使用DECODE在更新语句中进行性能调整

Question

I have below update statement which include a reference to a table and a DECODE statement. My table has around 500000 records which will alter by this statement and it is taking around 2 1/2 hours to finish. Any types of suggestions are welcome to enhance the performance of this.

Here comes my update statement

   UPDATE TABlE_1 t1
  SET t1.column_1= DECODE( (SELECT creator 
                            FROM table_2 t2
                            WHERE t2.key1 = t1.key1 
                            AND   t2.key2 = t1.key2), 
                            'AAAAAAA_table2', 
                            'aaaaa_table1', 
                            'BBBBBBB_table2', 
                            'bbbbb_table1', 
                            'ccccc_table1')
  WHERE t1.column_1 IS NULL;

Answer 1

If the number of options in the decode statement is not too large, you may be better off running a number of targeted update statements.

eg

    UPDATE table_1 t1 
    SET t1.column_1 = 'aaaaa_table_1' 
    WHERE EXISTS (SELECT 1 
                 FROM table_2 t2
                 WHERE t2.key1 = t1.key1 
                 AND   t2.key2 = t1.key2
                 AND creator = 'AAAAAA') 
   AND t1.column_1 IS NULL

And then repeat this for each mapping you are trying to perform.

Answer 2

You seem to have a rule that transforms the CREATOR value when inserting to TABLE_1. How about just applying the rule.

UPDATE table_1 t1
   SET t1.column_1 = (SELECT LOWER(SUBSTR(creator,1,5))||'_table1' 
                        FROM table_2 t2
                       WHERE t2.key1 = t1.key1 
                         AND t2.key2 = t1.key2)
  WHERE t1.column_1 IS NULL

Or use a merge statement

MERGE INTO table_1 t1
USING (SELECT LOWER(SUBSTR(creator,1,5))||'_table1' AS derived_creator
         FROM table_2) t2  
       ON (    t2.key1 = t1.key1 
           AND t2.key2 = t1.key2)
WHEN MATCHED THEN UPDATE SET t1.column_1 = t2.derived_creator

I haven't run these on a dataset so can't say that either would be quicker. However, I think that they both look neater. As an aside it is recommended that the CASE expression be used in SQL now instead of the DECODE function which would give you the SQL below.

UPDATE TABlE_1 t1
   SET t1.column_1= SELECT CASE creator 
                           WHEN 'AAAAAAA_table2' THEN 'aaaaa_table1'
                           WHEN 'BBBBBBB_table2' THEN 'bbbbb_table1'
                           ELSE 'ccccc_table1'
                           END                           
                      FROM table_2 t2
                     WHERE t2.key1 = t1.key1 
                       AND t2.key2 = t1.key2
  WHERE t1.column_1 IS NULL