1. CS4222 Principles of Database System
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CS4222 Principles of Database System
14. PL/PgSQL 2
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. Returning Variables RETURN expression;
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Returning Variables
RETURN expression;
RETURN NEXT expression/ RETURN QUERY query;
RETURN;
RETURN statement is required even for functions that return void
RETURN NEXT/ RETURN QUERY
They do not actually return from the function — they simply append zero or more rows to the function's result set.
Execution then continues with the next statement in the PL/pgSQL function. As successive RETURN NEXT or RETURN QUERY commands are executed, the result set is built up.
A final RETURN, which should have no argument, causes control to exit the function
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3. Example Select * from getAllFoo();
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Example
CREATE TABLE foo (fooid INT, foosubid INT, fooname TEXT);
INSERT INTO foo VALUES (1, 2, 'three');
INSERT INTO foo VALUES (4, 5, 'six');
CREATE OR REPLACE FUNCTION getAllFoo() RETURNS SETOF foo AS '
DECLARE r foo%rowtype;
BEGIN
FOR r IN SELECT * FROM foo WHERE fooid > 0
LOOP
RETURN NEXT r; -- return current row of SELECT
END LOOP;
RETURN;
END ;
' LANGUAGE 'plpgsql' ;
Select * from getAllFoo();
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4. Handling Errors and Exceptions
RAISE statement
Syntax
RAISE level ''message string'' [, identifier [...] ];
Possible level values:
DEBUG, NOTICE, EXCEPTION
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5. Severity Levels Level Message written to server’s log
Message prompted to the screen
Function runs to completion
Current transaction aborted
DEBUG
YES NO
NOTICE
EXCEPTION
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6. Typical Uses of Severity Levels
DEBUG
When debugging software in working environment
NOTICE
To inform user about some interesting event which does not adversely affect the execution
EXCEPTION
To abort transaction due to some foreseen event and inform user about that
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7. Raise statement example
CREATE FUNCTION raise_test() RETURNS integer AS '
DECLARE
an_integer INTEGER := 1;
BEGIN -- Raise a debug level message
RAISE DEBUG ''The raise_test function began.'';
an_integer := an_integer + 1;
RAISE NOTICE ''Variable an_integer was changed.'';
RAISE NOTICE ''Variable an_integer value is now %.'',an_integer;
-- Raise an exception.
RAISE EXCEPTION ''Variable % changed. Transaction aborted.'',an_integer;
RETURN 1;
END;
' LANGUAGE 'plpgsql';
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8. Results of the raise_test( ) function
SELECT raise_test();
NOTICE: Variable an_integer was changed.
NOTICE: Variable an_integer's value is now 2.
ERROR: Variable 2 changed. Aborting transaction.
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9. Cursors
Rather than executing a whole query at once, it is possible to set up a cursor that encapsulates the query, and then read the query result a few rows at a time
To create a cursor variable
One way to create a cursor variable is just to declare it as a variable of type refcursor
Another way is to use the cursor declaration syntax
name CURSOR [ ( arguments ) ] FOR query ;
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10. Examples DECLARE curs1 refcursor;
curs2 CURSOR FOR SELECT * FROM table1;
curs3 CURSOR (k integer) FOR SELECT * FROM table1 WHERE key=k;
Unbound cursor: curs1
Bound cursors: curs2, curs3
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11. Opening and closing cursors
Before a cursor can be used to retrieve rows, it must be opened
OPEN curs1 FOR SELECT * FROM table2;
OPEN curs2;
OPEN curs3(47);
Close curs1;
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12. Fetching From a Cursor
FETCH retrieves the next row from the cursor into a target
The target may be a row variable, a record variable, or a comma-separated list of simple variables, just like SELECT INTO.
As with SELECT INTO, the special variable FOUND may be checked to see whether a row was obtained or not.
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13. Fetching from a Cursor FETCH cursor INTO target; Examples:
FETCH curs1 INTO record1;
FETCH curs2 INTO v1, v2, v3;
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Cursor example
Write a function mean() to compute the average age of sailors.(Do not use avg() )
CREATE FUNCTION mean () RETURNS float8 AS '
DECLARE
mean float8 :=0;
tempAge int8; count int8 :=0;
ages CURSOR for select age from Sailors;
BEGIN
OPEN ages;
Loop
FETCH ages into tempAge;
Exit when not found;
count:=count+1; mean:=mean+tempAge;
END loop; CLOSE ages;
if count<> 0 then
mean := mean/count;
return mean;
else return -1;
end if;
END;
' LANGUAGE 'plpgsql';
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15. PERFORM
Use the PERFORM keyword to call a function and ignore its return data
Syntax: PERFORM function_identifier(arguments);
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16. PERFORM example
CREATE FUNCTION ship_item (text,text,text) RETURNS integer AS '
DECLARE
l_name ALIAS FOR $1; f_name ALIAS FOR $2; book_isbn ALIAS FOR $3;
-- Declare a variable to hold the book ID number. This variable -- is necessary to check for the existence of the provided ISBN.
book_id INTEGER;
-- Declare a variable to hold the customer ID number. This variable -- is necessary to check for the existence of the customer.
customer_id INTEGER;
BEGIN
-- Retrieve the customer ID number with a previously created -- function.
