Github user dyozie commented on a diff in the pull request:
https://github.com/apache/incubator-hawq-docs/pull/85#discussion_r95694042
--- 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.
+
+To be able to create a cast, you must own the source or the target data
type. To create a binary-compatible cast, you must be superuser.
+
+
+
+## <a id="topic1__section4"></a>Parameters
+
+<dt>\<sourcetype\> </dt>
+<dd>The name of the source data type of the cast.</dd>
+
+<dt>\<targettype\> </dt>
+<dd>The name of the target data type of the cast.</dd>
+
+<dt> \<funcname(argtypes)\> </dt>
+<dd>The function used to perform the cast. The function name may be
schema-qualified. If it is not, the function is looked up in the schema search
path. The function's result data type must match the target type of the cast.
+Cast implementation functions may have one to three arguments. The first
argument type must be identical to the cast's source type. The second argument,
if present, must be type `integer`; it receives the type modifier associated
with the destination type, or `-1` if there is none. The third argument, if
present, must be type `boolean`; it receives `true` if the cast is an explicit
cast, `false` otherwise. In some cases, the SQL specification demands different
behaviors for explicit and implicit casts. This argument is supplied for
functions that must implement such casts. Designing your own data types this
way is not recommended.
+Ordinarily, a cast must have different source and target data types.
However, declaring a cast with identical source and target types is allowed if
it has a cast implementation function with more than one argument. This is used
to represent type-specific length coercion functions in the system catalogs.
The named function is used to coerce a value of the type to the type modifier
value specified by its second argument. (Since the grammar presently permits
only certain built-in data types to have type modifiers, this feature is of no
use for user-defined target types.)
+When a cast has different source and target types and a function that
takes more than one argument, it represents a conversion from one type to
another and applying a length coercion in a single step. When no such entry is
available, coercion to a type that uses a type modifier involves two steps, one
to convert between data types and a second to apply the modifier.
+</dd>
+
+<dt>WITHOUT FUNCTION \<call\_handler\> </dt>
+<dd>Indicates that the source type and the target type are binary
compatible, so no function is required to perform the cast.</dd>
+
+<dt>AS ASSIGNMENT \<valfunction\> </dt>
+<dd>Indicates that the cast may be invoked implicitly in assignment
contexts.</dd>
+
+<dt>AS IMPLICIT \<valfunction\> </dt>
--- End diff --
Neither the syntax block nor text mentions \<valfunction\>. This also
doesn't appear in the GPDB version of this reference.
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