Github user dyozie commented on a diff in the pull request:
https://github.com/apache/incubator-hawq-docs/pull/101#discussion_r105447834
--- Diff: markdown/plext/using_plpgsql.html.md.erb ---
@@ -19,143 +19,278 @@ software distributed under the License is distributed
on an
KIND, either express or implied. See the License for the
specific language governing permissions and limitations
under the License.
--->
+-->
-SQL is the language of most other relational databases use as query
language. It is portable and easy to learn. But every SQL statement must be
executed individually by the database server.
+PL/pgSQL is a trusted procedural language that is automatically installed
and registered in all HAWQ databases. With PL/pgSQL, you can:
-PL/pgSQL is a loadable procedural language. PL/SQL can do the following:
+- Create functions
+- Add control structures to the SQL language
+- Perform complex computations
+- Use all of the data types, functions, and operators defined in SQL
-- create functions
-- add control structures to the SQL language
-- perform complex computations
-- inherit all user-defined types, functions, and operators
-- be trusted by the server
+SQL is the language most relational databases use as a query language.
While it is portable and easy to learn, every SQL statement is individually
executed by the database server. Your client application sends each query to
the database server, waits for it to be processed, receives and processes the
results, does some computation, then sends further queries to the server. This
back-and-forth requires interprocess communication and incurs network overhead
if your client is on a different host than the HAWQ master.
-You can use functions created with PL/pgSQL with any database that
supports built-in functions. For example, it is possible to create complex
conditional computation functions and later use them to define operators or use
them in index expressions.
+PL/pgSQL does not have these limitations. When creating functions with the
PL/pgSQL language, you can group computation blocks and queries inside the
database server, combining the power of a procedural language and the ease of
use of SQL, but with considerable savings of client/server communication
overhead. With PL/pgSQL:
-Every SQL statement must be executed individually by the database server.
Your client application must send each query to the database server, wait for
it to be processed, receive and process the results, do some computation, then
send further queries to the server. This requires interprocess communication
and incurs network overhead if your client is on a different machine than the
database server.
+- Extra round trips between client and server are eliminated
+- Intermediate, and perhaps unneeded, results do not have to be
marshaled or transferred between the server and client
+- You avoid multiple rounds of query parsing
+
-With PL/pgSQL, you can group a block of computation and a series of
queries inside the database server, thus having the power of a procedural
language and the ease of use of SQL, but with considerable savings of
client/server communication overhead.
+## <a id="plpgsql_structure"></a>PL/pgSQL Function Syntax
-- Extra round trips between client and server are eliminated
-- Intermediate results that the client does not need do not have to be
marshaled or transferred between server and client
-- Multiple rounds of query parsing can be avoided
+PL/pgSQL is a block-structured language. The complete text of a function
definition must be a block, which is defined as:
-This can result in a considerable performance increase as compared to an
application that does not use stored functions.
+``` sql
+[ <label> ]
+[ DECLARE
+ declarations ]
+BEGIN
+ statements
+END [ label ];
+```
-PL/pgSQL supports all the data types, operators, and functions of SQL.
+Each declaration and each statement within a block is terminated by a
semicolon. A block that appears within another block must have a semicolon
after `END`, as shown above; however the final `END` that concludes a function
body does not require a semicolon.
+
+You can specify all key words and identifiers in mixed upper and lower
case. Identifiers are implicitly converted to lowercase unless double-quoted.
+
+PL/pgSQL supports two types of comments. A double dash (`--`) starts a
comment that extends to the end of the line. A `/*` starts a block comment that
extends to the next occurrence of `*/`. Block comments cannot be nested, but
you can enclose double dash comments into a block comment and a double dash can
hide the block comment delimiters `/*` and `*/`.
+
+This example PL/pgSQL function adds thirteen to an integer:
+
+``` sql
+=> CREATE FUNCTION add_thirteen(i integer) RETURNS integer AS
+ $$
+ DECLARE
+ incvalue integer := 13;
+ BEGIN
+ -- add thirteen to i
+ RETURN i + incvalue;
+ END;
+ $$ LANGUAGE plpgsql;
+=> SELECT add_thirteen( 11 );
+ increment
+ -----------
+ 24
+ (1 row)
+```
-**Note:** PL/pgSQL is automatically installed and registered in all HAWQ
databases.
+**Note**: Do not to confuse the use of `BEGIN/END` for grouping statements
in PL/pgSQL with the database commands for transaction control. PL/pgSQL's
BEGIN/END are only for statement grouping; they do not start or end a
transaction.
-## <a id="supportedargumentandresultdatatypes"></a>Supported Data Types
for Arguments and Results
-Functions written in PL/pgSQL accept as arguments any scalar or array data
type supported by the server, and they can return a result containing this data
type. They can also accept or return any composite type (row type) specified by
name. It is also possible to declare a PL/pgSQL function as returning record,
which means that the result is a row type whose columns are determined by
specification in the calling query. See <a href="#tablefunctions"
class="xref">Table Functions</a>.
