Question

I am trying to set up a view which could potentially contain 1000’s of records. Some of the fields requires subqueries which return the exact same data. I was wondering if I can query this once, within the view, instead of running it each time.

Below is some sample tables / data:

DROP VIEW MYVIEW;
DROP TABLE MYTABLE;
DROP TABLE MYTABLE_PARENT;

-- TABLES FOR VIEW
CREATE TABLE MYTABLE_PARENT ( PRIMARY_KEY NUMBER PRIMARY KEY );
CREATE TABLE MYTABLE ( PRIMARY_KEY NUMBER, MYVAL VARCHAR(255), PARENT_PRIMARY_KEY NUMBER);
INSERT INTO MYTABLE_PARENT VALUES (1);
INSERT INTO MYTABLE_PARENT VALUES (2);
INSERT INTO MYTABLE VALUES (1, 'MYVAL1-1', 1);
INSERT INTO MYTABLE VALUES (2, 'MYVAL1-2', 1);
INSERT INTO MYTABLE VALUES (3, 'MYVAL2-1', 2);

-- VIEW
CREATE OR REPLACE FORCE VIEW MYVIEW AS 
  SELECT CS.PRIMARY_KEY AS PARENT_PK,

  -- THE BELOW STATEMENTS REUSE THE SAME INNER SUBQUERY, IF THE 
  -- VIEW CONTAINS 1000 ROWS, THE INNER SUBQUERY WILL BE EXECUTED 
  -- 1000 TIMES, RETURNING THE SAME DATA EACH TIME.
  (SELECT PRIMARY_KEY FROM MYTABLE WHERE MYVAL = 'MYVAL1-1' AND 
  PARENT_PRIMARY_KEY = (SELECT PRIMARY_KEY FROM MYTABLE_PARENT 
  WHERE PRIMARY_KEY = CS.PRIMARY_KEY)) AS OUTPUT_VAL_1,

  (SELECT PRIMARY_KEY FROM MYTABLE WHERE MYVAL = 'MYVAL1-2' AND 
  PARENT_PRIMARY_KEY = (SELECT PRIMARY_KEY FROM MYTABLE_PARENT 
  WHERE PRIMARY_KEY = CS.PRIMARY_KEY)) AS OUTPUT_VAL_2
-- DEFINE CS
  FROM MYTABLE_PARENT CS;

SELECT * FROM MYVIEW;  

The result from the select statement is the following:

PARENT_PK              OUTPUT_VAL_1           OUTPUT_VAL_2           
---------------------- ---------------------- ---------------------- 
1                      1                      2                      
2                                                                    

So, in the above query, i want to run the following query only once for each row in the view:

(SELECT PRIMARY_KEY FROM MYTABLE_PARENT WHERE PRIMARY_KEY = CS.PRIMARY_KEY)

Is there any way of optimizing the inner subqueries within the view?

Answer 1

For a single access each of mytable and mytable_parent, try:

CREATE OR REPLACE FORCE VIEW MYVIEW AS 
  SELECT CS.PRIMARY_KEY AS PARENT_PK,
         MAX(DECODE(MT.MYVAL, 'MYVAL1-1',MT.PRIMARY_KEY, TO_NUMBER(NULL))
                        AS OUTPUT_VAL_1,
         MAX(DECODE(MT.MYVAL, 'MYVAL1-2',MT.PRIMARY_KEY, TO_NUMBER(NULL))
                        AS OUTPUT_VAL_2
  FROM MYTABLE_PARENT CS
  LEFT JOIN MYTABLE MT ON MT.PARENT_PRIMARY_KEY = CS.PRIMARY_KEY AND
                          MT.MYVAL IN ('MYVAL1-1', 'MYVAL1-2')
  GROUP BY CS.PRIMARY_KEY