1. AN INTRODUCTION TO EXECUTION PLAN OF QUERIES These slides have been adapted from a presentation originally made by ORACLE. The full set of original slides are available at : http://www.slideshare.net/Jackieken/explaining-the-explain-plan

2. WHAT IS AN EXECUTION PLAN? Detailed steps necessary to execute a SQL statement Steps expressed as a set of database operators that consumes and produces rows Optimizer decides The order of the operators and their implementation using query transformations and physical optimization techniques The display is commonly shown in a tabular format, but a plan is in fact tree-shaped

3. TABULAR AND TREE REPRESENTATIONS Query SELECT prod_category, avg(amount_sold) FROM sales s, products p WHERE p.prod_id = s.prod_id GROUP BY prod_category; Tabular representation of plan ----------------------------------------------------------- Id Operation Name ----------------------------------------------------------- 0 SELECT STATEMENT 1 HASH GROUP BY 2 HASH JOIN 3 TABLE ACCESS FULL PRODUCTS 4 PARTITION RANGE ALL 5 TABLE ACCESS FULL SALES ---------------------------------------------------------- Tree-shaped representation of plan GROUP BY | JOIN ______|_______ | | TABLE ACCESS TABLE ACCESS PRODUCTS SALES

4. HOW TO GET AN EXECUTION PLAN ? EXPLAIN PLAN command Displays an execution plan for a SQL statement without actually executing the statement EXPLAIN PLAN SET STATEMENT_ID = ' ' FOR ; Plan stored in PLAN_TABLE

5. SELECT LPAD(' ', 2*LEVEL)||OPERATION||' '||OPTIONS||' '||OBJECT_NAME Query_Plan FROM PLAN_TABLE CONNECT BY PRIOR ID = PARENT_ID and STATEMENT_ID = ' ' START WITH ID=0 and STATEMENT_ID = ' ' ORDER BY ID; QUERY THE PLAN_TABLE

6. USE CASE Use Case: Plan review allows review of access paths and join types Access paths Is the data being accessed in the best way? Scan? Index lookup? Join type Are the right join algorithm types being used?

7. ACCESS PATHS Access Path describes method to get data out of the table The access path can be: Full table scan Table access by Rowid Index unique scan Index range scan (descending) Index skip scan Full index scan Fast full index scan Index joins Bitmap indexes

8. Look in Operation session to see how obj is being accessed EXAMPLE OF ACCESS PATHS What access method should be use for this Query? SELECT e.name, e.salary, d.dept_name FROM hr.employees e, hr.departments d WHERE d.dept_name IN ('Marketing‘,'Sales') AND e.department_id=d.department_id; Employees has 107 rows Departments has 27 rows Foreign key relationship between Employees and Departments on dept_id

9. JOIN ALGORITHM TYPE A Join retrieve data from more than one table Possible join algorithm types are Nested Loops joins Hash Joins Partition Wise Joins Sort Merge joins Cartesian Joins Outer Joins

10. JOIN ALGORITHM TYPE EXAMPLE What Join type should be use for this Query? SELECT e.name, e.salary, d.dept_name FROM hr.employees e, hr.departments d WHERE d.dept_name IN ('Marketing‘,'Sales') AND e.department_id=d.department_id; Employees has 107 rows Departments has 27 rows Foreign key relationship between Employees and Departments on dept_id Look in Operation session to see Join strategy