mysql replace sub-select with function - query never returns

Question

In the following original query:

SELECT COMPANYNAME,
    (
        SELECT SUM(RRP) * 0.1
        FROM CRM_RESALE_ITEM_VIEW
        INNER JOIN CRM_RESALE using (RESALE_ID)
        WHERE CRM_RESALE.CUSTOMER_ID = CRM_CUSTOMER_VIEW.CUSTOMER_ID
        ) AS DERRIVED_MAINTENANCE
FROM CRM_CUSTOMER_VIEW

I have replaced the DERRIVED_MAINTENANCE sub-select as follows:

SELECT COMPANYNAME,
    F_MAINTENANCE(CRM_CUSTOMER_VIEW.CUSTOMER_ID) AS DERRIVED_MAINTENANCE
FROM CRM_CUSTOMER_VIEW 

with a function:

BEGIN
    DECLARE DERRIVED_MAINTENANCE DECIMAL DEFAULT 0;

    SELECT SUM(RRP) * 0.1
    INTO DERRIVED_MAINTENANCE
    FROM CRM_RESALE_ITEM_VIEW
    INNER JOIN CRM_RESALE using (RESALE_ID)
    WHERE CRM_RESALE.CUSTOMER_ID = CUST_ID;

    RETURN DERRIVED_MAINTENANCE;
END

and now instead of taking 60 seconds, the query never returns. Can anyone see a reason for this?

CRM_CUSTOMER (CUSTOMER_ID) one-to-many with 
CRM_RESALE (RESALE_ID, CUSTOMER_ID) one-to-many with 
CRM_RESALE_ITEM_VIEW (RESALE_ID, ITEM_ID, RRP)

Answer 1

TL;DR : you may be better served by proper indexing. Offloading JOIN to a function may actually make things much, much worse. BUT indexing properly a VIEW is not straightforward, and you don't supply enough information to present a guaranteed solution. Below, you will find a proposal, and a test to evaluate .

Could it be that the function is returning, but it's taking very long. I have it returning in about the same time as the original query ( my definition is different from yours , double-check my code, I may have misunderstood):

Sample data

CREATE TABLE CRM_CUSTOMER_VIEW
    ( CUSTOMER_ID INTEGER, COMPANYNAME VARCHAR(50) );

INSERT INTO CRM_CUSTOMER_VIEW VALUES ( 1, 'ACME' ), ( 2, 'NASA' );

SELECT @N:=COUNT(*) FROM CRM_CUSTOMER_VIEW;
INSERT INTO CRM_CUSTOMER_VIEW SELECT CUSTOMER_ID + @N, CONCAT(SUBSTRING(COMPANYNAME, 1, 4), ' ', @N, '.', CUSTOMER_ID)
    FROM CRM_CUSTOMER_VIEW;
-- Repeat the two rows above to fill the table with, say, half a million records.

CREATE TABLE CRM_RESALE ( CUSTOMER_ID INTEGER, RESALE_ID INTEGER );    
SELECT @N:=1;

INSERT INTO CRM_RESALE SELECT CUSTOMER_ID, 5*CUSTOMER_ID+@N FROM CRM_CUSTOMER_VIEW;
SELECT @N:=@N+1;
-- Repeat five times the two rows above to get five items per customer

CREATE TABLE CRM_RESALE_ITEM_VIEW ( RESALE_ID INTEGER, RRP NUMERIC(7,3));
INSERT INTO CRM_RESALE_ITEM_VIEW SELECT RESALE_ID, 3.14159 FROM CRM_RESALE;

Now we run the query to get a baseline - no indexes, so quite expensive, even on a fast machine

SELECT COMPANYNAME,
    (
        SELECT SUM(RRP) * 0.1
        FROM CRM_RESALE_ITEM_VIEW
        INNER JOIN CRM_RESALE using (RESALE_ID)
        WHERE CRM_RESALE.CUSTOMER_ID = CRM_CUSTOMER_VIEW.CUSTOMER_ID
        ) AS DERRIVED_MAINTENANCE
FROM CRM_CUSTOMER_VIEW WHERE COMPANYNAME = 'ACME';

+-------------+----------------------+
| COMPANYNAME | DERRIVED_MAINTENANCE |
+-------------+----------------------+
| ACME        |               1.5710 |
+-------------+----------------------+
1 row in set (3.18 sec)

Now we move the inner query into a function of its own.

