1. SQL in a Programming Environment - Part II CIS 4301 Lecture Notes Lecture 23 - 13/4/2006

2. Lecture 23© CIS 4301 - Spring 20062 Application Development Often important to execute parts of the application logic directly in process space of DBMS Why? One solution: Programming Language for DBMS

3. Lecture 23© CIS 4301 - Spring 20063 DBMS Programming Language Many DBMS's provide their own mini- programming language that includes SQL, variables, control constructs, procedures, etc. Simple, general-purpose language Smoothes over the discontinuity between the programming language and SQL Runs in the DBMS Used to create procedures or functions to be stored with database schema A.k.a. Persistent Stored Modules PSM (SQL/PSM) A.k.a. Stored Procedures

4. Lecture 23© CIS 4301 - Spring 20064 DBMS Programming Languages E.g., Oracle’s Procedure Language SQL (PL/SQL) Primarily used for writing PSM’s and trigger actions User can execute PL/SQL interactively (SQLPlus) or call it from host language programs For rest of lecture, use SQL/PSM standard syntax

5. Lecture 23© CIS 4301 - Spring 20065 Persistent Stored Modules In SQL speak, application program is called module Collections of functions and procedure definitions, temp. relation declarations, optional local variable declarations Invoked by users or applications Stored inside DBMS, part of the schema Modules communicate via parameters or shared variables

6. Lecture 23© CIS 4301 - Spring 20066 Persistent Stored Modules Advantages Reduces, in some cases even eliminates, transfer of results to client – Example? Allows re-use of code (write once – use multiple times) Encapsulation of database schema shields application programmers from knowing details of how data is stored

7. Lecture 23© CIS 4301 - Spring 20067 Simple PSM Procedure CREATE PROCEDURE LucasMovies SELECT title, year FROM Movie NATURAL JOIN MovieExec WHERE name = ‘Lucas’;

8. Lecture 23© CIS 4301 - Spring 20068 PSM Procedure with Parameters CREATE PROCEDURE Move( IN oldAddr VARCHAR[255], IN newAddr VARCHAR[255] ) UPDATE MovieStar SET address = newAddr WHERE address = oldAddr; Parameters Mode may be one of IN, OUT, INOUT

9. Lecture 23© CIS 4301 - Spring 20069 A Few Explanations INOUT parameter combines properties of IN and OUT parameter Pass values into the PSM PSM can set their return values PSM enforces strict type conformance E.g., parameter of type INT cannot be called with an argument of type VARCHAR

10. Lecture 23© CIS 4301 - Spring 200610 PSM Functions and Procedures So far, we have seen two examples of PSM procedures Can have IN, OUT, INOUT parameters Optional local variable declarations Executable body of code defining the procedure PSM function defined in almost same way Use keyword function Specify return type Parameters only of type IN (i.e., no side-effects) When to use procedure/function? Same reasoning as in programming language applies – which is?

11. Lecture 23© CIS 4301 - Spring 200611 PSM Statements How do we invoke a PSM? From a host-language program: EXEC SQL CALL Move (:oldA, :newA); Statement of another PSM As an SQL command issued to SQLPLUS CALL Move (“LA”, “Boston”); Note, it is not permitted to call a PSM function! You can invoke functions only as part of expressions Other statements DECLARE ; declaration of local variable SET = ; assignment statement IF THEN ELSEIF THEN ELSEIF … ELSE END IF; if-then-elseif-else statement

12. Lecture 23© CIS 4301 - Spring 200612 Miscellaneous PSM standard has rich syntax Not shown but covered in textbook: Return statements for functions Local variables, assignments, branching, loops Exception handling Check out Oracle Help topic # 5, “Using Oracle PL/SQL”Using Oracle PL/SQL

13. Lecture 23© CIS 4301 - Spring 200613 SQL Environment So far, tacitly assumed that our program runs on the same server where the database is installed Did not worry about explicit connection to database Assumed default connection to the server and the database In reality, the situation is more complex

14. Lecture 23© CIS 4301 - Spring 200614 SQL Environment Connection: Want to run program involving SQL at a host where SQL client is running, need to open connection between client and server SQL-agent (execution of a module) SQL ClientSQL Server SQL Environment Connection Session (SQL operations that are performed while connection is active)  Modules  Generic SQL Interface  Embedded SQL  PSM modules

