1. Oracle9i Developer: PL/SQL Programming Chapter 11 Performance Tuning

2. Objectives Gain awareness of tuning concepts and issues Explore SQL statement tuning Explore PL/SQL statement tuning

3. Brewbean’s Challenge The application development group is beginning to test the Brewbean’s application Concerns arise regarding response time in some of the operations Where do they begin to review performance?

4. Tuning Issues Identify statements that are causing lengthy execution times Determine how the statement is being processed and identify potential modifications for improvement Determine if modifications improved processing efficiency

5. Identifying Coding Problems User testing feedback can narrow down the slow response areas V_$SQLAREA view contains execution statistics for all statements Number of reads per execution = disk reads / number of executions High number of reads highlights potential candidates for improvement

6. V_$SQLAREA View Query Note: A WHERE clause is used in this query to only display one SQL statement.

7. SQL TRACE Facility Can store statistics for a session in an operating system file Set server parameters to use Turn on and off: ALTER SESSION SET SQL_TRACE = TRUE; ALTER SESSION SET SQL_TRACE = FALSE; Use TKPROF command at operating system level to convert the file to a readable format

8. SQL_TRACE File (partial listing)

9. Oracle Optimizers Determine how a statement will be processed Two Optimizers available: rule-based and cost-based Rule-based Optimizer uses a list of rules Cost-based Optimizer uses object statistics Object must be analyzed for the cost- based Optimizer to work

10. Optimizer Mode ValueDescription CHOOSEIf statistics exist for any of the tables accessed the cost- basis is used with a goal of best throughput. If some of the tables do not have statistics then estimation is used. If no statistics are available, the rule-basis is used. ALL_ROWSAlways uses cost-basis with goal of best throughput. FIRST_ROWS_nAlways uses cost-basis with goal of best response time to return the first n number of rows. n can be 1, 10, 100 or 1000. FIRST_ROWSAvailable for backward compatibility. Uses a mix of cost and heuristics for fast delivery of the first few rows. RULEAlways uses rule-basis.

11. Explain Plan Execution plan for the statement that is generated by the Optimizer Display using AUTOTRACE SQL*Plus CommandDescription SET AUTOTRACE ONDisplays explain plan, statistics, and result set SET AUTOTRACE ON EXPLAINDisplays explain plan and result set SET AUTOTRACE ON STATISTICSDisplays statistics and result set SET AUTOTRACE TRACEONLYDisplays explain plan and statistics

12. Explain Plan Example

13. Explain Plan Example (continued)

14. Timing Feature Basic tool to measure processing times Displays execution time in hundredths of seconds To use: SET TIMING ON Always use second statement execution time to exclude statement caching time

15. Timing Example

16. SQL Statement Tuning Performance tuning focuses on SQL statements as these tend to be more resource intensive Overview of several issues: –Column selection –Optimizer used –Index use –Correlated subqueries –Join driving table –Optimizer hints

17. Column Selection Including unnecessary columns in SQL statements causes additional processing Could lead to unnecessary table scans and joins A common error is including a column from a table that causes an additional join when the column is available from tables already in the query

18. Optimizer Different execution plans will be produced by each of the optimizers Cost-based Optimizer is typically more efficient as it uses object statistics to determine execution path such as the use of indexes

19. Index Suppression Function on a WHERE clause column will suppress the use of an index

20. Index Suppression Comparing a character column to a numeric value will suppress index usage

21. Concatenated Indexes Concatenated indexes are indexes that include more than one column If all columns of the concatenated index are included in the criteria then the index will be used The Optimizer uses these indexes only when the leading column indexed is included in the criteria of the SQL statement

22. Concatenated Index Example Concatenated Index: CREATE INDEX bb_shopname_idx ON bb_shopper (lastname, firstname); Use of leading column of the index, index is used:

