Oracle Example Scripts for “Sorting and Grouping”


This section contains the create, insert and PL/SQL code to run the examples from Chapter 6 in an Oracle 11gR2 database.

Indexed Order By

DB2
Explain Plan
-----------------------------------------------------------
ID | Operation            |                     Rows | Cost
 1 | RETURN               |                          |  688
 2 |  FETCH SALES         |     394 of 394 (100.00%) |  688
 3 |   IXSCAN SALES_DT_PR | 394 of 1009326 (   .04%) |   24

Predicate Information
 3 - START (Q1.SALE_DATE = (CURRENT DATE - 1 DAYS))
      STOP (Q1.SALE_DATE = (CURRENT DATE - 1 DAYS))
Oracle
---------------------------------------------------------------
|Id | Operation                   | Name        | Rows | Cost |
---------------------------------------------------------------
| 0 | SELECT STATEMENT            |             |  320 |  300 |
| 1 |  TABLE ACCESS BY INDEX ROWID| SALES       |  320 |  300 |
|*2 |   INDEX RANGE SCAN          | SALES_DT_PR |  320 |    4 |
---------------------------------------------------------------

Gathering new statistics is good practice after changing indexes:

BEGIN
     DBMS_STATS.GATHER_TABLE_STATS(null, 'SALES', 
     METHOD_OPT=>'for all indexed columns', CASCADE => true);
END;
/

Indexed Group By

There is one particular problem in the Oracle database (11g) that appears when ordering the grouped result in reverse index order:

SELECT product_id, sum(eur_value)
  FROM sales
 WHERE sale_date = TRUNC(sysdate) - INTERVAL '1' DAY
 GROUP BY product_id
 ORDER BY product_id DESC;
--------------------------------------------------------------
|Id | Operation                   | Name       | Rows | Cost |
--------------------------------------------------------------
| 0 |SELECT STATEMENT             |            |   24 |  193 |
| 1 | SORT GROUP BY               |            |   24 |  193 |
| 2 |  TABLE ACCESS BY INDEX ROWID| SALES      |  321 |  192 |
|*3 |   INDEX RANGE SCAN          | SALES_DT_PR|  321 |    3 |
--------------------------------------------------------------

Although it can use the index when ordering in index order:

SELECT product_id, sum(eur_value)
  FROM sales
 WHERE sale_date = TRUNC(sysdate) - INTERVAL '1' DAY
 GROUP BY product_id
 ORDER BY product_id ASC;
--------------------------------------------------------------
|Id | Operation                   | Name       | Rows | Cost |
--------------------------------------------------------------
| 0 |SELECT STATEMENT             |            |   24 |  192 |
| 1 | SORT GROUP BY NOSORT        |            |   24 |  192 |
| 2 |  TABLE ACCESS BY INDEX ROWID| SALES      |  321 |  192 |
|*3 |   INDEX RANGE SCAN          | SALES_DT_PR|  321 |    3 |
--------------------------------------------------------------

There is no known workaround for this problem.

About the Author

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Markus Winand tunes developers for high SQL performance. He also published the book SQL Performance Explained and offers in-house training as well as remote coaching at http://winand.at/

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