Oracle 5.0 Reference Manual page 665

the variables from its outer context. For
each row of the outer context.
Cacheability of subqueries differs from caching of query results in the query cache (which is
described in Section 8.6.3.1, "How the Query Cache
query execution, whereas the query cache is used to store results only after query execution
finishes.
•
table
The name of the table to which the row of output refers. This can also be one of the following values:
• <unionM,N>: The row refers to the union of the rows with
• <derivedN>: The row refers to the derived table result for the row with an
derived table may result, for example, from a subquery in the
•
type
The join type. For descriptions of the different types, see
•
possible_keys
The
possible_keys
in this table. Note that this column is totally independent of the order of the tables as displayed in the
output from EXPLAIN. That means that some of the keys in
practice with the generated table order.
If this column is NULL, there are no relevant indexes. In this case, you may be able to improve
the performance of your query by examining the
column or columns that would be suitable for indexing. If so, create an appropriate index and check
the query with
EXPLAIN
To see what indexes a table has, use
•
key
The column indicates the key (index) that MySQL actually decided to use. If MySQL decides to
key
use one of the
possible_keys
It is possible that
key
can happen if none of the
columns selected by the query are columns of some other index. That is, the named index covers
the selected columns, so although it is not used to determine which rows to retrieve, an index scan is
more efficient than a data row scan.
For InnoDB, a secondary index might cover the selected columns even if the query also selects
the primary key because
NULL, MySQL found no index to use for executing the query more efficiently.
To force MySQL to use or ignore an index listed in the
INDEX, or
INDEX, USE
For MyISAM, NDB, and
indexes. For
MyISAM
Section 7.6, "MyISAM
•
key_len
The column indicates the length of the key that MySQL decided to use. The length is
key_len
if the column says NULL. Note that the value of
key
parts of a multiple-part key MySQL actually uses.
Output Format
EXPLAIN
UNCACHEABLE
column indicates which indexes MySQL can choose from use to find the rows
again. See Section 13.1.4,
SHOW INDEX FROM
indexes to look up rows, that index is listed as the key value.
will name an index that is not present in the
possible_keys
stores the primary key value with each secondary index. If
InnoDB
in your query. See
IGNORE INDEX
tables, running
BDB ANALYZE TABLE
tables,
myisamchk --analyze
Table Maintenance and Crash
645
SUBQUERY, the subquery is re-evaluated for
Operates"). Subquery caching occurs during
values of
id
FROM
EXPLAIN
possible_keys
clause to check whether it refers to some
WHERE
"ALTER TABLE
tbl_name.
possible_keys
indexes are suitable for looking up rows, but all the
possible_keys
Section 13.2.8.3, "Index Hint
helps the optimizer choose better
does the same as
Recovery".
enables you to determine how many
key_len
and N.
M
value of N. A
id
clause.
Join Types.
might not be usable in
Syntax".
value. This
column, use
FORCE
Syntax".
TABLE. See
ANALYZE
is
key
NULL