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C20.0046: Database Management Systems Lecture #21

C20.0046: Database Management Systems Lecture #21. M.P. Johnson Stern School of Business, NYU Spring, 2008. Agenda. Stored procedures? Triggers Transactions RAID? Implementation?. Integration with SQL. DECLARE l_book_count INTEGER; BEGIN SELECT COUNT(*) INTO l_book_count

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C20.0046: Database Management Systems Lecture #21

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  1. C20.0046: Database Management SystemsLecture #21 M.P. Johnson Stern School of Business, NYU Spring, 2008 M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  2. Agenda • Stored procedures? • Triggers • Transactions • RAID? • Implementation? M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  3. Integration with SQL DECLARE l_book_count INTEGER; BEGIN SELECT COUNT(*) INTO l_book_count FROM books WHERE author LIKE '%FEUERSTEIN, STEVEN%'; DBMS_OUTPUT.PUT_LINE ( 'Steven has written (or co-written) ' || l_book_count || ' books.'); END; M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  4. Dynamic PL/SQL • E.g.: write function to return number rows in an arbitrary table CREATE OR REPLACE FUNCTION rowCount ( tabname IN VARCHAR2) return integer as retval integer; begin execute immediate 'select count(*) from ' || tabname into retval; return retval; end; / M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  5. Dynamic PL/SQL for DDL • Ordinarily can’t do DDL in PL/SQL • But you can in dynamic PL/SQL • Here’s an e.g.: CREATE OR REPLACE procedure dropproc(procname in varchar2) as begin execute immediate 'drop procedure ' || procname; end; / M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  6. Live examples • Factorial function: • http://pages.stern.nyu.edu/~mjohnson/dbms/plsql/fact.sql • Converting between bases: • http://pages.stern.nyu.edu/~mjohnson/dbms/plsql/numsys.sql • Directory of examples: • http://pages.stern.nyu.edu/~mjohnson/dbms/plsql/ M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  7. SPs in MySQL (5.0) • Generally similar, though technical diffs • Need to temporarily redefine ; delimiter mysql> drop procedure if exists hello; myslq> delimiter / mysql> create procedure hello() -> begin -> select 'hi'; -> end; -> / mysql> delimiter ; mysql> call hello(); M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  8. New topic: Triggers • PL/SQL programs that run automatically (are “triggered”) when a certain event occurs • E.g.: on insert to some table • On system start-up • On delete from table • Big benefit: need not be called explicitly • However row in table x is deleted, the trigger gets called M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  9. Trigger events • Trigger code may be “triggered” by many kinds of events: • Oracle start-up/shut-down • Triggers may replace initialization scripts • Data updates: • Delete: maybe delete related rows • Inserts • Updates: maybe make other rows consistent • Delete: maybe prevent • DDL statements • Log creation of all objects, e.g. M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  10. Triggers • Constraints state what must remain true • DBMS decides when to check • Triggers are instructions to perform at explicitly specified times • Three aspects: • An event (e.g., update to an attribute) • A condition (e.g., a test of that update value) • An action (the trigger’s effect) (deletion, update, insertion) • When the event occurs, DBMS checks the constraint, and if it is satisfied, performs the action M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  11. DML trigger options • The trigger may be: • Statement-level (e.g., a DELETE WHERE statement) or • Row-level (e.g., for each row deleted) • The trigger may run • BEFORE • AFTER or • INSTEAD OF the statement (in Oracle, not in others) • It may be triggered by • INSERTs • UPDATEs • DELETEs M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  12. Trigger form CREATE [OR REPLACE] TRIGGER trigger name {BEFORE | AFTER | INSTEAD OF} {INSERT | DELETE | UPDATE | UPDATE OF column list} ON table name [FOR EACH ROW] [WHEN (...)] [DECLARE ... ] BEGIN ... executable statements ... [EXCEPTION ... ] END [trigger name]; M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  13. Trigger type examples • First run copy_tables.sql • statement_vs_row.sql • INSERT INTO to_table SELECT * FROM from_table; • before_vs_after.sql • INSERT INTO to_table SELECT * FROM from_table; • one_trigger_per_type.sql • INSERT INTO to_table VALUES (1); UPDATE to_table SET col1 = 10; DELETE to_table; M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  14. DML Trigger e.g. • Q: Why is this (maybe) better than client-side validation? CREATE OR REPLACE TRIGGER validate_employee_changes BEFORE INSERT OR UPDATE ON employee FOR EACH ROW BEGIN check_age (:NEW.date_of_birth); check_resume (:NEW.resume); END; M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  15. Triggers with WHEN CREATE OR REPLACE TRIGGER check_raise AFTER UPDATE OF salary, commission ON employee FOR EACH ROW WHEN ((OLD.salary != NEW.salary OR (OLD.salary IS NULL AND NEW.salary IS NULL)) OR (OLD.commission != NEW.commission OR (OLD.commission IS NULL AND NEW.commission IS NULL))) BEGIN ... END; • NB: WHEN applies only to row-level triggers M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  16. Triggers with WHEN • Parentheses are required • Can only call built-in functions in when • Packages like DBMS_OUTPUT are not allowed CREATE OR REPLACE TRIGGER valid_when_clause BEFORE INSERT ON frame FOR EACH ROW WHEN ( TO_CHAR(SYSDATE,'HH24') BETWEEN 9 AND 17 ) ... M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  17. Simple trigger example • R(id, data, last-modified) • data is a large string • Last-modified is a newly added date field • Goal: whenever data is modified, update last-modified date • Could modify all scripts/programs that touch this table • Bad idea • Better: