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MySQL 5.0 Reference Manual :: 8 Optimization :: 8.3 Optimizing SQL Statements :: 8.3.1 Optimizing SELECT Statements :: 8.3.1.3 Range Optimization

- 8.3.1 Optimizing SELECT Statements
- 8.3.1.1 Speed of SELECT Statements
- 8.3.1.2 How MySQL Optimizes WHERE Clauses
- 8.3.1.3 Range Optimization
- 8.3.1.4 Index Merge Optimization
- 8.3.1.5 Engine Condition Pushdown Optimization
- 8.3.1.6 IS NULL Optimization
- 8.3.1.7 LEFT JOIN and RIGHT JOIN Optimization
- 8.3.1.8 Nested-Loop Join Algorithms
- 8.3.1.9 Nested Join Optimization
- 8.3.1.10 Outer Join Simplification
- 8.3.1.11 ORDER BY Optimization
- 8.3.1.12 GROUP BY Optimization
- 8.3.1.13 DISTINCT Optimization
- 8.3.1.14 Optimizing Subqueries with EXISTS Strategy
- 8.3.1.15 Optimizing LIMIT Queries
- 8.3.1.16 How to Avoid Table Scans

The `range`

access method
uses a single index to retrieve a subset of table rows that
are contained within one or several index value intervals. It
can be used for a single-part or multiple-part index. The
following sections give a detailed description of how
intervals are extracted from the `WHERE`

clause.

For a single-part index, index value intervals can be
conveniently represented by corresponding conditions in the
`WHERE`

clause, so we speak of
*range conditions* rather than
“intervals.”

The definition of a range condition for a single-part index is as follows:

For both

`BTREE`

and`HASH`

indexes, comparison of a key part with a constant value is a range condition when using the`=`

,`<=>`

,`IN()`

,`IS NULL`

, or`IS NOT NULL`

operators.Additionally, for

`BTREE`

indexes, comparison of a key part with a constant value is a range condition when using the`>`

,`<`

,`>=`

,`<=`

,`BETWEEN`

,`!=`

, or`<>`

operators, or`LIKE`

comparisons if the argument to`LIKE`

is a constant string that does not start with a wildcard character.For all types of indexes, multiple range conditions combined with

`OR`

or`AND`

form a range condition.

“Constant value” in the preceding descriptions means one of the following:

Here are some examples of queries with range conditions in
the `WHERE`

clause:

SELECT * FROM t1 WHERE> 1 AND`key_col`

< 10; SELECT * FROM t1 WHERE`key_col`

= 1 OR`key_col`

IN (15,18,20); SELECT * FROM t1 WHERE`key_col`

LIKE 'ab%' OR`key_col`

BETWEEN 'bar' AND 'foo';`key_col`

Note that some nonconstant values may be converted to constants during the constant propagation phase.

MySQL tries to extract range conditions from the
`WHERE`

clause for each of the possible
indexes. During the extraction process, conditions that
cannot be used for constructing the range condition are
dropped, conditions that produce overlapping ranges are
combined, and conditions that produce empty ranges are
removed.

Consider the following statement, where
`key1`

is an indexed column and
`nonkey`

is not indexed:

SELECT * FROM t1 WHERE (key1 < 'abc' AND (key1 LIKE 'abcde%' OR key1 LIKE '%b')) OR (key1 < 'bar' AND nonkey = 4) OR (key1 < 'uux' AND key1 > 'z');

The extraction process for key `key1`

is as
follows:

Start with original

`WHERE`

clause:(key1 < 'abc' AND (key1 LIKE 'abcde%' OR key1 LIKE '%b')) OR (key1 < 'bar' AND nonkey = 4) OR (key1 < 'uux' AND key1 > 'z')

Remove

`nonkey = 4`

and`key1 LIKE '%b'`

because they cannot be used for a range scan. The correct way to remove them is to replace them with`TRUE`

, so that we do not miss any matching rows when doing the range scan. Having replaced them with`TRUE`

, we get:(key1 < 'abc' AND (key1 LIKE 'abcde%' OR TRUE)) OR (key1 < 'bar' AND TRUE) OR (key1 < 'uux' AND key1 > 'z')

Collapse conditions that are always true or false:

`(key1 LIKE 'abcde%' OR TRUE)`

is always true`(key1 < 'uux' AND key1 > 'z')`

is always false

Replacing these conditions with constants, we get:

(key1 < 'abc' AND TRUE) OR (key1 < 'bar' AND TRUE) OR (FALSE)

Removing unnecessary

`TRUE`

and`FALSE`

constants, we obtain:(key1 < 'abc') OR (key1 < 'bar')

Combining overlapping intervals into one yields the final condition to be used for the range scan:

(key1 < 'bar')

In general (and as demonstrated by the preceding example),
the condition used for a range scan is less restrictive than
the `WHERE`

clause. MySQL performs an
additional check to filter out rows that satisfy the range
condition but not the full `WHERE`

clause.

