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7.2.6 Index Merge Optimization
------------------------------
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* index-merge-intersection The Index Merge Intersection Access Algorithm
* index-merge-union The Index Merge Union Access Algorithm
* index-merge-sort-union The Index Merge Sort-Union Access Algorithm
The Index Merge method is used to retrieve rows with several `range'
scans and to merge their results into one. The merge can produce unions,
intersections, or unions-of-intersections of its underlying scans.
* If you have upgraded from a previous version of MySQL, you
should be aware that this type of join optimization is first introduced
in MySQL 5.0, and represents a significant change in behavior with
regard to indexes. (Formerly, MySQL was able to use at most only one
index for each referenced table.)
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.
Examples:
SELECT * FROM TBL_NAME WHERE KEY_PART1 = 10 OR KEY_PART2 = 20;
SELECT * FROM TBL_NAME
WHERE (KEY_PART1 = 10 OR KEY_PART2 = 20) AND NON_KEY_PART=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 method has several access algorithms (seen in the
`Extra' field of `EXPLAIN' output):
* `Using intersect(...)'
* `Using union(...)'
* `Using sort_union(...)'
The following sections describe these methods in greater detail.
deficiencies:
* If a range scan is possible on some key, an Index Merge is not
considered. For example, consider this query:
SELECT * FROM t1 WHERE (goodkey1 < 10 OR goodkey2 < 20) AND badkey < 30;
For this query, two plans are possible:
* An Index Merge scan using the `(goodkey1 < 10 OR goodkey2 <
20)' condition.
* A range scan using the `badkey < 30' condition.
However, the optimizer considers only the second plan. If that is
not what you want, you can make the optimizer consider Index Merge
by using `IGNORE INDEX' or `FORCE INDEX'. The following queries
are executed using Index Merge:
SELECT * FROM t1 FORCE INDEX(goodkey1,goodkey2)
WHERE (goodkey1 < 10 OR goodkey2 < 20) AND badkey < 30;
SELECT * FROM t1 IGNORE INDEX(badkey)
WHERE (goodkey1 < 10 OR goodkey2 < 20) AND badkey < 30;
* If your query has a complex `WHERE' clause with deep `AND'/`OR'
nesting and MySQL doesn't choose the optimal plan, try
distributing terms using the following identity laws:
(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 fulltext indexes. We plan to
extend it to cover these in a future MySQL release.
The choice between different possible variants of the Index Merge
access method and other access methods is based on cost estimates of
various available options.
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