by Markus Winand.

Oracle Example Scripts for “Sorting and Grouping”


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

Indexed Order By

SELECT sale_date, product_id, quantity
  FROM sales
 WHERE sale_date >= TRUNC(sysdate) - INTERVAL '1' DAY
 ORDER BY product_id

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 (at least 11g-19c) 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;

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;

There is no known workaround for this problem.

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