15. Lecture 23© CIS 4301 - Spring 200615 Call-Level Interfaces A more modern approach to the host- language/SQL connection is a call-level interface (CLI) C (or other language) program creates SQL statements as character strings and passes them to functions that are part of a library Similar to what really happens in embedded SQL implementations Two major approaches: SQL/CLI (standard of ODBC = open database connectivity) JDBC (Java database connectivity), links Java programs to DB in an O-O style

16. Lecture 23© CIS 4301 - Spring 200616 Introduction to JDBC 1.Load a driver for the database system we will use (creates an object called DriverManager ) Installation-dependent, see Oracle help pages for details 2.Establish connection to database: applying method getConnection to DriverManager creates variable of type Connection Java Statement: Connection myCon = DriverManager.getConnection(, name,password);

17. Lecture 23© CIS 4301 - Spring 200617 Intuition By applying the appropriate methods to a connection like myCon, we can create statement objects Place SQL statements “in” objects Bind values to SQL statement parameters Execute SQL statements Examine results a tuple at a time

18. Lecture 23© CIS 4301 - Spring 200618 Executing a SELECT Query in JDBC Query: SELECT netWorth FROM MovieExec; 1.Create statement object and execute directly Statement execStat=myCon.createStatement(); ResultSet Worths=execStat.executeQuery( “SELECT netWorth FROM MovieExec”); OR 2.Prepare query and execute later PreparedStatement execStat=myCon.createStatement(“SELECT netWorth FROM MovieExec”); ResultSet Worths = execStat.executeQuery();

19. Lecture 23© CIS 4301 - Spring 200619 Executing a Parameter-less Update Update: Insert new fact into StarsIn table 1.Create statement object and execute directly Statement starStat=myCon.createStatement(); starStat.executeUpdate(“INSERT INTO StarsIn VALUES (‘Remember the Titans’, 2000, ‘Denzel Washington’)” ); OR 2.Prepare update and execute later PreparedStatement starStat=myCon.createStatement(“INSERT INTO StarsIn VALUES (‘Remember the Titans’, 2000, ‘Denzel Washington’)”); starStat.executeUpdate();

20. Lecture 23© CIS 4301 - Spring 200620 Cursor Operations When we obtain result set object, we may run cursor through tuples of result ResultSet class provides useful methods Next(), returns FALSE if there is no next tuple getString(i), getInt(i), getFloat(i), etc. Returns the i th component of the tuple currently indicated by cursor While(Worths.next()) { Worth = Worths.getInt(1); /* process this net worth */ }

21. Lecture 23© CIS 4301 - Spring 200621 Accessing the Result Set JDBC offers a number of methods to find out where you are in the result set using getRow, isFirst, isBeforeFirst, isLast, isAfterLast Means to make scroll-able cursors allow free access of any row in the result set Default, cursors scroll forward only and are read only When creating a Statement for a Connection, you can change the type of ResultSet to a more flexible scrolling or updatable model: Statement stmt = MyCon.createStatement( ResultSet.TYPE_FORWARD_ONLY,ResultSet.CONCUR_READ_ONLY); ResultSet rs = stmt.executeQuery("SELECT * FROM MovieExec");

22. Lecture 23© CIS 4301 - Spring 200622 Accessing the Result Set The different options for types are TYPE_FORWARD_ONLY TYPE_SCROLL_INSENSITIVE TYPE_SCROLL_SENSITIVE You can choose whether the cursor is read-only or updatable using the options CONCUR_READ_ONLY CONCUR_UPDATABLE With default cursor, you can scroll forward using rs.next() With scroll-able cursors you have more options: rs.absolute(3); // moves to the third tuple rs.previous(); // moves back one tuple (tuple 2) rs.relative(2); // moves forward two tuples (tuple 4) rs.relative(-3); // moves back three tuples (tuple 1)

23. Lecture 23© CIS 4301 - Spring 200623 Parameter Passing How do we pass parameters into query? Use question mark in place of portion of query, then bind values to those parameters Need to create prepared statement Need to apply to statement objects methods such as setString(i, v) setInt(i, v) Bind value v, which must be of appropriate type, to the i th parameter in the query

