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CHAPTER 19 IM PR OVING DA TA BAS E PERFORM ANC E: S QL QUE RY OPTIMIZA TION
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$ @$ORACLE_HOME/rdbms/admin/utlxpls.sql The output of the utlxpls.sql script is exactly identical to that of the DBMS_XPLAN.DISPLAY, which was presented a few paragraphs prior.
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Interpreting the EXPLAIN PLAN Output
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Reading an EXPLAIN PLAN is somewhat confusing in the beginning, and it helps to remember these simple principles: Each step in the plan returns output in the form of a set of rows to the parent step. Read the plan outward starting from the line that is indented the most. If two operations are at the same level in terms of their indentation, read the top one first. The numbering of the steps in the plan is misleading. Start reading the EXPLAIN PLAN output from the inside out. That is, read the most indented operation first. In the example shown earlier in Listing 19-3 (I reproduce the plan output after the code), Oracle uses the INVENTORIES table as its driving table and uses the following execution path: SELECT STATEMENT HASH JOIN NESTED LOOPS TABLE ACCESS FULL INVENTORIES INDEX UNIQUE SCAN WAREHOUSES_PK INDEX FAST FULL SCAN PRD_DESC_PK The plan output is as follows: 1. Oracle does a full table scan of the INVENTORIES table. 2. Oracle performs an index unique scan of the WAREHOUSES table using its primary key index. 3. Oracle performs a nested loop operation to join the rows from steps 1 and 2. 4. Oracle performs an index fast full scan of the product_descriptions table using its primary key, PRD_DESC_PK. 5. In the final step, Oracle performs a hash join of the set from step 3 and the rows resulting from the index full scan of step 4. Using the output of the EXPLAIN PLAN, you can quickly see why some of your queries are taking much longer than anticipated. Armed with this knowledge, you can fine-tune a query until an acceptable performance threshold is reached. The wonderful thing about the EXPLAIN PLAN is that you never have to execute any statement in the database to trace the execution plan of the statement. The next section presents a few examples so you can feel more comfortable using the EXPLAIN PLAN utility.
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In this section, you ll learn how to interpret various kinds of execution plans derived by using the EXPLAIN PLAN utility. In the first example, consider what happens when you use a function on an indexed column. Oracle completely ignores the index! As you can see, the optimizer can make mistakes. Good programmers can help the optimizer get it right by using methods such as proper indexing of tables, optimizer hints, and so on.
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CHAPTER 19 IMPROV IN G DA TA BAS E PERFORMAN CE: SQL QUERY OPTIMIZA TION
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EXPLAIN PLAN set statement_id = 'example_plan1' FOR SELECT last_name FROM hr.employees WHERE upper(last_name) = 'FAY';
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Explained. SQL> example_plan1 ----------------------------SELECT STATEMENT TABLE ACCESS FULL EMPLOYEES SQL> The next example is a query similar to the preceding one, but without the upper function on last_name. This time, Oracle uses the index on the last_name column: SQL> EXPLAIN PLAN SET statement_id = 'example_plan1' 2 FOR 3 SELECT last_name FROM hr.employees 4*WHERE last_name='FAY'; Explained. SQL> example_plan1 -----------------------------SELECT STATEMENT INDEX RANGE SCAN EMP_NAME_IX SQL> In the third example, two tables (customers and orders) are joined to retrieve the query results: SQL> EXPLAIN PLAN SET statement_id 'newplan1' 2 FOR 3 SELECT o.order_id, 4 o.order_total, 5 c.account_mgr_id 6 FROM customers c, 7 orders o 8 WHERE o.customer_id=c.customer_id 9 AND o.order_date > '01-JUL-05' Explained. SQL> Listing 19-4 shows the EXPLAIN PLAN from the plan table. Listing 19-4. Another EXPLAIN PLAN Output SQL> 2 3 4 5 6 7* Enter old SELECT lpad(' ',level-1)||operation||' '||options||' '|| object_name "newplan" FROM plan_table CONNECT BY prior id = parent_id AND prior statement_id = statement_id START WITH id = 0 AND statement_id = '&1' ORDER BY id; value for 1: newplan1 6: START WITH id = 0 AND statement_id = '&1'
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