+## <a id="plpgsql_structure"></a>PL/pgSQL Statements and Control Structures
-PL/pgSQL functions can be declared to accept a variable number of
arguments by using the VARIADIC marker. This works exactly the same way as for
SQL functions. See <a href="#sqlfunctionswithvariablenumbersofarguments"
class="xref">SQL Functions with Variable Numbers of Arguments</a>.
+Refer to the PostgreSQL documentation for detailed information on the
statements and control structures supported by the PL/pgSQL language:
-PL/pgSQLfunctions can also be declared to accept and return the
polymorphic typesanyelement,anyarray,anynonarray, and anyenum. The actual data
types handled by a polymorphic function can vary from call to call, as
discussed in <a
href="http://www.postgresql.org/docs/8.4/static/extend-type-system.html#EXTEND-TYPES-POLYMORPHIC";
class="xref">Section 34.2.5</a>. An example is shown in <a
href="http://www.postgresql.org/docs/8.4/static/plpgsql-declarations.html#PLPGSQL-DECLARATION-ALIASES";
class="xref">Section 38.3.1</a>.
+- You can execute SQL commands in PL/pgSQL functions using `EXECUTE`,
`PERFORM`, and `SELECT ... INTO` statements. Refer to [Basic
Statements](https://www.postgresql.org/docs/8.2/static/plpgsql-statements.html)
for PL/pgSQL specifics in this area.
-PL/pgSQL functions can also be declared to return a "set" (or table) of
any data type that can be returned as a single instance. Such a function
generates its output by executing RETURN NEXT for each desired element of the
result set, or by using RETURN QUERY to output the result of evaluating a query.
+- [Control
Structures](https://www.postgresql.org/docs/8.2/static/plpgsql-control-structures.html)
identifies the data manipulation constructs supported by PL/pgSQL.
-Finally, a PL/pgSQL function can be declared to return void if it has no
useful return value.
-PL/pgSQL functions can also be declared with output parameters in place of
an explicit specification of the return type. This does not add any fundamental
capability to the language, but it is often convenient, especially for
returning multiple values. The RETURNS TABLE notation can also be used in place
of RETURNS SETOF .
+## <a id="supportedargumentandresultdatatypes"></a>PL/pgSQL Argument and
Result Data Types
-This topic describes the following PL/pgSQLconcepts:
+Functions written in PL/pgSQL accept as arguments any base or array data
type supported by the server, and they can return a result containing any of
these data types. PL/pgSQL functions can also accept and return any composite
type (row-type) specified by name.
-- [Table Functions](#tablefunctions)
-- [SQL Functions with Variable number of
Arguments](#sqlfunctionswithvariablenumbersofarguments)
-- [Polymorphic Types](#polymorphictypes)
+You can declare PL/pgSQL functions to accept and return the polymorphic
types `anyelement` and `anyarray` types. PL/pgSQL functions can also be
declared to return a set (or table) of any data type that can be returned as a
single instance. Finally, you can declare a PL/pgSQL function to return `void`
if it has no useful return value.
+In place of an explicit specification of the return type, you can declare
PL/pgSQL functions with output parameters. This does not add any fundamental
capability to the language, but it is often convenient, especially when
returning multiple values.
-## <a id="tablefunctions"></a>Table Functions
+Upcoming sections provide specific PL/pgSQL examples using base,
composite, and polymorphic argument and return types.
-Table functions are functions that produce a set of rows, made up of
either base data types (scalar types) or composite data types (table rows).
They are used like a table, view, or subquery in the FROM clause of a query.
Columns returned by table functions can be included in SELECT, JOIN, or WHERE
clauses in the same manner as a table, view, or subquery column.
+### <a id="plpgsql_namingargs"></a>Naming PL/pgSQL Function Arguments
-If a table function returns a base data type, the single result column
name matches the function name. If the function returns a composite type, the
result columns get the same names as the individual attributes of the type.
+Arguments passed to PL/pgSQL functions are named with identfiers `$1`,
`$2`, `$3`, etc. If you chose, you can also declare aliases for the `$<n>`
argument names.
-A table function can be aliased in the FROM clause, but it also can be
left unaliased. If a function is used in the FROM clause with no alias, the
function name is used as the resulting table name.
+One way to declare an alias is to give the argument a name in the PL/pgSQL
function signature. In the following example, the single input argument (`$1`)
is named `subtotal`. `subtotal` is used by name in the sales tax calculation in
the body of the function.
-Some examples:
+``` sql
+=> CREATE FUNCTION calculate_sales_tax(subtotal real) RETURNS real AS $$
+ BEGIN
+ RETURN subtotal * 0.06;
+ END;
+ $$ LANGUAGE plpgsql;
+=> SELECT calculate_sales_tax( 123.45 );
+ calculate_sales_tax
+ ---------------------
+ 7.407
+ (1 row)
+```
-```sql
-CREATE TABLE foo (fooid int, foosubid int, fooname text);
+You can also explicitly use the `DECLARE` block to declare an alias for a
function argument:
+
+``` sql
+DECLARE
+ subtotal ALIAS FOR $1;
+```
-CREATE FUNCTION getfoo(int) RETURNS SETOF foo AS $$
- SELECT * FROM foo WHERE fooid = $1;
-$$ LANGUAGE SQL;
+### <a id="plpgsql_inoutargs"></a>Input and Output PL/pgSQL Function
Arguments
-SELECT * FROM getfoo(1) AS t1;
+You can declare PL/pgSQL functions with both input (default) and output
arguments. Output arguments provide a convenient way of defining functions
that return several values or columns.
