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HAWQ-1251 Add CREATE CAST/CONVERSION pages to documentation (closes #85)
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janebeckman authored and dyozie committed Jan 13, 2017
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@@ -692,6 +692,7 @@
<ul>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/ABORT.html">ABORT</a></li>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/ALTER-AGGREGATE.html">ALTER AGGREGATE</a></li>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/ALTER-CONVERSION.html">ALTER CONVERSION</a></li>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/ALTER-DATABASE.html">ALTER DATABASE</a></li>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/ALTER-FUNCTION.html">ALTER FUNCTION</a></li>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/ALTER-OPERATOR.html">ALTER OPERATOR</a></li>
@@ -709,6 +710,8 @@
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/COMMIT.html">COMMIT</a></li>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/COPY.html">COPY</a></li>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/CREATE-AGGREGATE.html">CREATE AGGREGATE</a></li>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/CREATE-CAST.html">CREATE CAST</a></li>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/CREATE-CONVERSION.html">CREATE CONVERSION</a></li>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/CREATE-DATABASE.html">CREATE DATABASE</a></li>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/CREATE-EXTERNAL-TABLE.html">CREATE EXTERNAL TABLE</a></li>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/CREATE-FUNCTION.html">CREATE FUNCTION</a></li>
@@ -729,6 +732,8 @@
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/DEALLOCATE.html">DEALLOCATE</a></li>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/DECLARE.html">DECLARE</a></li>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/DROP-AGGREGATE.html">DROP AGGREGATE</a></li>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/DROP-CAST.html">DROP CAST</a></li>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/DROP-CONVERSION.html">DROP CONVERSION</a></li>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/DROP-DATABASE.html">DROP DATABASE</a></li>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/DROP-EXTERNAL-TABLE.html">DROP EXTERNAL TABLE</a></li>
<li><a href="/docs/userguide/2.1.0.0-incubating/reference/sql/DROP-FILESPACE.html">DROP FILESPACE</a></li>
@@ -0,0 +1,56 @@
---
title: ALTER CONVERSION
---

Changes the definition of a CONVERSION.

## <a id="topic1__section2"></a>Synopsis

``` pre
ALTER CONVERSION <name> RENAME TO <newname>

ALTER CONVERSION <name> OWNER TO <newowner>
```

## <a id="topic1__section3"></a>Description

`ALTER CONVERSION` changes the definition of a conversion.

You must own the conversion to use `ALTER CONVERSION`. To alter the owner, you must also be a direct or indirect member of the new owning role, and that role must have `CREATE` privilege on the conversion's schema. (These restrictions enforce that altering the owner does not do anything you could not do by dropping and recreating the conversion. However, a superuser can always alter ownership of any conversion.).

## <a id="topic1__section4"></a>Parameters

<dt>\<name\> </dt>
<dd>The (optionally schema-qualified) name of an existing conversion.</dd>

<dt>\<newname\> </dt>
<dd>The new name of the conversion.</dd>

<dt>\<newowner\> </dt>
<dd>The new owner of the conversion.</dd>


## <a id="topic1__section5"></a>Examples

To rename the conversion `iso_8859_1_to_utf8` to `latin1_to_unicode`:

``` pre
ALTER CONVERSION iso_8859_1_to_utf8 RENAME TO
latin1_to_unicode;
```

To change the owner of the conversion `iso_8859_1_to_utf8` to `joe`:

``` pre
ALTER CONVERSION iso_8859_1_to_utf8 OWNER TO joe;
```

## <a id="topic1__section6"></a>Compatibility

There is no ALTER CONVERSION statement in the SQL standard.

## <a id="topic1__section7"></a>See Also

[CREATE CONVERSION](CREATE-CONVERSION.html), [DROP CONVERSION](DROP-CONVERSION.html)


@@ -0,0 +1,96 @@
---
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 by 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</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 </dt>
<dd>Indicates that the cast may be invoked implicitly in assignment contexts.</dd>

<dt>AS IMPLICIT </dt>
<dd>Indicates that the cast may be invoked implicitly in any context.</dd>

## <a id="topic1__section5"></a>Notes

For this release of HAWQ, user-defined functions used in a user-defined cast must be defined as `IMMUTABLE`. Any compiled code (shared library files) for custom functions must be placed in the same location on every host in your HAWQ array (master and all segments). This location must also be in the `LD_LIBRARY_PATH` so that the server can locate the files.

Remember that if you want to be able to convert types both ways, you need to declare casts both ways explicitly.

We recommend that you follow the convention of naming cast implementation functions after the target data type, as the built-in cast implementation functions are named. Many users are used to being able to cast data types using a function-style notation, that is `typename(x)`.

## <a id="topic1__section6"></a>Examples

Create a cast from type `text` to type `int4` using the function `int4(text)`. (This cast is already predefined in the system.):

``` pre
CREATE CAST (text AS int4) WITH FUNCTION int4(text);
```

## <a id="topic1__section7"></a>Compatibility

The `CREATE CAST` command conforms to the SQL standard, except that SQL does not make provisions for binary-compatible types or extra arguments to implementation functions. `AS IMPLICIT` is also a HAWQ extension.

## <a id="topic1__section8"></a>See Also

[DROP CAST](DROP-CAST.html), [CREATE FUNCTION](CREATE-FUNCTION.html)

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---
title: CREATE CONVERSION
---

Defines a new encoding conversion.

