Github user dyozie commented on a diff in the pull request:
https://github.com/apache/incubator-hawq-docs/pull/85#discussion_r95692909
--- Diff: markdown/reference/sql/CREATE-CAST.html.md.erb ---
@@ -0,0 +1,101 @@
+---
+title: CREATE CAST
+---
+
+Defines a new cast.
+
+## <a id="topic1__section2"></a>Synopsis
+
+``` pre
+CREATE CAST (sourcetype AS targettype)
+ WITH FUNCTION funcname (argtypes)
+ [AS ASSIGNMENT | AS IMPLICIT]
+
+CREATE CAST (sourcetype AS targettype) WITHOUT FUNCTION
+ [AS ASSIGNMENT | AS IMPLICIT]
+```
+
+## <a id="topic1__section3"></a>Description
+
+`CREATE CAST` defines a new cast. A cast specifies how to perform a
conversion between two data types. For example,
+
+```pre
+SELECT CAST(42 AS text);
+```
+
+converts the integer constant 42 to type `text` by invoking a previously
specified function, in this `case text(int4)`. If no suitable cast has been
defined, the conversion fails.
+
+Two types may be binary compatible, which means that they can be converted
into one another without invoking any function. This requires that
corresponding values use the same internal representation. For instance, the
types `text` and `varchar` are binary compatible.
+
+By default, a cast can be invoked only by an explicit cast request, that
is an explicit `CAST(x AS typename)` or `x:: typename` construct.
+
+If the cast is marked `AS ASSIGNMENT` then it can be invoked implicitly
when assigning a value to a column of the target data type. For example,
supposing that `foo.f1` is a column of type text, then:
+
+``` pre
+INSERT INTO foo (f1) VALUES (42);
+```
+will be allowed if the cast from type integer to type text is marked `AS
ASSIGNMENT`, otherwise not. The term assignment cast is typically used to
describe this kind of cast.
+
+If the cast is marked `AS IMPLICIT` then it can be invoked implicitly in
any context, whether assignment or internally in an expression. The term
*implicit cast* is typically used to describe this kind of cast. For example,
since `||` takes text operands,
--- End diff --
"whether assignment" -> "whether by assignment" ?
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