user a trigger CREATE TRIGGER UpdateDateTrigger AFTER UPDATE OF data ON R • REFERENCING • NEW ROW AS NewTuple FOR EACH ROW BEGIN NewTuple.last-modified = sysdate; END; M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  18. Multiple DML actions • DML actions may be ORed together CREATE OR REPLACE TRIGGER three_for_the_price_of_one BEFORE DELETE OR INSERT OR UPDATE ON account_transaction FOR EACH ROW BEGIN IF INSERTING THEN :NEW.created_by := USER; :NEW.created_date := SYSDATE; ELSIF DELETING THEN audit_deletion(USER,SYSDATE); END; • To find actual action, check: • INSERTING • DELETING • UPDATING M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  19. More on UPDATING • UPDATING may be called for partic. columns CREATE OR REPLACE TRIGGER validate_update BEFORE UPDATE ON account_transaction FOR EACH ROW BEGIN IF UPDATING ('ACCOUNT_NO') THEN errpkg.raise('Account number cannot be updated'); END IF; END; M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  20. Extended auditing example • Tables: grades, grades_audit • Run: grades_tables.sql, grades_audit.sql • Cases: hacker changes grades, deletes others UPDATE grades SET grade = 'A+' WHERE student_id = 1 AND class_id = 101; DELETE grades WHERE student_id = 2 AND class_id = 101; M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  21. Extended auditing example • Run: grades_tables.sql, grades_audit2.sql • Cases: hacker changes student or class ids UPDATE grades SET student_id = 3 WHERE student_id = 1 AND class_id = 101; UPDATE grades SET student_id = 1 WHERE student_id = 2 AND class_id = 101; UPDATE grades SET student_id = 2 WHERE student_id = 3 AND class_id = 101; M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  22. DDL Triggers • Respond to DDL events • Creating/dropping tables, indices, etc. • ALTER TABLE etc. • General form: CREATE [OR REPLACE] TRIGGER trigger name {BEFORE | AFTER| {DDL event} ON {DATABASE | SCHEMA} DECLARE Variable declarations BEGIN ... some code... END; M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  23. DDL trigger e.g. • Town crier examples triggered by creates: • uninformed_town_crier.sql • informed_town_crier.sql M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  24. Available DDL events • CREATE, ALTER, DROP, GRANT, RENAME, REVOKE, TRUNCATE • DDL: any DDL event no_create.sql • Q: Does this work?? CREATE OR REPLACE TRIGGER no_create AFTER CREATE ON SCHEMA BEGIN RAISE_APPLICATION_ERROR (-20000, 'ERROR : Objects cannot be created in the production database.'); END; M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  25. DB Event triggers • Form similar to DDL triggers: • Triggering events: STARTUP, SHUTDOWN, SERVERERROR, LOGON, LOGOFF CREATE [OR REPLACE] TRIGGER trigger name {BEFORE | AFTER} {database event} ON {DATABASE | SCHEMA} DECLARE Variable declarations BEGIN ... some code... END; M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  26. DB event restrictions • Have BEFORE and AFTER as above, but they don’t always apply: • No BEFORESTARTUP/LOGON/SERVERERROR • No AFTERSHUTDOWN/LOGOFF M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  27. DB Trigger e.g. • Gather stats before shutdown: • Log error messages CREATE OR REPLACE TRIGGER on_shutdown BEFORE SHUTDOWN ON DATABASE BEGIN gather_system_stats; END; M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  28. New-old topic: Transactions • So far, have simply issued commands • Ignored xacts • Recall, though: an xact is an operation/set of ops executed atomically • In one instant • ACID test: • Xacts are atomic • Each xact (not each statement) must leave the DB consistent M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  29. Default xact behavior (in Oracle) • An xact begins upon login • By default, xact lasts until logoff • Except for DDL statements • They automatically commit • Examples with two views of tbl… • But with TYPE=innodb ! • mysql> set autocommit = 0 M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  30. Direct xact instructions • At any point, may explicitly COMMIT: • SQL> COMMIT; • Saves all statements entered up to now • Begins new xact • Conversely, can ROLLBACK • SQL> ROLLBACK; • Cancels all statements entered since start of xact • Example: delete from emp; or delete junk; M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  31. Direct xact instructions • Remember, DDL statements are auto-committed •  They cannot be rollbacked • Examples: • Q: Why doesn’t rollback “work”? drop table junk; rollback; truncate table junk; rollback; M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  32. Savepoints • Xacts are atomic • Can rollback to beginning of current xact • But might want to rollback only part way • Make 10 changes, make one bad change • Want to: roll back to before last change • Don’t have Word-like multiple undo • But do have savepoints M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  33. Savepoints • Create a savepoint: • emp example: SAVEPOINT savept_name; --changes SAVEPOINT sp1; --changes SAVEPOINT sp2; --changes SAVEPOINT sp3 --changes ROLLBACK TO SAVEPOINT sp2; ROLLBACK TO SAVEPOINT sp1; • Can skip savepoints • But can ROLLBACK only backwards • Can ROLLBACK only to last COMMIT M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  34. AUTOCOMMIT • Finally, can turn AUTOCOMMIT on: • Oralce: SQL> SET AUTOCOMMIT ON; • Mysql: mysql> SET AUTOCOMMIT=1; • Can put this in your config file • Can specify through JDBC, etc. • Then each statement is auto-committed as its own xact • Not just DDL statements M.P. Johnson, DBMS, Stern/NYU, Spring 2008

  35. RAID levels • RAID level 1: each disk gets a mirror • RAID level 4: one disk is xor of all others • Each bit is sum mod 2 of corresponding bits • E.g.: • Disk 1: 10110011 • Disk 2: 10101010 • Disk 3: 00111000 • Disk 4: • How to recover? • What’s the disadvantage of R4? • Various other RAID levels in text… M.P. Johnson, DBMS, Stern/NYU, Spring 2008

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