Github user dyozie commented on a diff in the pull request:
https://github.com/apache/incubator-hawq-docs/pull/85#discussion_r95693447
--- 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,
+
+``` pre
+SELECT 'The time is ' || now();
+```
+It is wise to be conservative about marking casts as implicit. An
overabundance of implicit casting paths can cause HAWQ to choose surprising
interpretations of commands, or to be unable to resolve commands at all because
there are multiple possible interpretations. A good rule of thumb is to make a
cast implicitly invokable only for information-preserving transformations
between types in the same general type category. For example, the cast from
int2 to int4 can reasonably be implicit, but the cast from float8 to int4
should probably be assignment-only. Cross-type-category casts, such as text to
int4, are best made explicit-only.
--- End diff --
Need to make all data types `code format` in this paragraph.
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