23. Concatenated Index Example Leading column of the index not included, index is not used:

24. Subquery Considerations Subquery processes inner query first and treats result set as an IN list A correlated subquery executes the outer query first and executes the inner query for each row returned in the outer query Correlated subqueries tend to be more efficient EXISTS operator will stop execution when match is found, IN does not stop

25. Correlated Subquery Example

26. Join Driving Table The driving table is the table used first to begin the join operation Goal for efficiency – smallest table to be the driving table Rule-based Optimizer sets the last table in the FROM clause as the driving table Cost-based Optimizer sets the first table in the FROM clause as the driving table

27. Optimizer Hints The execution plan of an SQL statement can be altered by including hints Hints include a comment with + sign Numerous hints are available and are grouped by the following categories: –The optimization approach for an SQL statement –The goal of the cost-based Optimizer for an SQL statement –The access path for a table accessed by the statement –The join order for a join statement –A join operation in a join statement

28. Optimizer Hint Examples Hint on optimization approach: SELECT /*+RULE*/ firstname, lastname FROM bb_shopper WHERE lastname = ‘Parker’; Hint on table access path: SELECT /*+FULL(b)*/ idshopper, firstname, lastname FROM bb_shopper b WHERE lastname = 'Parker';

29. PL/SQL Statement Tuning Attention to a number of common PL/SQL coding issues can lead to greater efficiency: –Minimize loop iterations –Use ROWID in updates –Perform variable comparisons –Order conditional statements –Use PLS_INTEGER data type –Pin program units

30. Minimize Loop Iterations Minimize both number of iterations and time for a single iteration Be sure that statements that do not have to be repeated for each iteration are located outside of the loop Be sure to minimize rows that need processing by loop by using WHERE and HAVING clauses

31. Use ROWID in UPDATES ROWID – physical address of row Fastest path to row retrieval 1 DECLARE 2 CURSOR prod_cur IS 3 SELECT idproduct, price, ROWID 4 FROM bb_product; 5 lv_new NUMBER(7,2); 6 BEGIN 7 FOR prod_rec IN prod_cur LOOP 8 lv_new := ROUND(prod_rec.price * :g_pct,2); 9 UPDATE bb_product 10 SET price = lv_new 11 WHERE rowid = prod_rec.ROWID; 12 END LOOP; 13 COMMIT; 14 END;

32. Variable Comparisons When comparing values of different data types, Oracle must implicitly convert values to the same type The conversion will add to the processing time Numeric to character comparison: FOR i IN 1..100000 LOOP IF i IN('5','10','15','20') THEN lv_cnt := lv_cnt + 1; END IF; END LOOP;

33. Ordering Conditions Developers tend to order conditional statements in IF or CASE statements in logical order which may not be most efficient If information is known regarding data values, place the most frequent true conditions first to minimize processing needed

34. PLS_INTEGER Data Type A numeric data type that can hold whole number values ranging from negative to positive 2,147,483,647 More efficient than the NUMBER data type

35. Pinning Program Units Stored program units that are executed are placed into the shared pool memory which serves as a cache Code is moved out of cache when memory space runs out Heavily used program units can be pinned into memory DBMS_SHARED_POOL packages enables pinning

36. Summary The goal of tuning is to minimize resources needed by an application to improve response time Performance tuning ranges from hardware configuration and database configuration to application code This section only addresses application code tuning The V_$SQLAREA view and SQL Trace facility enable the review of processing statistics

37. Summary Two optimizers exist: Rule-based and Cost-based The explain plan outlines the execution path the Optimizer generates AUTOTRACE enables display of the explain plan The TIMING command allows the display of execution time

38. Summary SQL tuning includes a review of: selected columns, indexes, order of tables in the FROM clause, correlated subquery use, and optimizer identification PL/SQL tuning includes a review of: loop iterations, use of ROWID, variable data types in comparisons, order of conditional statements, use of PLS_INTEGER data type, and potential pinning opportunities

39. Summary This section is only an introduction to tuning to expose you to the topic