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From dyozie <...@git.apache.org>
Subject [GitHub] incubator-hawq-docs pull request #101: HAWQ-1383 - plpgsql page cleanup, res...
Date Fri, 10 Mar 2017 17:45:38 GMT
Github user dyozie commented on a diff in the pull request:

    https://github.com/apache/incubator-hawq-docs/pull/101#discussion_r105448115
  
    --- 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.
    +
    +``` sql
    +=> CREATE TABLE table1 (order_id integer, order_item_id integer, product_name text
);
    +=> INSERT INTO table1 VALUES( 1, 13, 'Paper Towels' );
    +=> INSERT INTO table1 VALUES( 1, 17, 'Pencils' );
    +=> INSERT INTO table1 VALUES( 2, 19, 'Light Bulbs' );
    +=> CREATE FUNCTION get_order_product( oid integer ) RETURNS text AS $$
    +   DECLARE
    +     table1_row table1%ROWTYPE;
    +   BEGIN
    +     SELECT * FROM table1 WHERE table1.order_id = oid INTO table1_row;
    +     RETURN table1_row.order_item_id || table1_row.product_name;
    +   END;
    +   $$ LANGUAGE plpgsql;
    +=> SELECT get_order_product(2);
    +    get_order_product 
    +   -------------------
    +    19Light Bulbs
    +   (1 row)
     ```
     
    -Effectively, all the actual arguments at or beyond the VARIADIC position are gathered
up into a one-dimensional array, as if you had written
    +## <a id="plpgsqlexamples"></a>PL/pgSQL Functions as Table Sources
    +
    +You can use PL/pgSQL functions in the same way you specify a table, view, or subquery
in the `FROM` clause of a query. These functions are referred to as table functions, and can
return both base and composite types. Functions that return base types produce a one-column
table. Functions that return composite types produce a table column for each attribute of
the composite type. You can use the columns returned by table functions in `SELECT`, `JOIN`,
or `WHERE` clauses in the same manner as you would a table, view or subquery column.
    --- End diff --
    
    "table, view" -> "table, view,"


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