Implement Spark decimal add and subtract (facebookincubator#5791)

Summary:
Use Arrow Gandiva BasicDecimal128 algorithm to compute value.
Arrow implementation:
https://github.com/apache/arrow/blob/release-12.0.1-rc1/cpp/src/gandiva/precompiled/decimal_ops.cc#L211-L231

Spark result precision and scale maybe different with Presto, because it will use adjustPrecisionScale to change the precision and scale when precision is beyond 38.
And this implement can compute data without overflow in some situation.

Pull Request resolved: facebookincubator#5791

Reviewed By: kgpai, pedroerp

Differential Revision: D52220210

Pulled By: Yuhta

fbshipit-source-id: 96f9be2c36d37d11aa71df4308a6a0c9a3bd50a3

velox/docs/functions/spark/math.rst

@@ -27,6 +27,20 @@ Mathematical Functions
     Returns the result of adding x to y. The types of x and y must be the same.
     For integral types, overflow results in an error. Corresponds to sparks's operator ``+``.
 
+.. spark:function:: add(x, y) -> decimal
+
+    Returns the result of adding ``x`` to ``y``. The argument types should be DECIMAL, and can have different precisions and scales.
+    Fast path is implemented for cases that should not overflow. For the others, the whole parts and fractional parts of input decimals are added separately and combined finally.
+    The result type is calculated with the max precision of input precisions, the max scale of input scales, and one extra digit for possible carrier.
+    Overflow results in null output. Corresponds to Spark's operator ``+``.
+
+    ::
+
+        SELECT CAST(1.1232100 as DECIMAL(38, 7)) + CAST(1 as DECIMAL(10, 0)); -- DECIMAL(38, 6) 2.123210
+        SELECT CAST(-999999999999999999999999999.999 as DECIMAL(30, 3)) + CAST(-999999999999999999999999999.999 as DECIMAL(30, 3)); -- DECIMAL(31, 3) -1999999999999999999999999999.998
+        SELECT CAST(99999999999999999999999999999999.99998 as DECIMAL(38, 6)) + CAST(-99999999999999999999999999999999.99999 as DECIMAL(38, 5)); -- DECIMAL(38, 6) -0.000010
+        SELECT CAST(-99999999999999999999999999999999990.0 as DECIMAL(38, 3)) + CAST(-0.00001 as DECIMAL(38, 7)); -- DECIMAL(38, 6) NULL
+
 .. spark:function:: bin(x) -> varchar
 
     Returns the string representation of the long value ``x`` represented in binary.
@@ -179,6 +193,18 @@ Mathematical Functions
     Returns the result of subtracting y from x. The types of x and y must be the same.
     For integral types, overflow results in an error. Corresponds to Spark's operator ``-``.
 
+.. spark:function:: subtract(x, y) -> decimal
+
+    Returns the result of subtracting ``y`` from ``x``. Reuses the logic of add function for decimal type.
+    Corresponds to Spark's operator ``-``.
+
+    ::
+
+        SELECT CAST(1.1232100 as DECIMAL(38, 7)) - CAST(1 as DECIMAL(10, 0)); -- DECIMAL(38, 6) 0.123210
+        SELECT CAST(-999999999999999999999999999.999 as DECIMAL(30, 3)) - CAST(-999999999999999999999999999.999 as DECIMAL(30, 3)); -- DECIMAL(31, 3) 0.000
+        SELECT CAST(99999999999999999999999999999999.99998 as DECIMAL(38, 6)) - CAST(-0.00001 as DECIMAL(38, 5)); -- DECIMAL(38, 6) 99999999999999999999999999999999.999990
+        SELECT CAST(-99999999999999999999999999999999990.0 as DECIMAL(38, 3)) - CAST(0.00001 as DECIMAL(38, 7)); -- DECIMAL(38, 6) NULL
+
 .. spark:function:: unaryminus(x) -> [same as x]
 
     Returns the negative of `x`.  Corresponds to Spark's operator ``-``.