Why is my query so slow? Trying to find the null fields on a left join in mysql

Question

EDIT: Updated with suggestions from Bill Karwin below. Still very slow.

I'm trying to write a query that will find all items on an order that are entered to a warehouse that doesn't have a record for that item in that warehouse. As an example, if item XYZ is entered for warehouse A, but warehouse A doesn't actually carry item XYZ, I want the order item to show up in my report.

I'm able to run the query just fine, but it seems to take forever (50 seconds). It seems to be hanging mainly on the "is null" condition I have in the where clause. If I remove the condition with the "is null" and run it, it executes in about 4.8s. Here's my query:

SELECT
saw_order.Wo,
saw_orderitem.Item,
saw_orderitem.Stock,
saw_order.`Status`,
saw_order.`Date`,
saw_orderitem.Warehouse,
saw_stockbalance.Balno,
saw_stockbalance.Stock
FROM
saw_order
Inner Join saw_orderitem ON saw_order.Wo = saw_orderitem.Wo
Inner Join saw_stock ON saw_orderitem.Stock = saw_stock.Stock
Left Join saw_stockbalance ON saw_orderitem.Stock = saw_stockbalance.Stock 
    AND saw_orderitem.Warehouse = saw_stockbalance.Warehouse
WHERE
saw_order.`Status` Between 3 and 81 and
saw_stockbalance.Stock Is Null

When I explain the query above, I see:

+----+-------------+------------------+--------+------------------------------+---------+---------+-------------------------------------------------------+-------+-------------------------+
| id | select_type | table            | type   | possible_keys                | key     | key_len | ref                                                   | rows  | Extra                   |
+----+-------------+------------------+--------+------------------------------+---------+---------+-------------------------------------------------------+-------+-------------------------+
|  1 | SIMPLE      | saw_stock        | index  | PRIMARY                      | PRIMARY | 17      | NULL                                                  | 32793 | Using index             |
|  1 | SIMPLE      | saw_orderitem    | ref    | PRIMARY,Stock,StockWarehouse | Stock   | 17      | saws.saw_stock.Stock                                  |    68 |                         |
|  1 | SIMPLE      | saw_order        | eq_ref | PRIMARY,Status               | PRIMARY | 4       | saws.saw_orderitem.Wo                                 |     1 | Using where             |
|  1 | SIMPLE      | saw_stockbalance | ref    | Stock,Warehouse              | Stock   | 20      | saws.saw_orderitem.Stock,saws.saw_orderitem.Warehouse |     1 | Using where; Not exists |
+----+-------------+------------------+--------+------------------------------+---------+---------+-------------------------------------------------------+-------+-------------------------+

I'm pretty sure I have indexes for all of the fields of the respective tables in my joins, but can't figure out how to rewrite the query to make it go faster.

EDIT: Here are the indexes I have set up on my tables:

mysql> show index from saw_order;
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table     | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| saw_order |          0 | PRIMARY  |            1 | Wo          | A         |      553425 | NULL     | NULL   |      | BTREE      |         |
| saw_order |          1 | Customer |            1 | Customer    | A         |       14957 | NULL     | NULL   |      | BTREE      |         |
| saw_order |          1 | Other    |            1 | Other       | A         |         218 | NULL     | NULL   |      | BTREE      |         |
| saw_order |          1 | Site     |            1 | Site        | A         |           8 | NULL     | NULL   |      | BTREE      |         |
| saw_order |          1 | Date     |            1 | Date        | A         |        1594 | NULL     | NULL   |      | BTREE      |         |
| saw_order |          1 | Status   |            1 | Status      | A         |          15 | NULL     | NULL   |      | BTREE      |         |
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
6 rows in set

