Brief description of DECFLOAT datatype

doc/sql.extensions/README.data_types

@@ -101,3 +101,82 @@ BINARY, VARBINARY, BINARY VARYING (FB 4.0)
     2. Can be distinguished from text types by value 1 in RDB$FIELD_SUB_TYPE.
     3. Character set is set to OCTETS for backward compatibility.
     4. In API are similar to corresponding text types, getSubType() returns 0.
+
+
+DECFLOAT (FB 4.0)
+--------------
+
+  Function:
+    DB2-compliant numeric type. DECFLOAT precisely (unlike FLOAT or DOUBLE PRECISION that provide
+    binary approximation) stores decimal values being therefore ideal choice for business appli-
+    cations. Firebird according to IEEE standard has both 16- and 34-digit decimal float encodings.
+    All intermediate calculations are performed with 34-digit values.
+
+  Author:
+    Alex Peshkoff <peshkoff@mail.ru>
+
+  Syntax rules:
+    DECFLOAT(16)
+    DECFLOAT(34)
+
+  Storage:
+    64-bit / 128-bit, format according to IEEE 754.
+
+  Example(s):
+    1. DECLARE VARIABLE VAR1 DECFLOAT(34);
+    2. CREATE TABLE TABLE1 (FIELD1 DECFLOAT(16));
+
+  Note(s):
+    1.	A number of standard functions can be used with DECFLOAT datatype. It is:
+    	ABS, CEILING, EXP, FLOOR, LN, LOG, LOG10, POWER, SIGN, SQRT.
+    	Agregate functions SUM, AVG, MAX and MIN also work with DECFLOAT data.
+    	All statistics aggregates (like but not limited to STDDEV or CORR) work with DECFLOAT data.
+
+    2.	Firebird supports four functions, specially designed to support DECFLOAT data:
+    	- COMPARE_DECFLOAT - compares two DECFLOAT values to be equal, different or unordered.
+    		Returns SMALLINT value which can be as follows:
+    			0 - values are equal
+    			1 - first value is less than second
+    			2 - first value is greater than second
+    			3 - values unordered (i.e. one or both is NAN / SNAN)
+    		Unlike comparison operators ('<', '=', '>', etc.) comparison is exact - i.e.
+    		COMPARE_DECFLOAT(2.17, 2.170) returns 2, not 0.
+    	- NORMALIZE_DECFLOAT - has single DECFLOAT argument returned in it's simplest form.	That
+    		means that for any nonzero value trailing zero are removed with appropriate correction
+    		of an exponent. For example NORMALIZE_DECFLOAT(12.00) returns 12 and 
+    		NORMALIZE_DECFLOAT(120) returns 1.2E+2.
+    	- QUANTIZE - has two DECFLOAT arguments. The returned value is first argument scaled using
+    		second value as a pattern. For example QUANTIZE(1234, 9.999) returns 1234.000.
+    	- TOTALORDER - compares two DECFLOAT values including any special value. The comparison is
+    		exact. Returns SMALLINT value which can be as follows:
+    			-1 - first value is less than second
+    			 0 - values are equal
+    			 1 - first value is greater than second
+    		DECFLOAT values are ordered as follows:
+    			-nan < -snan < -inf < -0.1 < -0.10 < -0 < 0 < 0.10 < 0.1 < inf < snan < nan
+
+	3.	Firebird supports new session control operator SET DECFLOAT. It has following forms:
+			SET DECFLOAT ROUND <mode> - controls rounding mode used in operations with DECFLOAT
+				values. Valid modes are: CEILING (towards +infinity), UP (away from 0), HALF_UP
+				(to nearest, if equidistant - up), HALF_EVEN (to nearest, if equidistant - ensure
+				last digit in the result to be even), HALF_DOWN (to nearest, if equidistant - down),
+				DOWN (towards 0), FLOOR (towards -infinity), REROUND (up if digit to be rounded is
+				0 or 5, down in other cases).
+
+			SET DECFLOAT TRAPS TO <comma-separated traps list - may be empty> - controls which
+				exceptional conditions cause a trap. Valid traps are: Division_by_zero, Inexact,
+				Invalid_operation, Overflow and Underflow. By default traps are set to:
+				Division_by_zero, Invalid_operation, Overflow, Underflow.
+
+			SET DECFLOAT BIND <bind-type> - controls how are DECFLOAT values represented in outer
+				world (i.e. in messages or in XSQLDA). Valid binding types are: NATIVE (use IEEE754
+				binary representation), CHAR/CHARACTER (use ASCII string), DOUBLE PRECISION (use
+				8-byte FP representation - same as used for DOUBLE PRECISION fields) or BIGINT
+				with possible comma-separated SCALE clause (i.e. 'BIGINT, 3'). Various bindings
+				are useful if one plans to use DECFLOAT values with some old client not supporting
+				native format. One can choose between strings (ideal precision, but poor support
+				for further processing), floating point values (ideal support for further processing
+				but poor precision) or scaled integers (good support for further processing and
+				required precision but range of values is very limited). When using is a tool like
+				generic purporse GUI client choice of CHAR binding is OK in most cases.
+