The range condition extraction algorithm can handle nested
`AND`

/`OR`

constructs of arbitrary depth, and its output does not
depend on the order in which conditions appear in
`WHERE`

clause.

Currently, MySQL does not support merging multiple ranges
for the `range`

access
method for spatial indexes. To work around this limitation,
you can use a `UNION`

with
identical `SELECT`

statements,
except that you put each spatial predicate in a different
`SELECT`

.

Range conditions on a multiple-part index are an extension of range conditions for a single-part index. A range condition on a multiple-part index restricts index rows to lie within one or several key tuple intervals. Key tuple intervals are defined over a set of key tuples, using ordering from the index.

For example, consider a multiple-part index defined as
`key1(`

, and the
following set of key tuples listed in key order:
* key_part1*,

`key_part2`

`key_part3`

`key_part1`

`key_part2`

NULL 1 'abc' NULL 1 'xyz' NULL 2 'foo' 1 1 'abc' 1 1 'xyz' 1 2 'abc' 2 1 'aaa'`key_part3`

The condition

defines this interval:
* key_part1*
= 1

(1,-inf,-inf) <= (,`key_part1`

,`key_part2`

) < (1,+inf,+inf)`key_part3`

The interval covers the 4th, 5th, and 6th tuples in the preceding data set and can be used by the range access method.

By contrast, the condition

does not define a single interval and cannot
be used by the range access method.
* key_part3* =
'abc'

The following descriptions indicate how range conditions work for multiple-part indexes in greater detail.

For

`HASH`

indexes, each interval containing identical values can be used. This means that the interval can be produced only for conditions in the following form:`key_part1`

`cmp`

AND`const1`

`key_part2`

`cmp`

AND ... AND`const2`

`key_partN`

`cmp`

;`constN`

Here,

,`const1`

, … are constants,`const2`

is one of the`cmp`

`=`

,`<=>`

, or`IS NULL`

comparison operators, and the conditions cover all index parts. (That is, there areconditions, one for each part of an`N`

-part index.) For example, the following is a range condition for a three-part`N`

`HASH`

index:= 1 AND`key_part1`

IS NULL AND`key_part2`

= 'foo'`key_part3`

For the definition of what is considered to be a constant, see Section 8.3.1.3.1, “The Range Access Method for Single-Part Indexes”.

For a

`BTREE`

index, an interval might be usable for conditions combined with`AND`

, where each condition compares a key part with a constant value using`=`

,`<=>`

,`IS NULL`

,`>`

,`<`

,`>=`

,`<=`

,`!=`

,`<>`

,`BETWEEN`

, or`LIKE '`

(where'`pattern`

`'`

does not start with a wildcard). An interval can be used as long as it is possible to determine a single key tuple containing all rows that match the condition (or two intervals if'`pattern`

`<>`

or`!=`

is used).The optimizer attempts to use additional key parts to determine the interval as long as the comparison operator is

`=`

,`<=>`

, or`IS NULL`

. If the operator is`>`

,`<`

,`>=`

,`<=`

,`!=`

,`<>`

,`BETWEEN`

, or`LIKE`

, the optimizer uses it but considers no more key parts. For the following expression, the optimizer uses`=`

from the first comparison. It also uses`>=`

from the second comparison but considers no further key parts and does not use the third comparison for interval construction:= 'foo' AND`key_part1`

>= 10 AND`key_part2`

> 10`key_part3`

The single interval is:

('foo',10,-inf) < (

,`key_part1`

,`key_part2`

) < ('foo',+inf,+inf)`key_part3`

It is possible that the created interval contains more rows than the initial condition. For example, the preceding interval includes the value

`('foo', 11, 0)`

, which does not satisfy the original condition.If conditions that cover sets of rows contained within intervals are combined with

`OR`

, they form a condition that covers a set of rows contained within the union of their intervals. If the conditions are combined with`AND`

, they form a condition that covers a set of rows contained within the intersection of their intervals. For example, for this condition on a two-part index:(

= 1 AND`key_part1`

< 2) OR (`key_part2`

> 5)`key_part1`

The intervals are:

(1,-inf) < (

,`key_part1`

) < (1,2) (5,-inf) < (`key_part2`

,`key_part1`

)`key_part2`

In this example, the interval on the first line uses one key part for the left bound and two key parts for the right bound. The interval on the second line uses only one key part. The

`key_len`

column in the`EXPLAIN`

output indicates the maximum length of the key prefix used.In some cases,

`key_len`

may indicate that a key part was used, but that might be not what you would expect. Suppose thatand`key_part1`

can be`key_part2`

`NULL`

. Then the`key_len`

column displays two key part lengths for the following condition:>= 1 AND`key_part1`

< 2`key_part2`

But, in fact, the condition is converted to this:

>= 1 AND`key_part1`

IS NOT NULL`key_part2`

Section 8.3.1.3.1, “The Range Access Method for Single-Part Indexes”, describes how optimizations are performed to combine or eliminate intervals for range conditions on a single-part index. Analogous steps are performed for range conditions on multiple-part indexes.

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