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8.2.1.3 Index Merge Optimization
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.
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 8.9.2, “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.
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 AND key_part2 = const2 ... AND 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.