prec = 24, values 25 <= bin_prec <= 53 are all handled as bin_prec = 53.

  • Most Firebird tools will report FLOAT(1) — FLOAT(24) as FLOAT, and FLOAT(25) — FLOAT(53) as DOUBLE PRECISION.