The document discusses view triggers in PostgreSQL. It describes how view triggers were implemented previously using rules and shadow tables, which had performance and concurrency issues. It then explains that view triggers are now implemented directly using trigger functions and triggers, avoiding the problems of the previous approach. This provides an easier way to execute code in response to INSERT, UPDATE, and DELETE operations on a view.

1. View Triggers!
JDCon East
March 25, 2011
Copyright © 2011
David Fetter dfetter@vmware.com
All Rights Reserved

5. ...or does it?

6. What is a View?

7. What is a View?
• Cached query

8. What is a View?
• Cached query
• Modified at runtime

9. What is a Trigger?
• Executable code
• Call and Response
• Scope: row or statement

10. What is a View Trigger?
• Cached query brings rows
• Code executes on each

11. Trigger Contexts
SCOPE
WHEN EVENT
ROW STATEMENT
INSERT
UPDATE TABLES TABLES/VIEWS
BEFORE DELETE
TRUNCATE ― TABLES
INSERT
UPDATE TABLES TABLES/VIEWS
AFTER DELETE
TRUNCATE ― ―
INSERT
UPDATE VIEWS ―
DELETE
INSTEAD OF
TRUNCATE ― ―

12. Why?

13. Query
Rewrite
RULEs :P

14. Actually,

15. Query
Rewrite
SUthis is a family presentation

16. Like C Pre-Processor
Macros, only Less Fun

17. No more RULEs! :)

18. View Triggers Before
• For each VIEW
• Shadow table
• 3 RULEs
• INSERT RULE
• UPDATE RULE
• DELETE RULE
• Trigger on Shadow Table

20. Shadow Table
CREATE TABLE v_t_shadow (
old_column,
old_list,
old_here,
new_column,
new_list,
new_here,
);

21. INSERT RULE
CREATE RULE t_v_insert
ON INSERT TO t_v
DO INSTEAD
INSERT INTO t_v_shadow (
action,
new_column,
new_list,
new_here)
VALUES (
'I',
NEW.t_v_column,
NEW.list,
NEW.here
);

22. DELETE RULE
CREATE RULE t_v_delete
ON DELETE TO t_v
DO INSTEAD
INSERT INTO t_v_shadow (
action,
column,
list,
here)
VALUES (
'D',
OLD.t_v_column,
OLD.list,
OLD.here
);

23. UPDATE RULE
CREATE RULE t_v_update
ON UPDATE TO t_v
DO INSTEAD
INSERT INTO t_v_shadow (
action,
column,
list,
here)
VALUES (
'U',
OLD.t_v_column,
OLD.list,
OLD.here,
NEW.t_v_column,
NEW.list,
NEW.here
);

24. Shadow Table Trigger Function
CREATE OR REPLACE FUNCTION shadow_table_t_v()
RETURNS TRIGGER
LANGUAGE plpgsql
AS $$
BEGIN
IF NEW.action = 'I' THEN
...
ELSIF NEW.action = 'D' THEN
...
ELSIF NEW.action = 'U' THEN
...
END IF;
RETURN NULL;
END;
$$;

25. Shadow Trigger
CREATE TRIGGER shadow_table_t_v
BEFORE
INSERT OR UPDATE OR DELETE
ON shadow_t_v
FOR EACH ROW
EXECUTE PROCEDURE shadow_trigger_t_v();

26. •Performance?
• Not So Much™

27. •Concurrency?
• What's that?

28. With Triggers on Views

29. With Triggers on Views
• That's

30. With Triggers on Views
• That's
• All

31. With Triggers on Views
• That's
• All
• Just

32. With Triggers on Views
• That's
• All
• Just
•A

33. With Triggers on Views
• That's
• All
• Just
•A
• Bad

34. With Triggers on Views
• That's
• All
• Just
•A
• Bad
• Memory

35. View Triggers Now
• Create trigger function
• Create trigger
• DONE!

36. Table
CREATE TABLE triangle_victims (
first_name TEXT,
last_name TEXT NOT NULL,
age INTEGER,
notes TEXT
);

37. View
CREATE VIEW triangle_victims_v
AS SELECT
COALESCE(
first_name || ' ',
'') ||
last_name AS "full_name",
age,
notes
FROM triangle_victims;

38. Trigger Function
CREATE OR REPLACE FUNCTION write_triangle_victims_v()
RETURNS TRIGGER
LANGUAGE plpgsql
AS $$
DECLARE
name_split TEXT[];
BEGIN
IF TG_OP = 'INSERT' THEN
name_split = string_to_array(NEW.full_name, ' ');
INSERT INTO triangle_victims (first_name, last_name, age, notes)
VALUES (
array_to_string(
name_split[1:array_upper(name_split,1)-1], ' '
),
name_split[array_upper(name_split,1)],
NEW.age,
NEW.notes
);
RETURN NEW;
ELSIF TG_OP = 'DELETE' THEN /* Similar code goes here */
ELSIF TG_OP = 'UPDATE' THEN /* And slightly more complicated here */
END IF;
END;
$$;

39. Triangle Shirtwaist Fire
http://en.wikipedia.org/wiki/Triangle_Shirtwaist_Factory_fire
http://law2.umkc.edu/faculty/projects/ftrials/triangle/trianglevictims2.html

40. • Questions?
• Comments?

41. Thank You!
JDCon East
March 25, 2011
Copyright © 2011
David Fetter dfetter@vmware.com
All Rights Reserved