Question

I have the following query:

SELECT t.id
FROM account_transaction t
JOIN transaction_code tc ON t.transaction_code_id = tc.id
JOIN account a ON t.account_number = a.account_number
GROUP BY tc.id

When I do an EXPLAIN the first row shows, among other things, this:

table: t
type: ALL
possible_keys: account_id,transaction_code_id,account_transaction_transaction_code_id,account_transaction_account_number
key: NULL
rows: 465663

Why is key NULL?

Answer 1

It might be because the statistics is broken, or because it knows that you always have a 1:1 ratio between the two tables.

You can force an index to be used in the query, and see if that would speed up things. If it does, try to run ANALYZE TABLE to make sure statistics are up to date.

By specifying USE INDEX (index_list), you can tell MySQL to use only one of the named indexes to find rows in the table. The alternative syntax IGNORE INDEX (index_list) can be used to tell MySQL to not use some particular index or indexes. These hints are useful if EXPLAIN shows that MySQL is using the wrong index from the list of possible indexes.

You can also use FORCE INDEX, which acts like USE INDEX (index_list) but with the addition that a table scan is assumed to be very expensive. In other words, a table scan is used only if there is no way to use one of the given indexes to find rows in the table.

Each hint requires the names of indexes, not the names of columns. The name of a PRIMARY KEY is PRIMARY. To see the index names for a table, use SHOW INDEX.

From http://dev.mysql.com/doc/refman/5.1/en/index-hints.html