DELIMITER //
CREATE FUNCTION DERRIVED_MAINTENANCE ( CUSTID INTEGER )
RETURNS NUMERIC(7,3) DETERMINISTIC
BEGIN
SELECT SUM(RRP) * 0.1 INTO @SRRP
        FROM CRM_RESALE_ITEM_VIEW
        INNER JOIN CRM_RESALE using (RESALE_ID)
        WHERE CRM_RESALE.CUSTOMER_ID = CUSTID;

RETURN @SRRP;
END//
DELIMITER ;

mysql> SELECT DERRIVED_MAINTENANCE(1);
+-------------------------+
| DERRIVED_MAINTENANCE(1) |
+-------------------------+
|                   1.571 |
+-------------------------+
1 row in set (3.60 sec)

If I run the query against FIVE rows, I get five times as long since the function gets called five times.

SELECT CUSTOMER_ID, DERRIVED_MAINTENANCE(CUSTOMER_ID) FROM CRM_CUSTOMER_VIEW WHERE CUSTOMER_ID < 5;
+-------------+-----------------------------------+
| CUSTOMER_ID | DERRIVED_MAINTENANCE(CUSTOMER_ID) |
+-------------+-----------------------------------+
|           1 |                             1.571 |
|           2 |                             1.571 |
|           3 |                             1.571 |
|           4 |                             1.571 |
+-------------+-----------------------------------+
4 rows in set (14.45 sec)

If I index the tables, though, using a covering index -- this I can do since they're tables; you can do this for views too, but you need to index differently, and maybe you could benefit from a different aggregate view. Can't really advise without knowing more

CREATE INDEX CRM_RESALE_ITEM_VIEW_NDX ON CRM_RESALE_ITEM_VIEW(RESALE_ID, RRP);
CREATE INDEX CRM_RESALE_NDX ON CRM_RESALE (CUSTOMER_ID, RESALE_ID);

Now I can either use a VIEW instead of the function, or call the function:

CREATE VIEW CRM_RESALE_FULL_VIEW AS
        SELECT CUSTOMER_ID, SUM(RRP) * 0.1 AS DERRIVED_MAINTENANCE
        FROM CRM_RESALE_ITEM_VIEW
        INNER JOIN CRM_RESALE using (RESALE_ID)
    GROUP BY CUSTOMER_ID;

SELECT COMPANYNAME, DERRIVED_MAINTENANCE FROM     CRM_CUSTOMER_VIEW JOIN CRM_RESALE_FULL_VIEW USING (CUSTOMER_ID)     WHERE COMPANYNAME LIKE 'ACME 1024.20%';
+---------------+----------------------+
| COMPANYNAME   | DERRIVED_MAINTENANCE |
+---------------+----------------------+
| ACME 1024.201 |               1.5710 |
| ACME 1024.203 |               1.5710 |
| ACME 1024.205 |               1.5710 |
| ACME 1024.207 |               1.5710 |
| ACME 1024.209 |               1.5710 |
+---------------+----------------------+
5 rows in set (1.11 sec)

SELECT COMPANYNAME, DERRIVED_MAINTENANCE FROM     CRM_CUSTOMER_VIEW JOIN CRM_RESALE_FULL_VIEW USING (CUSTOMER_ID)     WHERE COMPANYNAME LIKE 'ACME 1024.2%';
+---------------+----------------------+
| COMPANYNAME   | DERRIVED_MAINTENANCE |
+---------------+----------------------+
| ACME 1024.21  |               1.5710 |
...
| ACME 1024.299 |               1.5710 |
+---------------+----------------------+
55 rows in set (1.31 sec)

or call the function

SELECT CUSTOMER_ID, DERRIVED_MAINTENANCE(CUSTOMER_ID) FROM CRM_CUSTOMER_VIEW WHERE CUSTOMER_ID > 42 AND CUSTOMER_ID < 47;

SELECT CUSTOMER_ID, DERRIVED_MAINTENANCE(CUSTOMER_ID) FROM CRM_CUSTOMER_VIEW WHERE CUSTOMER_ID > 42 AND CUSTOMER_ID < 47;
+-------------+-----------------------------------+
| CUSTOMER_ID | DERRIVED_MAINTENANCE(CUSTOMER_ID) |
+-------------+-----------------------------------+
|          43 |                             1.571 |
|          44 |                             1.571 |
|          45 |                             1.571 |
|          46 |                             1.571 |
+-------------+-----------------------------------+
4 rows in set (0.03 sec)

Indexing alone can yield a performance increase of two orders of magnitude.