Explaining the explain_plan
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- 2. <Insert Picture Here> Explaining the Explain Plan: Interpreting Execution Plans for SQL Statements Maria Colgan Senior Principal Product Manager
- 3. Agenda • What is an execution plan <Insert Picture Here> • How to generate a plan • What is a good plan for the optimizer • Understanding execution plans – Cardinality – Access paths – Join type – Join order • Execution plan examples 3
- 4. What is an Execution plan? • Execution plans show the detailed steps necessary to execute a SQL statement • These steps are expressed as a set of database operators that consumes and produces rows • The order of the operators and their implementation is decided by the optimizer using a combination of query transformations and physical optimization techniques • The display is commonly shown in a tabular format, but a plan is in fact tree-shaped 4
- 5. What is an Execution plan? 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 Tree-shaped representation of plan ----------------------------------------------------------- Id Operation Name GROUP BY ----------------------------------------------------------- 0 SELECT STATEMENT 1 HASH GROUP BY HASH JOIN 2 HASH JOIN 3 TABLE ACCESS FULL PRODUCTS 4 PARTITION RANGE ALL 5 TABLE ACCESS FULL SALES TABLE ACCESS TABLE ACCESS ---------------------------------------------------------- PRODUCTS SALES 5
- 6. Agenda • What is an execution plan <Insert Picture Here> • How to generate a plan • What is a good plan for the optimizer • Understanding execution plans – Cardinality – Access paths – Join type – Join order • Execution plan examples 6
- 7. How to get an Execution Plan Two methods for looking at the execution plan 1. EXPLAIN PLAN command – Displays an execution plan for a SQL statement without actually executing the statement 2. V$SQL_PLAN – A dictionary view introduced in Oracle 9i that shows the execution plan for a SQL statement that has been compiled into a cursor in the cursor cache Either way use DBMS_XPLAN package to display plans Under certain conditions the plan shown with EXPLAIN PLAN can be different from the plan shown using V$SQL_PLAN 7
- 8. How to get an Execution Plan Example 1 • EXPLAIN PLAN command & dbms_xplan.display function SQL> EXPLAIN PLAN FOR SELECT prod_name, avg(amount_sold) FROM sales s, products p WHERE p.prod_id = s.prod_id GROUP BY prod_name; SQL> SELECT plan_table_output FROM table(dbms_xplan.display('plan_table',null,'basic')); ------------------------------------------ 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 ------------------------------------------- 8
- 9. How to get an Execution Plan Example 2 • Generate & display execution plan for last SQL stmts executed in a session SQL>SELECT prod_category, avg(amount_sold) FROM sales s, products p WHERE p.prod_id = s.prod_id GROUP BY prod_category; SQL> SELECT plan_table_output FROM table(dbms_xplan.display_cursor(null,null,'basic')); ------------------------------------------ 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 ------------------------------------------- 9
- 10. Agenda • What is an execution plan <Insert Picture Here> • How to generate a plan • What is a good plan for the optimizer • Understanding execution plans – Cardinality – Access paths – Join type – Join order • Execution plan examples 10
- 11. What’s a Good Plan for the Optimizer? The Optimizer has two different goals • Serial execution: It’s all about cost – The cheaper, the better • Parallel execution: it’s all about performance – The faster, the better Two fundamental questions: • What is cost? • What is performance? 11
- 12. What is Cost? • A magically number the optimizer makes up? • Resources required to execute a SQL statement? • Result of complex calculations? • Estimate of how long it will take to execute a statement? Actual Definition • Cost represents units of work or resources used • Optimizer uses CPU & IO as units of work • Cost is an estimate of the amount of CPU and the number of disk I/Os, used to perform an operation Cost is an internal Oracle measurement 12
- 13. What is performance? • Getting as many queries completed as possible? • Getting fastest possible elapsed time using the fewest resources? • Getting the best concurrency rate? Actual Definition • Performance is fastest possible response time for a query • Goal is to complete the query as quickly as possible • Optimizer does not focus on resources needed to execute the plan 13
- 14. Agenda • What is an execution plan <Insert Picture Here> • How to generate a plan • What is a good plan for the optimizer • Understanding execution plans – Cardinality – Access paths – Join type – Join order • Execution plan examples 14
- 15. Cardinality What is it? • Estimate of number rows that will be returned by each operation How does the Optimizer Determine it? • Cardinality for a single column equality predicate = total num of rows num of distinct values – For example: A table has 100 rows, a column has 10 distinct values => cardinality=10 rows • More complicated predicates have more complicated cardinality calculation Why should you care? • Influences everything! Access method, Join type, Join Order etc 15
- 16. How to Identify Cardinality in an Execution Plan Cardinality the estimated # of rows returned Determine correct cardinality using a SELECT COUNT(*) from each table applying any WHERE Clause predicates belonging to that table 16