## <a id="topic1__section2"></a>Synopsis

``` pre
CREATE [DEFAULT] CONVERSION <name> FOR <source_encoding> TO
<dest_encoding> FROM <funcname>
```

## <a id="topic1__section3"></a>Description

`CREATE CONVERSION` defines a new conversion between character set encodings. Conversion names may be used in the convert function to specify a particular encoding conversion. Also, conversions that are marked `DEFAULT` can be used for automatic encoding conversion between client and server. For this purpose, two conversions, from encoding A to B and from encoding B to A, must be defined.

To create a conversion, you must have `EXECUTE` privilege on the function and `CREATE` privilege on the destination schema.


## <a id="topic1__section4"></a>Parameters

<dt>DEFAULT</dt>
<dd>Indicates that this conversion is the default for this particular source to destination encoding. There should be only one default encoding in a schema for the encoding pair.</dd>

<dt>\<name\> </dt>
<dd>The name of the conversion. The conversion name may be schema-qualified. If it is not, the conversion is defined in the current schema. The conversion name must be unique within a schema.</dd>

<dt>\<source_encoding\> </dt>
<dd>The source encoding name.</dd>

<dt>\<dest_encoding\> </dt>
<dd>The destination encoding name.</dd>

<dt>\<funcname\> </dt>
<dd>The function used to perform the conversion. The function name may be schema-qualified. If it is not, the function will be looked up in the path. The function must have the following signature:</dd>

``` pre
conv_proc(
integer, -- source encoding ID
integer, -- destination encoding ID
cstring, -- source string (null terminated C string)
internal, -- destination (fill with a null terminated C string)
integer -- source string length
) RETURNS void;
```

## <a id="topic1__section5"></a>Notes

For this release of HAWQ, user-defined functions used in a user-defined cast must be defined as `IMMUTABLE`. Any compiled code (shared library files) for custom functions must be placed in the same location on every host in your HAWQ array (master and all segments). This location must also be in the `LD_LIBRARY_PATH` so that the server can locate the files.


## <a id="topic1__section6"></a>Examples

To create a conversion from encoding `UTF8` to `LATIN1` using `myfunc`:

``` pre
CREATE CONVERSION myconv FOR 'UTF8' TO 'LATIN1' FROM myfunc;
```

## <a id="topic1__section7"></a>Compatibility

There is no CREATE CONVERSION statement in the SQL standard.

## <a id="topic1__section8"></a>See Also

[DROP CONVERSION](DROP-CONVERSION.html), [CREATE FUNCTION](CREATE-FUNCTION.html)
@@ -0,0 +1,48 @@
---
title: DROP CAST
---

Removes a cast.

## <a id="topic1__section2"></a>Synopsis

``` pre
DROP CAST [IF EXISTS] (<sourcetype> AS <targettype>) [CASCADE | RESTRICT]
```

## <a id="topic1__section3"></a>Description

`DROP CAST` will delete a previously defined cast. To be able to drop a cast, you must own the source or the target data type. These are the same privileges that are required to create a cast.

## <a id="topic1__section4"></a>Parameters

<dt>IF EXISTS </dt>
<dd>Do not throw an error if the cast does not exist. A notice is issued in this case.</dd>

<dt>\<sourcetype\> </dt>
<dd>The name of the source data type of the cast to be removed.</dd>

<dt>\<targettype\> </dt>
<dd>The name of the target data type of the cast to be removed.</dd>

<dt>CASCADE</dt>
<dt>RESTRICT</dt>
<dd>These keywords have no effect since there are no dependencies on casts.</dd>

## <a id="topic1__section5"></a>Examples

To drop the cast from type `text` to type `int`:

``` pre
DROP CAST (text AS int);
```

## <a id="topic1__section6"></a>Compatibility

The `DROP CAST` command conforms to the SQL standard.

## <a id="topic1__section7"></a>See Also

[CREATE CAST](CREATE-CAST.html)


@@ -0,0 +1,48 @@
---
title: DROP CONVERSION
---

Removes a CONVERSION.

## <a id="topic1__section2"></a>Synopsis

``` pre
DROP CONVERSION [IF EXISTS] <name> [CASCADE | RESTRICT]
```

## <a id="topic1__section3"></a>Description

`DROP CONVERSION` removes a previously defined conversion. To be able to drop a conversion, you must own the conversion.

## <a id="topic1__section4"></a>Parameters

<dt>IF EXISTS </dt>
<dd>Do not throw an error if the conversion does not exist. A notice is issued in this case.</dd>

<dt>\<name\> </dt>
<dd>The name of the conversion. The conversion name may be schema-qualified.</dd>

<dt>\<targettype\> </dt>
<dd>The name of the target data type of the cast to be removed.</dd>

<dt>CASCADE</dt>
<dt>RESTRICT</dt>
<dd>These keywords have no effect since there are no dependencies on conversions.</dd>

## <a id="topic1__section5"></a>Examples

Drop the conversion named `myname`:

``` pre
DROP CONVERSION myname;
```

## <a id="topic1__section6"></a>Compatibility

There is no `DROP CONVERSION` statement in the SQL standard.

## <a id="topic1__section7"></a>See Also

[CREATE CONVERSION](CREATE-CONVERSION.html), [ALTER CONVERSION](ALTER-CONVERSION.html)


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