Oracle 5.0 Reference Manual page 667

[646] can be used for indexed columns that are compared using the
ref
following examples, MySQL can use a
SELECT * FROM ref_table
SELECT * FROM ref_table,other_table
WHERE ref_table.key_column=other_table.column;
SELECT * FROM ref_table,other_table
WHERE ref_table.key_column_part1=other_table.column
AND ref_table.key_column_part2=1;
• [647]
fulltext
The join is performed using a
• [647]
ref_or_null
This join type is like
ref
contain values. This join type optimization is used most often in resolving subqueries. In the
NULL
following examples, MySQL can use a
SELECT * FROM ref_table
WHERE key_column=expr
See Section 8.3.1.6,
• [647]
index_merge
This join type indicates that the Index Merge optimization is used. In this case, the
output row contains a list of indexes used, and
the indexes used. For more information, see
•
unique_subquery
This type replaces
ref
value IN (SELECT primary_key
unique_subquery
for better efficiency.
•
index_subquery
This join type is similar to
nonunique indexes in subqueries of the following form:
value IN (SELECT key_column
• [647]
range
Only rows that are in a given range are retrieved, using an index to select the rows. The
in the output row indicates which index is used. The
used. The column is
ref
[647] can be used when a key column is compared to a constant using any of the
range
[875], [876], [876],
= <> >
[877], or
BETWEEN IN()
SELECT * FROM tbl_name
WHERE key_column
SELECT * FROM tbl_name
WHERE key_column
Output Format
EXPLAIN
ref
WHERE key_column=expr;
index.
FULLTEXT
[646], but with the addition that MySQL does an extra search for rows that
ref_or_null
OR key_column IS NULL;
"IS NULL Optimization".
[647]
[646] for some subqueries of the following form:
IN
FROM single_table
[647] is just an index lookup function that replaces the subquery completely
[647]
unique_subquery
FROM single_table
for this type.
NULL
[876], [876],
>= <
[878] operators:
= 10;
BETWEEN 10 and 20;
647
[646] join to process ref_table:
[647] join to process ref_table:
contains a list of the longest key parts for
key_len
Section 8.3.1.4, "Index Merge
WHERE some_expr)
[647]. It replaces
IN
WHERE some_expr)
contains the longest key part that was
key_len
[876], [877],
<= IS NULL
or operator. In the
= <=>
column in the
key
Optimization".
subqueries, but it works for
key
[876],
<=>
column