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Excerpts from this Manual

9.2.1.3 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; it does not merge 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);

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.

Note

The Index Merge optimization algorithm has the following known deficiencies:

  • If your query has a complex WHERE clause with deep AND/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.

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 9.9.2, “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.

The Index Merge Intersection Access Algorithm

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 exactly N 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.

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 a 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 Index Merge Union Access Algorithm

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 exactly N 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;
The Index Merge Sort-Union Access Algorithm

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.


User Comments
  Posted by Ratheesh Bhat on March 1, 2007
Beware of having low selectivity indexes on your table. A complex AND/OR WHERE clause will surely make your query very very slow if Index_Merge optimization is being used with an intersect() algorithm.
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