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Stored Procedures VS Functions


Ref: http://www.sqlines.com/postgresql/stored_procedures_functions

A stored procedure and user-defined function (UDF) is a set of SQL and procedural statements (declarations, assignments, loops, flow-of-control etc.) that stored on the database server and can be invoked using the SQL interface.

Quick Example:

   -- Function increments the input value by 1
   CREATE OR REPLACE FUNCTION increment(i INT) RETURNS INT AS $$
    BEGIN
      RETURN i + 1;
    END;
    $$ LANGUAGE plpgsql;
 
    -- An example how to use the function (Returns: 11)
    SELECT increment(10);

In PostgreSQL, both stored procedures and user-defined functions are created with CREATE FUNCTION statement. There are differences between the notion of stored procedures and functions in database systems:

Stored ProcedureFunction
Use in an expression
Return a value
Return values as OUT parameters
Return a single result set (as a table function)
Return multiple result sets

So in most cases, the purpose of a stored procedure is to:

  • Perform actions without returning any result (INSERT, UPDATE operations i.e.)
  • Return one or more scalar values as OUT parameters
  • Return one or more result sets

Usually the purpose of a user-defined function is to process the input parameters and return a new value.

Reporting Tools

Many reporting tools (Crystal Reports, Reporting Services, BI tools etc.) allow you to specify a query (SQL SELECT statement) or a stored procedure returning a result set to define a data set for a report.

Stored procedures are very useful when you need to perform complex calculations before the data is available for a report.

Stored Procedures in PostgreSQL

Usually stored procedures do not return any value, or return one or more result sets.

No Value Returned

If a stored procedure does not return any value, you can specify void as the return type:

    -- Procedure to insert a new city
    CREATE OR REPLACE FUNCTION add_city(city VARCHAR(70), state CHAR(2)) 
    RETURNS void AS $$
    BEGIN
      INSERT INTO cities VALUES (city, state);
    END;
    $$ LANGUAGE plpgsql;

You can use SELECT statement to invoke the add_city procedure:

   -- Add a new city
   SELECT add_city('St.Louis', 'MO');

You can also use PERFORM add_city() statement to invoke add_city from another procedure or function.

Return a Single Result Set - Return a Cursor

To return a result set from a PostgreSQL procedure, you have to specify refcursor return type, open and return a cursor:

   CREATE OR REPLACE FUNCTION show_cities() RETURNS refcursor AS $$
    DECLARE
      ref refcursor;
    BEGIN
      OPEN ref FOR SELECT city, state FROM cities;
      RETURN ref;
    END;
    $$ LANGUAGE plpgsql;

Important Note: The cursor remains open until the end of transaction, and since PostgreSQL works in auto-commit mode by default, the cursor is closed immediately after the procedure call, so it is not available to the caller. To work with cursors you have to start a transaction (turn auto-commit off).

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