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

Getting More Information About Databases
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Table 12.8
Type
Possible Select Types as Shown in Output from EXPLAIN
Description
SIMPLE
PRIMARY
UNION
DEPENDENT UNION
UNION RESULT
SUBQUERY
DEPENDENT SUBQUERY
DERIVED
UNCACHEABLE SUBQUERY
UNCACHEABLE UNION
Plain old SELECT, as in this example
Outer (first) query where subqueries and unions are used
Second or later query in a union
Second or later query in a union, dependent on the primary
query
The result of a UNION
Inner subquery
Inner subquery, dependent on the primary query (that is, a correlated
subquery)
Subquery used in FROM clause
A subquery whose result cannot be cached and must be reevaluated
for each row
The second or later select in a UNION that belongs to an
uncacheable subquery
The column table just lists the tables used to answer the query. Each row in the result
gives more information about how that particular table is used in this query. In this case,
you can see that the tables used are orders, order_items, customers, and books.(You
know this already by looking at the query.)
The type column explains how the table is being used in joins in the query.The set
of values this column can have is shown in Table 12.9.These values are listed in order
from fastest to slowest in terms of query execution.The table gives you an idea of how
many rows need to be read from each table to execute a query.
Table 12.9
Type
Possible Join Types as Shown in Output from EXPLAIN
Description
const or system
eq_ref
fulltext
ref
ref_or_null
The table is read from only once.This happens when the table has
exactly one row.The type system is used when it is a system table, and
the type const otherwise.
For every set of rows from the other tables in the join, you read one
row from this table.This type is used when the join uses all the parts of
the index on the table, and the index is UNIQUE or is the primary key.
A join has been performed using a fulltext index.
For every set of rows from the other tables in the join, you read a set of
table rows that all match.This type is used when the join cannot choose
a single row based on the join condition—that is, when only part of the
key is used in the join, or if it is not UNIQUE or a primary key.
This is like a ref query, but MySQL also looks for rows that are NULL.
(This type is used mostly in subqueries.)