Query difference. UNION vs OR

Question

Connected to:
Oracle9i Enterprise Edition Release 9.2.0.8.0 - 64bit Production
With the Partitioning, Oracle Label Security, OLAP and Oracle Data Mining options
JServer Release 9.2.0.8.0 - ProductionHello,
Sorry for a question on EOD friday.
I have three tables.(original tables have millions of rows)
Here is the sample create statements.
CREATE TABLE table1
AS
   (SELECT LEVEL col1, 10 col2
      FROM DUAL
    CONNECT BY LEVEL <= 500000
    UNION ALL
    SELECT LEVEL col1, 20 col2
      FROM DUAL
    CONNECT BY LEVEL <= 10000);
CREATE TABLE table2
AS
   (SELECT LEVEL col1,
           10 col2,
           'MAIN' col3,
           0 col4,
           LEVEL * 111 col5
      FROM DUAL
    CONNECT BY LEVEL <= 500000
    UNION ALL
    SELECT 20 col1,
           20 col2,
           'PRAC' col3,
           1 col4,
           999 col5
      FROM DUAL);
CREATE TABLE table3
AS
   (SELECT CEIL (LEVEL / 6) col1, LEVEL col2
      FROM DUAL
    CONNECT BY LEVEL <= 3000000);And I have this query written long ago by some one.
SELECT /*+ full(t1) full(t2) full(t3) */
      t1.col1,t1.col2,t2.col5,t3.col2
  FROM table1 t1, table2 t2,table3 t3
WHERE t1.col1 = t2.col1
   AND t1.col2 = t2.col2
   AND t1.col1 = t3.col1
UNION
SELECT /*+ full(t1) full(t2) full(t3) */
      t1.col1,t1.col2,t2.col5,t3.col2
  FROM table1 t1, table2 t2,table3 t3
WHERE t2.col3 = 'PRAC'
   AND t2.col4 = 1
   AND t1.col2 = t2.col2
   AND t1.col1 = t3.col1;     A little background on the business: For every row in table 1 there will be a row in table2 where both col1 and col2 matches.
and there might be one more row where only col2 matches and t2.col3='PRAC' and t2.col4=1.
That row is a common contract and will be shared by multiple rows in table1. You can see that when you execute the above select.
And for every row in table1 there are 6 rows in table3.
Now, After seeing the above select, I have changed it to the following simply which gives me same results and started testing it.
Which I have no doubts is simple and more efficient.
SELECT /*+ full(t1) full(t2) full(t3)*/
      t1.col1,t1.col2,t2.col5,t3.col2
  FROM table1 t1, table2 t2,table3 t3
WHERE (t1.col1 = t2.col1
     OR  (t2.col3 = 'PRAC'
      AND t2.col4 = 1))
   AND t1.col2 = t2.col2
   AND t1.col1=t3.col1
ORDER BY t1.col1;But surprisingly, the new query is taking for ever. What is wrong with a full table scan and an OR clause?
In the old query we have a simple joins in both parts of the union and my new query will return either 1 or 2 rows for every row in table1.
Not sure whats wrong.
G.
Edited by: Ganesh Srivatsav on Apr 29, 2011 3:51 PM
Edited by: Ganesh Srivatsav on Apr 29, 2011 4:57 PM

Answer 1

here is what I got
SELECT /*+ full(t1) full(t2) */
      t1.col1,t1.col2,t2.col5
  FROM table1 t1, table2 t2
WHERE t1.col1 = t2.col1
   AND t1.col2 = t2.col2
UNION
SELECT /*+ full(t1) full(t2) */
      t1.col1,t1.col2,t2.col5
  FROM table1 t1, table2 t2
WHERE t2.col3 = 'PRAC'
   AND t2.col4 = 1
   AND t1.col2 = t2.col2;
Elapsed: 00:00:00.04
plan
SELECT STATEMENT  CHOOSECost: 11  Bytes: 349  Cardinality: 5                      
     8 SORT UNIQUE  Cost: 11  Bytes: 349  Cardinality: 5                 
          7 UNION-ALL            
               3 HASH JOIN  Cost: 5  Bytes: 65  Cardinality: 1       
                    1 TABLE ACCESS FULL TABLE DSAMSTRC.TABLE2 Cost: 2  Bytes: 234  Cardinality: 6 
                    2 TABLE ACCESS FULL TABLE DSAMSTRC.TABLE1 Cost: 2  Bytes: 182  Cardinality: 7 
               6 HASH JOIN  Cost: 5  Bytes: 284  Cardinality: 4       
                    4 TABLE ACCESS FULL TABLE DSAMSTRC.TABLE2 Cost: 2  Bytes: 45  Cardinality: 1 
                    5 TABLE ACCESS FULL TABLE DSAMSTRC.TABLE1 Cost: 2  Bytes: 182  Cardinality: 7 
SELECT /*+ full(t1) full(t2) */
      t1.col1,t1.col2,t2.col5
  FROM table1 t1, table2 t2
WHERE (t1.col1 = t2.col1
     OR  (t2.col3 = 'PRAC'
      AND t2.col4 = 1))
   AND t1.col2 = t2.col2
ORDER BY t1.col1;
Elapsed: 00:00:00.03
Plan
SELECT STATEMENT  CHOOSECost: 6  Bytes: 336  Cardinality: 4                 
     4 SORT ORDER BY  Cost: 6  Bytes: 336  Cardinality: 4            
          3 HASH JOIN  Cost: 5  Bytes: 336  Cardinality: 4       
               1 TABLE ACCESS FULL TABLE DSAMSTRC.TABLE2 Cost: 2  Bytes: 348  Cardinality: 6 
               2 TABLE ACCESS FULL TABLE DSAMSTRC.TABLE1 Cost: 2  Bytes: 182  Cardinality: 7