mysql> show index from saw_orderitem;
+---------------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table         | Non_unique | Key_name       | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+---------------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| saw_orderitem |          0 | PRIMARY        |            1 | Wo          | A         | NULL        | NULL     | NULL   |      | BTREE      |         |
| saw_orderitem |          0 | PRIMARY        |            2 | Item        | A         |     1842359 | NULL     | NULL   |      | BTREE      |         |
| saw_orderitem |          1 | Stock          |            1 | Stock       | A         |       27093 | NULL     | NULL   |      | BTREE      |         |
| saw_orderitem |          1 | Product        |            1 | Product     | A         |         803 | NULL     | NULL   |      | BTREE      |         |
| saw_orderitem |          1 | GGroup         |            1 | GGroup      | A         |         114 | NULL     | NULL   |      | BTREE      |         |
| saw_orderitem |          1 | ShipVia        |            1 | ShipVia     | A         |         218 | NULL     | NULL   |      | BTREE      |         |
| saw_orderitem |          1 | Warehouse      |            1 | Warehouse   | A         |           9 | NULL     | NULL   |      | BTREE      |         |
| saw_orderitem |          1 | StockWarehouse |            1 | Stock       | A         |       27093 | NULL     | NULL   |      | BTREE      |         |
| saw_orderitem |          1 | StockWarehouse |            2 | Warehouse   | A         |       49793 | NULL     | NULL   |      | BTREE      |         |
+---------------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
9 rows in set

mysql> show index from saw_stock;
+-----------+------------+-------------------+--------------+-------------------+-----------+-------------+----------+--------+------+------------+---------+
| Table     | Non_unique | Key_name          | Seq_in_index | Column_name       | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-----------+------------+-------------------+--------------+-------------------+-----------+-------------+----------+--------+------+------------+---------+
| saw_stock |          0 | PRIMARY           |            1 | Stock             | A         |       32793 | NULL     | NULL   |      | BTREE      |         |
| saw_stock |          1 | Class             |            1 | Class             | A         |         655 | NULL     | NULL   | YES  | BTREE      |         |
| saw_stock |          1 | DateFirstReceived |            1 | DateFirstReceived | A         |        2732 | NULL     | NULL   |      | BTREE      |         |
+-----------+------------+-------------------+--------------+-------------------+-----------+-------------+----------+--------+------+------------+---------+
3 rows in set

mysql> show index from saw_stockbalance;
+------------------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table            | Non_unique | Key_name  | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+------------------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| saw_stockbalance |          0 | PRIMARY   |            1 | Balno       | A         |      146315 | NULL     | NULL   |      | BTREE      |         |
| saw_stockbalance |          1 | Stock     |            1 | Stock       | A         |       36578 | NULL     | NULL   |      | BTREE      |         |
| saw_stockbalance |          1 | Stock     |            2 | Warehouse   | A         |      146315 | NULL     | NULL   |      | BTREE      |         |
| saw_stockbalance |          1 | Warehouse |            1 | Warehouse   | A         |          11 | NULL     | NULL   |      | BTREE      |         |
+------------------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
4 rows in set

Any ideas?

Answer 1

I'd try to make it use a covering index. That is, instead of testing if Balno is null, test if one of the columns in your left outer join conditions is null. E.g. Stock or Warehouse.

You should also define an index over the two columns (Stock, Warehouse) in both tables saw_orderitem and saw_stockbalance.

Answer 2

Try:

SELECT t.wo,
       soi.item,
       soi.stock,
       t.Status,
       t.Date,
       soi.warehouse,
       NULL 'balno',  --ssb.Balno,
       NULL 'stock', --ssb.Stock
  FROM SAW_ORDER t
  JOIN SAW_ORDERITEM soi ON soi.wo = t.wo
  JOIN SAW_STOCK ss ON ss.stock = soi.stock
 WHERE t.status BETWEEN 3 AND 81 
   AND NOT EXISTS (SELECT NULL
                     FROM SAW_STOCKBALANCE ssb
                    WHERE ssb.stock != soi.stock
                      AND ssb.warehouse = soi.warehouse)

The problem with the query is that you are checking for nulls on a LEFT JOIN...