-SELECT * FROM foo
- WHERE foosubid IN (
- SELECT foosubid
- FROM getfoo(foo.fooid) z
- WHERE z.fooid = foo.fooid
- );
+Output arguments are named (`$<n>`) and aliased in the same way as input
arguments. You identify output arguments in the function signature using the
`OUT` keyword.
-CREATE VIEW vw_getfoo AS SELECT * FROM getfoo(1);
+In this example, you re-write the `calculate_sales_tax()` function to
return the tax in an output argument.
-SELECT * FROM vw_getfoo;
+``` sql
+=> CREATE FUNCTION calculate_sales_tax(subtotal real, OUT tax real) AS $$
+ BEGIN
+ tax := subtotal * 0.06;
+ END;
+ $$ LANGUAGE plpgsql;
+=> SELECT calculate_sales_tax( 123.45 );
```
-In some cases, it is useful to define table functions that can return
different column sets depending on how they are invoked. To support this, the
table function can be declared as returning the pseudotype record. When such a
function is used in a query, the expected row structure must be specified in
the query itself, so that the system can know how to parse and plan the query.
Consider this example:
+Notice that you do not include the output arguments when you invoke the
`calculate_sales_tax()` function. HAWQ considers only the input arguments to
define the function's calling signature.
-```sql
-SELECT *
- FROM dblink('dbname=mydb', 'SELECT proname, prosrc FROM pg_proc')
- AS t1(proname name, prosrc text)
- WHERE proname LIKE 'bytea%';
+## <a id="plpgsqltypes"></a>Identifying Column and Row Data Types
+
+You may need your PL/pgSQL function to operate on column or row data of
which you do not know the data types. PL/pgSQL provides `%TYPE` and `%ROWTYPE`
keywords for this purpose.
+
+For additional information on PL/pgSQL declarations, see
[Declarations](https://www.postgresql.org/docs/8.2/static/plpgsql-declarations.html)
in the PostgreSQL documentation.
+
+### <a id="plpgsqltypes_column"></a>Column Type
+
+Use the `<variable>%TYPE` notation to access the data type of a variable.
You would use this syntax when you want to declare a variable with the same
type as a specific table column.
+
+For example, if you have a column named `order_id` in your `orders` table
and you want to declare a variable with the same data type as `orders.order_id`:
+
+``` sql
+DECLARE
+ local_order_id orders.order_id%TYPE
```
-The `dblink` function executes a remote query (see `contrib/dblink`). It
is declared to return `record` since it might be used for any kind of query.
The actual column set must be specified in the calling query so that the parser
knows, for example, what `*` should expand to.
+`%TYPE` is particularly valuable in polymorphic functions, as the data
types required for internal variables may change from one function invocation
to the next.
+### <a id="plpgsqltypes_row"></a>Row Type
-## <a id="sqlfunctionswithvariablenumbersofarguments"></a>SQL Functions
with Variable Numbers of Arguments
+A variable of a composite type is called a row-type variable. Row-type
variables can hold a whole row of a query result, providing that the query's
column set matches the declared type of the variable.
-SQL functions can be declared to accept variable numbers of arguments, so
long as all the "optional" arguments are of the same data type. The optional
arguments will be passed to the function as an array. The function is declared
by marking the last parameter as VARIADIC; this parameter must be declared as
being of an array type. For example:
+You can declare a row-type variable to have the same type as the rows of
an existing table or view using the `<table_name>%ROWTYPE` notation. The fields
of the row-type variable inherit the table's field sizes and precisions. You
access the individual fields of a row-type variable using dot notation, for
example `<row_variable>.<field>`.
-```sql
-CREATE FUNCTION mleast(VARIADIC numeric[]) RETURNS numeric AS $$
- SELECT min($1[i]) FROM generate_subscripts($1, 1) g(i);
-$$ LANGUAGE SQL;
-
-SELECT mleast(10, -1, 5, 4.4);
- mleast
---------
- -1
-(1 row)
+When a function argument is a composite type, the corresponding argument
`$<n>` is a row-type variable; you would use the `$<n>.<field>` syntax to
access a specific field or column in the row.
+
+**Note**: Only user-defined columns of a table row are accessible via a
row-type variable; system and OID columns are not available.
+
+An example using a row-type variable follows. `table1` has integer fields
named `order_id` and `order_item_id` and a text field named `product_name`. You
will create and execute a `get_order_product()` function to return the order
item and product concatenated together in text string.
--- End diff --
"in text string" -> "in a text string"
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