Question

Can anyone tell me whats wrong with this MySQL query ?

select distinct(a.productId)
  from product a
  left join product_keyword b
    on b.productId = a.productId
  left join keywords c
    on c.keywordId = b.keywordId
  where a.productName LIKE '%truck%' OR c.value LIKE '%truck%'
  limit 100;

Actually I need to join 3 tables (product, product_keyword and keywords) and search based on user input. One product can be multiple keywords and I store it (keywordId from table keywords in product_keyword).

Can anyone help me please?

Answer 1

When you use the % wildcard in the prefix of a LIKE search, MySQL isn’t able to utilize any indexes for the search. Instead, MySQL must scan all of the rows.

You should at least have indexes on the join columns (productID and keywordID) so that MySQL is able to more quickly perform the join operations. However, if the result set is too large, MySQL will perform a scan for the JOINs as well.

Most likely, MySQL is scanning each row in product, then performing the JOIN to product_keyword, then performing the join to keywords. Then, it checks to see if it can exclude the row based on the WHERE clause. Once it returns 100 rows, it stops.