The Index Merge access
method retrieves rows with multiple
range
scans and merges
their results into one. This access method merges index scans
from a single table only, not scans across multiple tables.
The merge can produce unions, intersections, or
unions-of-intersections of its underlying scans.
Example queries for which Index Merge may be used:
SELECT * FROM tbl_name WHERE key1 = 10 OR key2 = 20;
SELECT * FROM tbl_name
WHERE (key1 = 10 OR key2 = 20) AND non_key = 30;
SELECT * FROM t1, t2
WHERE (t1.key1 IN (1,2) OR t1.key2 LIKE 'value%')
AND t2.key1 = t1.some_col;
SELECT * FROM t1, t2
WHERE t1.key1 = 1
AND (t2.key1 = t1.some_col OR t2.key2 = t1.some_col2);
The Index Merge optimization algorithm has the following known limitations:
If your query has a complex
WHERE
clause with deepAND
/OR
nesting and MySQL does not choose the optimal plan, try distributing terms using the following identity transformations:(x AND y) OR z => (x OR z) AND (y OR z) (x OR y) AND z => (x AND z) OR (y AND z)
Index Merge is not applicable to full-text indexes.
In EXPLAIN
output, the Index
Merge method appears as
index_merge
in the
type
column. In this case, the
key
column contains a list of indexes used,
and key_len
contains a list of the longest
key parts for those indexes.
The Index Merge access method has several algorithms, which
are displayed in the Extra
field of
EXPLAIN
output:
Using intersect(...)
Using union(...)
Using sort_union(...)
The following sections describe these algorithms in greater detail. The optimizer chooses between different possible Index Merge algorithms and other access methods based on cost estimates of the various available options.
This access algorithm is applicable when a
WHERE
clause is converted to several
range conditions on different keys combined with
AND
, and each condition is one
of the following:
An
N
-part expression of this form, where the index has exactlyN
parts (that is, all index parts are covered):key_part1 = const1 AND key_part2 = const2 ... AND key_partN = constN
Any range condition over the primary key of an
InnoDB
table.
Examples:
SELECT * FROM innodb_table
WHERE primary_key < 10 AND key_col1 = 20;
SELECT * FROM tbl_name
WHERE key1_part1 = 1 AND key1_part2 = 2 AND key2 = 2;
The Index Merge intersection algorithm performs simultaneous scans on all used indexes and produces the intersection of row sequences that it receives from the merged index scans.
If all columns used in the query are covered by the used
indexes, full table rows are not retrieved
(EXPLAIN
output contains
Using index
in Extra
field in this case). Here is an example of such a query:
SELECT COUNT(*) FROM t1 WHERE key1 = 1 AND key2 = 1;
If the used indexes do not cover all columns used in the query, full rows are retrieved only when the range conditions for all used keys are satisfied.
If one of the merged conditions is a condition over the
primary key of an InnoDB
table, it is not
used for row retrieval, but is used to filter out rows
retrieved using other conditions.
The criteria for this algorithm are similar to those for the
Index Merge intersection algorithm. The algorithm is
applicable when the table's WHERE
clause is converted to several range conditions on different
keys combined with OR
, and each
condition is one of the following:
An
N
-part expression of this form, where the index has exactlyN
parts (that is, all index parts are covered):key_part1 = const1 OR key_part2 = const2 ... OR key_partN = constN
Any range condition over a primary key of an
InnoDB
table.A condition for which the Index Merge intersection algorithm is applicable.
Examples:
SELECT * FROM t1
WHERE key1 = 1 OR key2 = 2 OR key3 = 3;
SELECT * FROM innodb_table
WHERE (key1 = 1 AND key2 = 2)
OR (key3 = 'foo' AND key4 = 'bar') AND key5 = 5;
This access algorithm is applicable when the
WHERE
clause is converted to several
range conditions combined by
OR
, but the Index Merge union
algorithm is not applicable.
Examples:
SELECT * FROM tbl_name
WHERE key_col1 < 10 OR key_col2 < 20;
SELECT * FROM tbl_name
WHERE (key_col1 > 10 OR key_col2 = 20) AND nonkey_col = 30;
The difference between the sort-union algorithm and the union algorithm is that the sort-union algorithm must first fetch row IDs for all rows and sort them before returning any rows.
Use of Index Merge is subject to the value of the
index_merge
,
index_merge_intersection
,
index_merge_union
, and
index_merge_sort_union
flags of the
optimizer_switch
system
variable. See Section 10.9.2, “Switchable Optimizations”. By
default, all those flags are on
. To
enable only certain algorithms, set
index_merge
to
off
, and enable only such of the others
as should be permitted.
In addition to using the
optimizer_switch
system
variable to control optimizer use of the Index Merge
algorithms session-wide, MySQL supports optimizer hints to
influence the optimizer on a per-statement basis. See
Section 10.9.3, “Optimizer Hints”.