Submitted by: @mrotteveel

Is related to JDBC591
Replaces CORE5734
Replaces CORE5733

Currently Firebird has two documented floating point datatypes:

- FLOAT (a 32 bit single precision)
- DOUBLE PRECISION (a 64 bit double precision)

Firebird also has the - undocumented - datatypes
- REAL (a 32 bit single precision), essentially an alias for FLOAT. REAL is a SQL standard type.
- LONG FLOAT (a 64 bit double precision), essentially an alias for DOUBLE PRECISION

In addition, Firebird supports - again, undocumented - FLOAT(p) with p in decimal precision, where
- p in [0,7] is a 32 bit single precision float
- p in [8,..) is a 64 bit double precision float

The SQL standard however defines FLOAT differently. From SQL:2016 6.1 <data type>:

"""
Syntax Rules

20) If a <precision> is omitted, then an implementation-defined <precision> is implicit.

21) The value of a <precision> shall be greater than 0 (zero). [..]
[..]
29) FLOAT specifies the data type approximate numeric, with binary precision equal to or greater than the value of the specified <precision>. The maximum value of <precision> is implementation-defined. <precision> shall not be greater than this value.

30) REAL specifies the data type approximate numeric, with implementation-defined precision.

31) DOUBLE PRECISION specifies the data type approximate numeric, with implementation-defined precision that is greater than the implementation-defined precision of REAL.

32) For the <approximate numeric type>s FLOAT, REAL, and DOUBLE PRECISION, the maximum and minimum values of the exponent are implementation-defined.
"""

I propose that for Firebird 4 we bring this inline with the standard:

1. Change and document FLOAT(p) to apply precision in binary digits, that is:
- p in [1, 24] is a 32 bit single precision
- p in [25, 53] is a 64 bit double precision
- p < 1 and p > 53 are an error (in the future we could consider adding support for higher precision formats like binary128 (p in [54, 113]) and binary256 (p in [114, 237]))
2. Document that FLOAT(p) with 1<=p<=24 is a synonym for FLOAT.
3. Document that FLOAT(p) with 25<=p<=53 is a synonym for DOUBLE PRECISION.
4. Document REAL as a synonym for FLOAT
5. Document LONG FLOAT and LONG FLOAT(p) with 1<=p<=53 as a deprecated synonym for DOUBLE PRECISION

This change will break backwards compatibility for FLOAT(p), but as the precision syntax for FLOAT was never documented, I think that is an acceptable tradeoff to gain better standard compliance.

Commits: 426aeb5 FirebirdSQL/jaybird@0fb5a02