SELECT INTO customer_id get_customer_id(l_name,f_name);
IF customer_id = -1 THEN RETURN -1; END IF;
-- Retrieve the ID number of the book with the specified ISBN.
SELECT INTO book_id book_id FROM editions WHERE isbn = book_isbn;
-- If the book does not exist in the system, return -1.
IF NOT FOUND THEN RETURN -1; END IF;
-- If the book and customer both exist, add the shipment.
PERFORM add_shipment(customer_id,book_isbn);
-- Return 1 to indicate the function was successful. RETURN 1;
END; ' LANGUAGE 'plpgsql';
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17. What is a trigger?
A trigger is a specification that the database should automatically execute a particular function whenever a certain type of operation is performed
Triggers can be defined to execute either before or after any INSERT, UPDATE, or DELETE operation, either once per modified row, or once per SQL statement.
If a trigger event occurs, the trigger's function is called at the appropriate time to handle the event
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18. Motivation Maintain database integrity Some applications of triggers :
Constraints embedded in SQL (e.g., referential integrity constraints and CHECK constraints) are not omni capable
Some applications of triggers :
Maintaining functional dependencies (e.g. in BCNF)
Keeping referential integrities for specialization/generalization hierarchies
To enforce business rules, e.g.,
Give a customer additional discount if he has made enough purchase in the past month
A bank deals with overdrafts by setting the account balance to 0 and creating a loan in the amount of the overdraft
These business rules could also be implemented on application level
The choice whether a rule is implemented in application or in database is a design issue
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19. Design a trigger Create a trigger function
Specify the actions to be taken when the trigger is invoked
Use CREATE TRIGGER to create a trigger
Specify the events that invoke the trigger
Insert, delete, update
The trigger function must be created before the trigger is created
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20. Create Triggers CREATE TRIGGER name
{ BEFORE | AFTER } { event [ OR ... ] }
ON table [ FOR [ EACH ] { ROW | STATEMENT } ]
EXECUTE PROCEDURE function_name ( )
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21. Options: The Condition
AFTER/BEFORE
Determine whether the function is called before or after the event
Three events
INSERT/DELETE/UPDATE.
UPDATE can be UPDATE ON a particular attribute
Use OR to connect multiple events
A trigger can only be set on the entire table (in PostgreSQL)
Cannot set triggers on columns
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22. Before/After triggers
Before triggers
The trigger function is run before the triggering statement completes (INSERT, UPDATE, DELETE)
Eliminate unnecessary processing of the triggering statement
After triggers
The trigger function is called after the trigger statement completes
If an exception is raised in the trigger function, the trigger statement is canceled
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23. Trigger level Triggers are either row-level or statement-level.
FOR EACH ROW indicates row-level; its absence indicates statement-level.
Row level triggers are executed once for each modified tuple.
Statement-level triggers execute once for an SQL statement regardless of how many tuples are modified.
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24. Trigger Type Combinations
BEFORE statement trigger
Before executing the triggering statement, the trigger action is run.
BEFORE row trigger
Before modifying each row affected by the triggering statement and before checking appropriate integrity constraints, the trigger action is run, if the trigger restriction was not violated.
AFTER row trigger
After modifying each row affected by the triggering statement and possibly applying appropriate integrity constraints, the trigger action is run for the current row provided the trigger restriction was not violated.
AFTER statement trigger
After executing the triggering statement and applying any deferred integrity constraints, the trigger action is run
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25. Order of Execution Execute any BEFORE STATEMENT trigger on table
For each row affected by statement
Execute any BEFORE ROW level trigger
Execute command (INSERT DELETE UPDATE)
Apply referential integrity constraints (ON DELETE, ON UPDATE)
Execute any AFTER ROW level trigger
Execute any AFTER STATEMENT trigger on table
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26. CREATE Trigger examples
CREATE TRIGGER emp_stamp
BEFORE INSERT OR UPDATE ON emp
FOR EACH ROW EXECUTE PROCEDURE emp_stamp();
Example 2
CREATE TRIGGER check_shipment
BEFORE INSERT OR UPDATE
ON Shipments
FOR EACH ROW EXECUTE PROCEDURE check_shipment_addition();
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27. Trigger functions
CREATE OR REPLACE FUNCTION funcname ( ) RETURNS TRIGGER AS '
–- Always return trigger!
{[DECLARE <variable name> <variable type]}…;
--Local variables
BEGIN
<function body>
RETURN <retvalue>;
END;
' LANGUAGE 'plpgsql';
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28. About Trigger funtions
Must be defined before the trigger is created
Must be declared as no parameter and return a trigger type
Return
NULL, or
the tuple to be modified
Every trigger function created has access to a number of special variables that exist to provide information about the calling trigger, and to allow the trigger function to manipulate table data.
