MySQL Query does not use index in table join

Question

I am trying to list all the book_sales information for a particular book author. So I have a query and it\'s not using Index to lookup records.

The followin

Answer 1

Try this

SELECT sale_amount, price
FROM book_sales,books
LEFT JOIN books ON(book_sales.book_id = books.book_id)
WHERE books.author_id =1

Answer 2

As you can see in the EXPLAIN, "book_id" is listed as a possible key. If MySQL doesn't use it, it's just that the optimizer doesn't think it would speed up the query. Which is true if "book_sales" only has 2 rows, and 100% of those rows share the same "book_id". It's called cardinality btw. How to Avoid Table Scans (MySQL Manual)

Try filling it with more rows and you should see that MySQL will use an index for the join.

Edit: the query

SELECT sale_amount, price
FROM books, book_sales
FORCE INDEX ( book_id ) 
WHERE book_sales.book_id = books.book_id
AND books.author_id =1

...will not work either in that case because the optimizer still recognizes that reading the index is suboptimal and switches the table order to avoid doing so. You can force the table order by using STRAIGHT_JOIN. This is, however, a bit of a hack because it forces MySQL to execute the query in a way that is not the best.

      EXPLAIN
       SELECT sale_amount, price
         FROM books
STRAIGHT_JOIN book_sales FORCE INDEX (book_id) ON book_sales.book_id = books.book_id
        WHERE books.author_id = 1