Oracle 5.0 Reference Manual page 664

•
EXPLAIN
Output Columns
EXPLAIN
This section describes the output columns produced by EXPLAIN. Later sections provide additional
information about the
Each output row from
summarized in
Table 8.1.
EXPLAIN
Column
[644]
id
select_type
[645]
table
[645]
type
possible_keys
[645]
key
[645]
key_len
[646]
ref
[646]
rows
[646]
Extra
•
id
The
SELECT
be if the row refers to the union result of other rows. In this case, the
NULL
value like
and N.
•
select_type
The type of SELECT, which can be any of those shown in the following table.
select_type
SIMPLE
PRIMARY
UNION
DEPENDENT UNION
UNION RESULT
SUBQUERY
DEPENDENT
SUBQUERY
DERIVED
UNCACHEABLE
SUBQUERY
DEPENDENT
Subqueries".
DEPENDENT SUBQUERY
DEPENDENT
EXPLAIN
Output Interpretation
and
type Extra
provides information about one table. Each row contains the values
EXPLAIN
Table 8.1, "EXPLAIN
Output Columns
Meaning
The
SELECT
[644] The
SELECT
The table for the output row
The join type
[645] The possible indexes to choose
The index actually chosen
The length of the chosen key
The columns compared to the index
Estimate of rows to be examined
Additional information
identifier. This is the sequential number of the
to indicate that the row refers to the union of the rows with
<unionM,N>
Value Meaning
Simple
Outermost
Second or later
Second or later
Result of a UNION.
First
SELECT
First
SELECT
Derived table
A subquery for which the result cannot be cached and must be re-
evaluated for each row of the outer query
typically signifies the use of a correlated subquery. See
evaluation differs from
SUBQUERY, the subquery is re-evaluated only once for each set of different values of
Output Format
columns.
Output Columns", and described in more detail following the table.
identifier
type
(not using
SELECT UNION
SELECT
statement in a
SELECT
statement in a UNION, dependent on outer query
SELECT
in subquery
in subquery, dependent on outer query
(subquery in
SELECT
UNCACHEABLE SUBQUERY
644
within the query. The value can
SELECT
table
or subqueries)
UNION
clause)
FROM
Section 13.2.9.7, "Correlated
evaluation. For
column shows a
values of
id M