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29. Special Variables NEW OLD
denotes table/row AFTER operation (update/insert/delete) is executed
OLD
denotes table/row BEFORE operation (update/insert/delete) is executed
NEW and OLD are similar to THIS pointer in C++;
For DELETE triggers we’ll use OLD, since after deletion NEW is empty
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30. Other Special Variables Accessible in Triggered Function
Name
Meaning
Allowed values
TG_NAME
Trigger name
string
TG_WHEN
Trigger action time
BEFORE or AFTER
TG_LEVEL
Level of triggered action
ROW or STATEMENT
TG_OP
Trigger event
INSERT, UPDATE or DELETE
TG_RELNAME
Name of table on which trigger is executed
String
TG_RELID
Object ID of triggered table
Integer
TG_NARGS
Number of arguments of triggered function
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31. Trigger function example
CREATE FUNCTION check_shipment_addition () RETURNS trigger AS '
DECLARE
id_number INTEGER;
book_isbn TEXT;
BEGIN
SELECT id INTO id_number FROM customers WHERE id = NEW.customer_id;
IF NOT FOUND
THEN RAISE EXCEPTION ''Invalid customer ID number.'';
END IF;
SELECT isbn INTO book_isbn FROM editions WHERE isbn = NEW.isbn;
IF NOT FOUND THEN RAISE EXCEPTION ''Invalid ISBN.''; END IF;
-- If the previous checks succeeded, update the stock amount.
IF TG_OP = ''INSERT'' THEN
UPDATE stock SET stock = stock -1 WHERE isbn = NEW.isbn; END IF; RETURN NEW;
END;
' LANGUAGE 'plpgsql';
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32. A complete example
This example trigger ensures that any time a row is inserted or updated in the employee table, the current user name and time are stamped into the row.
It also checks that an employee's name is given and that the salary is a positive value.
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33. A complete example CREATE TABLE emp ( empname text; salary integer,
last_date timestamp,
last_user text );
CREATE TRIGGER emp_stamp
BEFORE INSERT OR UPDATE ON emp
FOR EACH ROW EXECUTE PROCEDURE emp_stamp();
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34. A complete example CREATE FUNCTION emp_stamp() RETURNS trigger AS '
BEGIN
-- Check that empname and salary are given
IF NEW.empname IS NULL
THEN RAISE EXCEPTION ''empname cannot be null''; END IF;
IF NEW.salary IS NULL
THEN RAISE EXCEPTION ''% cannot have null salary'',
NEW.empname;
END IF;
IF NEW.salary < 0 THEN
RAISE EXCEPTION ''% cannot have a negative salary'', NEW.empname;
NEW.last_date := ''now'';
NEW.last_user := current_user;
RETURN NEW;
END;
' LANGUAGE plpgsql;
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35. Another trigger example
Create a function for states that uses the new RECORD variable to perform the following actions:
Reject a state code that is not exactly two alphabetic characters
Reject a state name that contains nonalphabetic characters
Reject a state name less than three characters in length
Uppercase the state code
Capitalize the state name
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36. Example Function CREATE TABLE statename ( code text, name text )
CREATE FUNCTION trigger_insert_update_statename()
RETURNS trigger AS '
BEGIN
IF new.code !~ ''^[A-Z|a-z][A-Z|a-z]$''
THEN RAISE EXCEPTION ''State code must be two alphabetic characters.'';
END IF;
IF new.name !~ ''^[A-Z|a-z ]*$''
THEN RAISE EXCEPTION ''State name must be only alphabetic characters.'';
IF length(trim(new.name)) < 3
THEN RAISE EXCEPTION ''State name must longer than two characters.'';
new.code := upper(new.code); -- uppercase statename.code
new.name := initcap(new.name); -- capitalize statename.name
RETURN new;
END;
' LANGUAGE 'plpgsql';
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37. Example Trigger CREATE TRIGGER trigger_statename
BEFORE INSERT OR UPDATE
ON statename
FOR EACH ROW
EXECUTE PROCEDURE trigger_insert_update_statename();
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38. Example Execution INSERT INTO statename VALUES ('a', 'alabama');
INSERT INTO statename VALUES ('al', 'alabama2');
INSERT INTO statename VALUES ('al', 'al');
INSERT INTO statename VALUES ('al', 'alabama');
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39. Exercise
CREATE TABLE Teach ( fid integer, cid integer, year date ) INSERT INTO Teach VALUES (100, 1111, '2010'); INSERT INTO Teach VALUES (100, 2222, '2010'); INSERT INTO Teach VALUES (200, 3333, '2010'); INSERT INTO Teach VALUES (200, 4444, '2010'); INSERT INTO Teach VALUES (200, 5555, '2010'); Create a trigger to ensure each faculty only teaches no more than 3 courses a year.
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