PHP and MySQL Web Development 4th Ed-tqw-_darksiderg

302 Chapter 12 Advanced MySQL Administration
Table 12.9
Type
Continued
Description
index_merge
unique_subquery
index_subquery
range
index
ALL
A specific optimization, the Index Merge, has been used.
This join type is used to replace ref for some IN subqueries where
one unique row is returned.
This join type is similar to unique_subquery but is used for indexed
nonunique subqueries.
For every set of rows from the other tables in the join, you read a set of
table rows that fall into a particular range.
The entire index is scanned.
Every row in the table is scanned.
In the previous example, you can see that one of the tables is joined using eq_ref
(books), one is joined using ref (order_items), and the other two (orders and customers)
are joined using ALL—that is, by looking at every single row in the table.
The rows column backs this up: It lists (roughly) the number of rows of each table
that has to be scanned to perform the join.You can multiply these numbers together to
get the total number of rows examined when a query is performed.You multiply these
numbers because a join is like a product of rows in different tables. Check out Chapter
10,“Working with Your MySQL Database,” for details. Remember that this is the
number of rows examined, not the number of rows returned, and that it is only an estimate;
MySQL can’t know the exact number without performing the query.
Obviously, the smaller you can make this number, the better. At present, you have a
negligible amount of data in the database, but when the database starts to increase in size,
this query would increase in execution time.We return to this matter shortly.
The possible_keys column lists, as you might expect, the keys that MySQL might
use to join the table. In this case, you can see that the possible keys are all PRIMARY keys.
The key column is either the key from the table MySQL actually used or NULL if no
key was used. Notice that, although there are possible PRIMARY keys for the customers
and orders table, none were used in this query.
The key_len column indicates the length of the key used.You can use this number
to tell whether only part of a key was used.The key length is relevant when you have
keys that consist of more than one column. In this case, where the keys were used, the
full key was used.
The ref column shows the columns used with the key to select rows from the table.
Finally, the Extra column tells you any other information about the way the join was
performed. Some possible values you might see in this column are shown in Table 12.10.
For a complete list of the more than 15 different possibilities, see the MySQL Manual at
http://dev.mysql.com/doc/refman/5.1/en/using-explain.html.