- 17. Check Cardinality Estimates SELECT /*+ gather_plan_statistics */ p.prod_name, SUM(s.quantity_sold) FROM sales s, products p WHERE s.prod_id =p.prod_id GROUP By p.prod_name ; SELECT * FROM table ( DBMS_XPLAN.DISPLAY_CURSOR(FORMAT=>'ALLSTATS LAST')); Compare the estimated number of rows returned for each operation in the plan to actual rows returned 17
- 18. Check Cardinality using SQL Monitor SQL Monitor is the easiest way to compare the estimated number of rows returned for each operation in a parallel plan to actual rows returned 18
- 19. Solutions to incorrect Cardinality estimates Cause Solution Stale or missing statistics DBMS_STATS Data Skew Create a histogram Multiple single column predicates Create a column group using on a table DBMS_STATS.CREATE_EXTENDED_STATS Function wrapped column Create statistics on the funct wrapped column using DBMS_STATS.CREATE_EXTENDED_STATS Multiple columns used in a join Create a column group on join columns using DBMS_STATS.CREATE_EXTENDED_STAT Complicated expression Use dynamic sampling level 4 or higher containing columns from multiple tables 19
- 20. Agenda • What is an execution plan <Insert Picture Here> • How to generate a plan • What is a good plan for the optimizer • Understanding execution plans – Cardinality – Access paths – Join type – Join order • Execution plan examples 20
- 21. Access Paths – Getting the data Access Path Explanation Reads all rows from table & filters out those that do not meet the where clause Full table scan predicates. Used when no index, DOP set etc Rowid specifies the datafile & data block containing the row and the location of Table access by Rowid the row in that block. Used if rowid supplied by index or in where clause Only one row will be returned. Used when stmt contains a UNIQUE or a Index unique scan PRIMARY KEY constraint that guarantees that only a single row is accessed Accesses adjacent index entries returns ROWID values Used with equality on Index range scan non-unique indexes or range predicate on unique index (<.>, between etc) Skips the leading edge of the index & uses the rest Advantageous if there are Index skip scan few distinct values in the leading column and many distinct values in the non- leading column Processes all leaf blocks of an index, but only enough branch blocks to find 1st Full index scan leaf block. Used when all necessary columns are in index & order by clause matches index struct or if sort merge join is done Scans all blocks in index used to replace a FTS when all necessary columns Fast full index scan are in the index. Using multi-block IO & can going parallel Hash join of several indexes that together contain all the table columns that are Index joins referenced in the query. Wont eliminate a sort operation uses a bitmap for key values and a mapping function that converts each bit Bitmap indexes position to a rowid. Can efficiently merge indexes that correspond to several conditions in a WHERE clause 21
- 22. How to Identify Access Path in an Execution Plan Look in Operation section to see how an object is being accessed If the wrong access method is being used check cardinality, join order… 22
- 23. Access Path examples A table countries contains 10K rows & has a primary key on country_id – What plan would you expect for these queries? Select country_id, name from countries where country_id in ('AU','FR','IE‘); Select country_id, name from countries where country_id between 'AU' and 'IE'; 23
- 24. Access Path examples A table countries contains 10K rows & has a primary key on country_id – What plan would you expect for these queries? Select country_id, name from countries where name='USA'; 24
- 25. Common Access Path issues Issue Cause Uses a table scan instead of index DOP on table but not index, value of MBRC Picks wrong index Stale or missing statistics Cost of full index access is cheaper than index look up followed by table access Picks index that matches most # of column 25
- 26. Agenda • What is an execution plan <Insert Picture Here> • How to generate a plan • What is a good plan for the optimizer • Understanding execution plans – Cardinality – Access paths – Join type – Join order • Execution plan examples 26
- 27. Join Types Join Type Explanation For every row in the outer table, Oracle accesses all the rows in the Nested Loops joins inner table Useful when joining small subsets of data and there is an efficient way to access the second table (index look up) The smaller of two tables is scan and resulting rows are used to build Hash Joins a hash table on the join key in memory. The larger table is then scan, join column of the resulting rows are hashed and the values used to probing the hash table to find the matching rows. Useful for larger tables & if equality pred Consists of two steps: Sort Merge joins 1. Both the inputs are sorted on the join key. 2. The sorted lists are merged together. Useful when the join condition between two tables is an inequality condition or one of the table is already ordered e.g. index access Joins every row from one data source with every row from the other Cartesian Joins data source, creating the Cartesian Product of the two sets. Only good if tables are very small. Only choice if there is no join condition specified in query Returns all rows that satisfy the join condition and also returns all of Outer Joins the rows from the table without the (+) for which no rows from the other table satisfy the join condition 27
- 28. How to Identify Join Type in an Execution Plan Look in the Operation section to check the right join type is used If wrong join type is used check stmt is written correctly & cardinality estimates 28
