MySQL - Slow Query when kept a View - mysql

Question

In MySQL, I have a simple join between 2 tables. Something like
select a.id, SUM(b.qty) from a inner join b on a.id=b.id
where a.id=12345
group by a.id
It runs normal as a query. But when I keep the query
select a.id, SUM(b.qty) from a inner join b on a.id=b.id
group by a.id
in a view called view_ab, the view takes enormous amount of time when i run the following query on the view.
select * from view_ab where id = 12345
Both these tables are large tables. Unable to figure out the reason for such a drop in performance. Please help resolve this performance issue
EDIT:
This is the view SQL
CREATE VIEW view_ab AS SELECT
r.drid AS drid,
SUM(s.return_qty) AS return_qty
FROM tbl_deliveryroute r INNER JOIN tbl_deliveryroute_sku s ON r.drid =
s.drid GROUP BY r.drid;
This is the query
SELECT
r.drid AS drid,
SUM(s.return_qty) AS return_qty
FROM tbl_deliveryroute r INNER JOIN tbl_deliveryroute_sku s ON r.drid =
s.drid WHERE r.drid=12718651
GROUP BY r.drid;
This is the query on the VIEW
SELECT * FROM view_ab WHERE drid=12718651;
Execution plan of the view
EXPLAIN EXTENDED SELECT * FROM view_ab WHERE drid=12718651;
id
select_type
table
partitions
type
possible_keys
key
key_len
ref
rows
filtered
Extra
1
PRIMARY
(NULL)
ref
4
const
10
100.00
(NULL)
2
DERIVED
s
(NULL)
ALL
idx_tbl_deliverroute_sku_drid
(NULL)
(NULL)
(NULL)
15060913
100.00
USING TEMPORARY; USING filesort
2
DERIVED
r
(NULL)
eq_ref
PRIMARY,FK_tbl_deliveryroute_1
PRIMARY
4
humdemotest.s.drid
1
100.00
USING INDEX
EXPLAIN EXTENDED SELECT
r.drid AS drid,
SUM(s.return_qty) AS return_qty
FROM tbl_deliveryroute r INNER JOIN tbl_deliveryroute_sku s ON r.drid =
s.drid WHERE r.drid=12718651
GROUP BY r.drid;
id
select_type
table
partitions
type
possible_keys
key
key_len
ref
rows
filtered
Extra
1
SIMPLE
r
(NULL)
const
PRIMARY
PRIMARY
4
const
1
100.00
USING INDEX
1
SIMPLE
s
(NULL)
ref
idx_tbl_deliverroute_sku_drid
idx_tbl_deliverroute_sku_drid
4
const
22
100.00
(NULL)

Answer 1

From what I am seeing, you don't even need a join since you are dealing with a join on the same key column from A-B, the key already exists in table B, just query group by that. Also, I would have an index on your DeliveryRoute_SKU on its route ID column
SELECT
s.drid,
sum( s.return_qty ) Return_Qty
from
tbl_DeliveryRoute_Sku s
where
s.drID = 12718651
group by
s.drID;
Since you are only doing the key and the sum, you don't even NEED the other table. Now if you needed other columns from the first table OTHER THAN the key, then yes, you would need the join. You could even simplify a step further since you are only querying a single key ID
SELECT
sum( s.return_qty ) Return_Qty
from
tbl_DeliveryRoute_Sku s
where
s.drID = 12718651;

Answer 2

The reason the view is slow is simple. You are executing:
SELECT *
FROM view_ab
WHERE drid = 12718651;
What you want to execute is:
select a.id, SUM(b.qty)
from a inner join
b
on a.id = b.id
where a.id = 12345
group by a.id;
What is actually being executed is:
select ab.*
from (select a.id, SUM(b.qty)
from a inner join
b
on a.id = b.id
group by a.id
) ab
where ab.id = 12345;
That is, the entire aggregation is performed first. Then the where is applied. What you want is for the predicate to be pushed up (MySQL calls this merging). You can review the documentation on this subject.
One solution would seem to be rephrasing the query as a correlated subquery:
select a.id,
(select sum(b.qty) from b where b.id = a.id) as qty
from a
where a.id = 12345;
Alas, subqueries in the select have the same effect, so this doesn't work.
I don't know of a solution using a view. You can avoid using views for this. The ultimate solution would be to implement a trigger to store the summarized results in another table -- effectively materializing the view.