24. Lecture 23© CIS 4301 - Spring 200624 Parameter Passing PreparedStatement studioStat=myCon.createStatement(“INSERT INTO Studio(name, address) VALUES (?,?)”); /* get values for variables studioName and studioAddress from user */ studioStat.setString(1, studioName); studioStat.setString(2, studioAddr); studioStat.executeUpdate();

25. Lecture 23© CIS 4301 - Spring 200625 More Info… For more info on how to use JDBC with CISE’s Oracle installation see: www.cise.ufl.edu/~jhammer/classes/Oracle/Intro_JDBC.htm

26. Transactions in SQL

27. Lecture 23© CIS 4301 - Spring 200627 Multi-User Database Access In many applications, database may need to perform 100’s of ops/sec Initiated at different sites Perhaps involving same part of database at the same time Operations may interact in strange ways E.g., banking, airline reservation Let’s look at two specific problems

28. Lecture 23© CIS 4301 - Spring 200628 Example: Order of Operations void chooseSeat() { /* C code for obtaining flight date and seat, store in three variables */ EXEC SQL SELECT occupied INTO :occ FROM Flights WHERE fltNum = :flight AND fltDate = :date AND fltSeat = :seat; if (!occ){ EXEC SQL UPDATE FLights SET occupied = TRUE WHERE fltNum = :flight AND fltDate = :date AND fltSeat = :seat; /* Code to record seat assignments and to inform user of assignment */ } else /* C code to notify user of unavailability and ask for another seat selection. */ }

29. Lecture 23© CIS 4301 - Spring 200629 What Could Go Wrong? User 1 finds seat empty time User 2 finds seat empty User 1 sets seat to occupied User 2 sets seat to occupied Possible to execute chooseSeat function correctly, yet have global result not be correct Problem can be solved by SQL mechanism that serves to serialize the execution of the two function executions Execution of functions is serial if one function executes completely before any other function begins Execution is serializable if they behave as if they were run serially, even though execution may overlap in time

30. Lecture 23© CIS 4301 - Spring 200630 Example: Execution of Operations as Unity EXEC SQL BEGIN DECLARE SECTION; int acct1, acct2; /* the two accounts */ int balance1; /* amount of money in the first account*/ int amount; /* transfer amount EXEC SQL END DECLARE SECTION; Example: Transfer Money Function transfer() that reads two accounts and an amount of money Checks that the first has at least that much money If so, transfers money from first account to second

31. Lecture 23© CIS 4301 - Spring 200631 void transfer() { /* C code for obtaining account info and transfer amount */ EXEC SQL SELECT balance INTO :balance1 FROM Accounts WHERE acctNo = :acct1; if (balance1 >= amount){ EXEC SQL UPDATE Accounts SET balance = balance + :amount; WHERE acctNo = :acct2; EXEC SQL Update Accounts SET balance = balance - :amount; WHERE acctNo = :acct1; } else /* C code to print message that there were insufficient funds to make transfer. */ } Cont’d

32. Lecture 23© CIS 4301 - Spring 200632 What Could Go Wrong? If failure occurs between two updates,database will be left in inconsistent state See that certain operations needs to be done atomically Either all operations complete or none goes through Simple solution: make changes in local workspace and commit to database only after all work is done correctly

33. Lecture 23© CIS 4301 - Spring 200633 Solution: Transactions Solution to previous two problems is to group database operations into transactions Collection of one or more operations on the database that must be executed atomically In addition, SQL requires that transactions are executed in serializable manner How to control transactions in embedded SQL or CLI? Transaction begins as soon as SQL statement begins Optionally: START TRANSACTION Two ways to end transaction: COMMIT causes transaction to end successfully (exception: deferred constraints) ROLLBACK causes transaction to abort; any changes made to database are undone (rolled back) Transactions in generic SQL interface start and end with SQL statement (exception: triggers)

34. Lecture 23© CIS 4301 - Spring 200634 void transfer() { /* C code for obtaining account info and transfer amount */ EXEC SQL SELECT balance INTO :balance1 FROM Accounts WHERE acctNo = :acct1; if (balance1 >= amount){ EXEC SQL UPDATE Accounts SET balance = balance + :amount; WHERE acctNo = :acct2; EXEC SQL Update Accounts SET balance = balance - :amount; WHERE acctNo = :acct1; EXEC SQL COMMIT; } else { /* C code to print message that there were insufficient funds to make transfer. */ EXEC SQL ROLLBACK;} } Transfer() with Transactions transaction starts here

