Query Optimization Operators

This section describes the syntax for migrating Teradata query optimization operators. The migration syntax determines how the keywords and features are migrated.

Use the inToExists parameter to configure the migration from IN orNOT IN to EXISTS or NOT EXISTS.

This parameter defaults to FALSE. To enable the query optimization feature, this parameter must be set to TRUE.

When being converted to GaussDB(DWS) SQL queries, Teradata queries containing the IN and NOT IN operators have been optimized, and IN and NOT IN have been converted to EXISTS and NOT EXISTS, respectively. The IN and NOT IN operators support single or multiple columns. DSC will migrate the IN or NOT IN statement only when it exists in the WHERE or ON clause. The following example shows the conversion from IN to EXISTS, which is also applicable to the conversion from NOT IN to NOT EXISTS.

Simple conversion from IN to EXISTS

In the following example, the keyword IN is provided in the input file. During the migration, DSC replaces IN with EXISTS to optimize query performance.

The IN and NOT IN statements with nested IN and NOT IN keywords cannot be migrated. In this case, the scripts will be invalid after migration.

       
            UPDATE tab1
   SET b = 123
 WHERE b IN ('abc') 
   AND b IN ( SELECT  i 
                FROM tab2 
                WHERE j NOT IN (SELECT m 
                                 FROM tab3
                               )
             )
;

When an IN or NOT IN statement containing subqueries is being migrated, comments between the IN or NOT IN operator and the subqueries (see the example) cannot be migrated.

Example:

       
            SELECT * 
  FROM categories 
 WHERE category_name
    IN --comment 
        ( SELECT category_name 
            FROM categories1 ) 
 ORDER BY category_name;

Migrating IN or NOT IN statements whose object names contain $ and #

DSC will not migrate the query if the TABLE name or TABLE ALIAS starts with $.

         
              SELECT Customer_Name
  FROM Customer_t $A
 WHERE Customer_ID IN( SELECT Customer_ID FROM Customer_t );

If the COLUMN name starts with #, DSC may fail to migrate the query.

         
              SELECT Customer_Name
  FROM Customer_t
 WHERE #Customer_ID IN( SELECT #Customer_ID FROM Customer_t );

Input: IN

   
        SELECT ...
   FROM tab1 t
   WHERE t.col1 IN (SELECT icol1 FROM tab2 e)
 ORDER BY col1

Output:

   
        SELECT ...
   FROM tab1 t
   WHERE EXISTS (SELECT icol1
                    FROM tab2 e
                   WHERE icol1 = t.col1
                 )
 ORDER BY col1;

Input: IN with multiple columns and Aggregate functions

   
        SELECT deptno, job_id, empno, salary, bonus
   FROM emp_t
  WHERE ( deptno, job_id, CAST(salary AS NUMBER(10,2))+CAST(bonus AS NUMBER(10,2)) ) 
                IN ( SELECT deptno, job_id, 
                      MAX(CAST(salary AS NUMBER(10,2))+CAST(bonus AS NUMBER(10,2)))
                        FROM emp_t 
                       WHERE hire_dt >= CAST( '20170101' AS DATE FORMAT 'YYYYMMDD' ) 
                       GROUP BY deptno, job_id )
    AND hire_dt IS NOT NULL;

Output:

   
        SELECT deptno, job_id, empno, salary, bonus
   FROM emp_t MAlias1
  WHERE EXISTS ( SELECT deptno, job_id, 
                            MAX(CAST(salary AS NUMBER(10,2))+CAST(bonus AS NUMBER(10,2)))
                      FROM emp_t 
                     WHERE hire_dt >= CAST( '20170101' AS DATE)
                           AND deptno  = MAlias1.deptno
                           AND job_id   = MAlias1.job_id
                     GROUP BY deptno, job_id 
                    HAVING MAX(CAST(salary AS NUMBER(10,2))+CAST(bonus AS NUMBER(10,2)))
                            = CAST(MAlias1.salary AS NUMBER(10,2))+CAST(MAlias1.bonus AS NUMBER(10,2)) )
    AND hire_dt IS NOT NULL;