- 29. Join Type Example 1 What Join type should be use for this Query? SELECT e.last_name, e.salary, d.department_name FROM hr.employees e, hr.departments d WHERE d.departments_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 29
- 30. Join Type Example 2 What Join type should be use for this Query? SELECT o.customer_id, l.unit_price * l.quantity FROM oe.orders o ,oe.order_items l WHERE l.order_id = o.order_id; Orders has 105 rows Order Items has 665 rows 30
- 31. Join Type Example 3 What Join type should be use for this Query? SELECT o.order_id,0.order_date,e.name FROM oe.orders o , hr.employees e; Orders has 105 rows Employees has 107 rows 31
- 32. Join Type Example 3 Cartesian product not always bad • Chosen when the number of rows being joined is low • Commonly used to join multiple small dimensions to one large fact table s.prod_id=p.prod_id s.cust_id=c.cust_id PRODUCTS SALES CUSTOMERS s.promo_id=c.promo_id PROMOTIONS 32
- 33. Join Type Example 3 Cartesian product • By joining the three small dimension tables together first we minimize the number of rows being carried through the joins 33
- 34. Join Type Example 4 What Join type should be use for this Query? SELECT d.department_id,e.emp_id FROM hr.employees e FULL OUTER JOIN hr.departments d ON e.department_id = d.department_id ORDER BY d.department_id; Employees has 107 rows Departments has 27 rows Foreign key relationship between Employees and Departments on dept_id 34
- 35. What causes the wrong Join Type to be selected Issue Cause Nested loop Cardinality estimate on the left side is under selected instead estimated triggers Nested loop to be selected of hash join Hash join selected In case of a hash join the Optimizer doesn’t taken instead of nested into consideration the benefit of caching. Rows on loop the left come in a clustered fashion or (ordered) so the probe into the right is less expensive Cartesian Joins Cardinality underestimation 35
- 36. Agenda • What is an execution plan <Insert Picture Here> • How to generate a plan • What is a good plan for the optimizer • Understanding execution plans – Cardinality – Access paths – Join type – Join order • Execution plan examples 36
- 37. Join Orders The order in which the tables are join in a multi table stmt • Ideally start with the table that will eliminate the most rows • Strongly affected by the access paths available Some basic rules • Joins guaranteed to produce at most one row always go first – Joins between two row sources that have only one row each • When outer joins are used the table with the outer join operator must come after the other table in the predicate • If view merging is not possible all tables in the view will be joined before joining to the tables outside the view 37
- 38. How to Identify Join order in an Execution Plan 1 2 3 4 5 Want to start with the table that reduce the result set the most If the join order is not correct, check the statistics, cardinality & access methods 38
- 39. Finding the Join Order for complex SQL statement • It can be hard to determine Join Order for Complex SQL statements but it is easily visible in the outline data of plan SELECT * FROM table(dbms_xplan.display_cursor(FORMAT=>’TYPICAL +outline’); The leading hint tells you the join order 39
- 40. What causes the wrong Join Order Causes Incorrect single table cardinality estimates Incorrect join cardinality estimates 40
- 41. Agenda • What is an execution plan <Insert Picture Here> • How to generate a plan • What is a good plan for the optimizer • Understanding execution plans – Cardinality – Access paths – Join type – Join order • Execution plan examples 41
- 42. Example SQL Statement • Find all the employees who make as much or more than their manager SELECT e1.last_name, e1.job_title, e1.total_comp FROM (SELECT e.manager_id, e.last_name, j.job_title, (e.salary+e.commission_pct) total_comp FROM employees e, jobs j, departments d WHERE d.department_name = 'Sales' AND e.department_id = d.department_id AND e.job_id = j.job_id ) e1, (SELECT e.employee_id, (e.salary+e.commission_pct) tc FROM employees e, departments d WHERE d.department_name = ‘Sales' AND e.department_id = d.department_id ) e2 WHERE e1.manager_id = e2.employee_id AND e1.total_comp >= e2.tc; 42
- 43. Is it a good Execution plan? 1. Is the estimated number of rows being returned accurate? 3.Are the access method correct? 2. Are the cardinality estimates accurate? Means no stats gathered strong indicator this won’t be best possible plan 43
- 44. Example Cont’d Execution plan 1 2 3 4 5 4. Are the right join types being used? 5. Is the join order correct? Is the table that eliminates the most rows accessed first? 44
- 45. What does the Plan Tree look like? NESTED LOOP NESTED LOOP INDEX UNIQUE SCAN - TABLE ACCESS JOBS INDEX UNIQUE SCAN - HASH JOIN TABLE ACCESS DEPARTMENT MERGE JOIN TABLE ACCESS CARTESIAN EMPLOYEES TABLE ACCESS TABLE ACCESS DEPARTMENT EMPLOYEES 45
- 46. Example Cont’d Execution plan - Solution 1. Only 1 row is actually returned and the cost is 4 lower now 4. Join types have changed to be all NL 1 5. The join 2 order has changed 3 4 5 3. Access methods have changed for 2. Cardinalities are correct some tables and with each join number of rows reduced 46
- 47. What does the Plan Tree look like? NESTED LOOP INDEX UNIQUE SCAN - NESTED LOOP TABLE ACCESS DEPARTMENT NESTED LOOP INDEX UNIQUE SCAN - TABLE ACCESS JOBS INDEX RANGE SCAN - NESTED LOOP TABLE ACCESS EMPLOYEES TABLE ACCESS INDEX RANGE SCAN - DEPARTMENT TABLE ACCESS EMPLOYEES 47
- 48. Q&A 48
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