35. Lecture 23© CIS 4301 - Spring 200635 Transaction Properties Transactions obey the "ACID properties“ Atomicity Consistency Isolation Durability How are ACID properties achieved? Take COP 4720

36. Lecture 23© CIS 4301 - Spring 200636 Read-Only Transactions Reading and writing may lead to serialization problems Enforcement of serializability may result in lower transaction throughput When transaction only reads data, DBMS has more freedom to let transactions execute in parallel EXEC SQL SET TRANSACTION READ ONLY Must be executed before transaction begins

37. Lecture 23© CIS 4301 - Spring 200637 Weaker Properties There's a lot of overhead to guaranteeing the ACID properties Sometimes full isolation (i.e., full serializability) is not required Following is a discussion of some of the ways we can use transactions with less overhead

38. Lecture 23© CIS 4301 - Spring 200638 Dirty Reads Dirty data is a common term for data written by transaction that has not yet committed Dirty read is read of dirty data Risk in reading dirty data is that transaction that wrote may abort Dirty data removed from database to roll back to clean state But: What happens to transaction that has read dirty data and then committed? Sometimes dirty read matters, sometimes doesn’t Sometimes makes sense to take risk to avoid: Time-consuming work by DBMS to avoid dirty reads Loss of parallelism that results from waiting until there isno possibility of dirty read

39. Lecture 23© CIS 4301 - Spring 200639 Example 1: New Money Transfer Program P (1) Add money to account 2 (2) Test if account 1 has enough money If false, remove money from account 2 and abort If true, subtract money from account 1 and commit Strange but ok if executed serializably What if dirty reads are possible? Imagine three accounts A1 ($100), A2 ($200), and A3 ($300) T1 executes P to transfer $150 from A1 to A2 while T2 runs P to transfer $250 from A2 to A3 P

40. Lecture 23© CIS 4301 - Spring 200640 Possible Scenario T2 adds $250 to A3, now has $550 T1 adds $150 to A2, now has $350 T2 executes test on A2 ($350) and allows transfer from A2 to A3 to occur T1 executes test of on A1 ($150) which has not enough funds T2 subtracts $250 from A2 and commits; A2 ($100) T1 subtracts $150 from A2 which now has -$50; T1 commits

41. Lecture 23© CIS 4301 - Spring 200641 Example 2: New Seat Selection Program S (1)Find available seat; reserve it by setting occupied flag to 1; if none, abort (2)Ask customer for approval If yes, commit If no, release seat and set occupied flag to 0; repeat step 1 to get other seat Problem: If two transactions execute at roughly the same time, one might reserve seat S, which is later rejected by customer; second transaction may not get option to chose seat for that point in time; may need retry Dirty read has occurred Not as serious as in previous scenario; re-running transaction will give seat to second traveler S

42. Lecture 23© CIS 4301 - Spring 200642 Isolation Level: Allowing Dirty Reads SET TRANSACTION READ WRITE ISOLATION LEVEL READ UNCOMMITTED; Must specify, ow assumed to be Read-only

43. Lecture 23© CIS 4301 - Spring 200643 Other Isolation Levels Total of four isolation levels SERIALIZABLE (default) REPEATABLE READ READ COMMITTED READ UNCOMMITTED (Dirty Read) SET TRANSACTION ISOLATION LEVEL... Subtle point: the isolation level of a transaction affects only what data that transaction may see Does not affect what any other transaction sees

44. Lecture 23© CIS 4301 - Spring 200644 Isolation Level: Read Committed Let’s run seat choosing function with isolation level “read committed” What happens when other transactions run in parallel?

45. Lecture 23© CIS 4301 - Spring 200645 Isolation Level: Repeatable Read If tuple is retrieved first time, then we can be sure it is retrieved again if query is repeated However, new tuples may be visible as well during subsequent executions (phantom tuples) Example: If we execute seat-choosing problem under repeatable read isolation, if a seat is available on the first query, then it is available under subsequent queries In addition, if more seats become available (e.g., bigger plane), additional seats also visible during subsequent queries