Question

I am working with a legacy database schema that looks like this:

product_table table has fields:

uid (int, primary key)
name (varchar 50)

category table has fields:

uid (int, primary key)

name (varchar 50)

---

Allright, now the product_table has 4 MM relations to the category table:

product_table__category_1__mm has fields:

uid_local (int, contains product_table.uid)
uid_foreign (int, contains category.uid)

product_table__category_2__mm has fields:

uid_local (int, contains product_table.uid)
uid_foreign (int, contains category.uid)

product_table__category_3__mm has fields:

uid_local (int, contains product_table.uid)
uid_foreign (int, contains category.uid)

product_table__category_4__mm has fields:

uid_local (int, contains product_table.uid)
uid_foreign (int, contains category.uid)

(yes, all the 4 MM tables have the same fields and all relate to the category table)

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So, if I want to pull in all four joins and filter based on int values selected by the user it would look something like this:

select
product_table.*

from
product_table

inner join product_table__category_1__mm mm_1 on mm_1.uid_local = product_table.uid
inner join category cat_1 on cat_1.uid = mm_1.uid_foreign and cat_1.uid in (7, 8)

inner join product_table__category_2__mm mm_2 on mm_2.uid_local = product_table.uid
inner join category cat_2 on cat_2.uid = mm_2.uid_foreign and cat_2.uid in (63, 52)

inner join product_table__category_3__mm mm_3 on mm_3.uid_local = product_table.uid
inner join category cat_3 on cat_3.uid = mm_3.uid_foreign and cat_3.uid in (84, 12)

inner join product_table__category_4__mm mm_4 on mm_4.uid_local = product_table.uid
inner join category cat_4 on cat_4.uid = mm_4.uid_foreign and cat_4.uid in (16, 33)

group by product_table.uid ### --> in order to get unique results

Now thats a big query, but I cant change the db design as it is already widely in use.

Any idea on how to make this query faster? Where would you put indexes?

Answer 1

You need to get rid of GROUP BY here.

MySQL is not good in optimizing it.

Rewrite your query as this:

SELECT  *
FROM    product_table
WHERE   EXISTS
        (
        SELECT  NULL
        FROM    product_table__category_1__mm mm_1
        JOIN    category cat_1
        ON      cat_1.uid = mm_1.uid_foreign
        WHERE   mm_1.uid_local = product_table.uid
                AND mm_1.uid_foreign IN (7, 8)
        )
        AND
        …        

Create the indexes:

product_table__category_*__mm (uid_local, uid_foreign)

or, better, declare them PRIMARY KEYs on product_table__category_*__mm:

ALTER TABLE product_table__category_*__mm ADD CONSTRAINT pk_pc*mm_local_foreign (uid_local, uid_foreign)