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From dyozie <...@git.apache.org>
Subject [GitHub] incubator-hawq-docs pull request #85: Feature/hawq-1251
Date Wed, 11 Jan 2017 23:10:52 GMT
Github user dyozie commented on a diff in the pull request:

    https://github.com/apache/incubator-hawq-docs/pull/85#discussion_r95693995
  
    --- 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>
    --- 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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