[数据库]MySQL Range Optimization
18.104.22.168 Range Optimization
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 descriptions of conditions under which the optimizer uses range access.
22.214.171.124.1 The Range Access Method for Single-Part Indexes
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:
HASH indexes, comparison of a key part with a constant value is a range condition when using the
IS NULL, or
IS NOT NULL operators.
BTREE indexes, comparison of a key part with a constant value is a range condition when using the
<> 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
AND form a range condition.
对于BTREE索引和HASH索引来说，索引的范围优化基本上只适用于等值查询。譬如=, <=>, IN(), IS NULL, IS NOT NULL操作符。
相对于HASH索引，BTREE索引同样支持非等值查询，譬如>, <, >=, <=, BETWEEN, !=, <>和LIKE（注意，like的常量值不能以通配符开头）
“Constant value” in the preceding descriptions means one of the following:
A constant from the query string
A column of a
system table from the same join
The result of an uncorrelated subquery
Any expression composed entirely from subexpressions of the preceding types
SELECT * FROM tbl_name WHERE primary_key=1;SELECT * FROM tbl_name WHERE primary_key_part1=1 AND primary_key_part2=2;
Here are some examples of queries with range conditions in the
SELECT * FROM t1 WHERE
key_col > 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';
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
(key1 < 'abc' AND (key1 LIKE 'abcde%' OR key1 LIKE '%b')) OR(key1 < 'bar' AND nonkey = 4) OR(key1 < 'uux' AND key1 > 'z')
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:
Replacing these conditions with constants, we get:
(key1 < 'abc' AND TRUE) OR (key1 < 'bar' AND TRUE) OR (FALSE)
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
The range condition extraction algorithm can handle nested
OR constructs of arbitrary depth, and its output does not depend on the order in which conditions appear in
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
126.96.36.199.2 The Range Access Method for Multiple-Part Indexes
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_part3 NULL 1 'abc' NULL 1 'xyz' NULL 2 'foo' 1 1 'abc' 1 1 'xyz' 1 2 'abc' 2 1 'aaa'
defines this interval:
key_part1 = 1
(1,-inf,-inf) <= (
key_part3) < (1,+inf,+inf)
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.
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:
const2, … are constants,
cmp is one of the
IS NULL comparison operators, and the conditions cover all index parts. (That is, there are
N conditions, one for each part of an
N-part index.) For example, the following is a range condition for a three-part
key_part1 = 1 AND
key_part2 IS NULL AND
key_part3 = 'foo'
For the definition of what is considered to be a constant, see Section 188.8.131.52.1, “The Range Access Method for Single-Part Indexes”.
BTREE index, an interval might be usable for conditions combined with
AND, where each condition compares a key part with a constant value using
LIKE ' (where
' 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
!= is used).
The optimizer attempts to use additional key parts to determine the interval as long as the comparison operator is
IS NULL. If the operator is
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:
key_part1 = 'foo' AND
key_part2 >= 10 AND
key_part3 > 10
The single interval is:
('foo',10,-inf) < (
key_part3) < ('foo',+inf,+inf)
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.
譬如：key_part1 = 'foo' AND key_part2 >= 10 AND key_part3 > 10
所以，它的变换形式为('foo',10,-inf) < (key_part1,key_part2,key_part3) < ('foo',+inf,+inf)，
而不是('foo',10,10) < (key_part1,key_part2,key_part3) < ('foo',+inf,+inf)
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:
key_part1 = 1 AND
key_part2 < 2) OR (
key_part1 > 5)
The intervals are:
(1,-inf) < (
key_part2) < (1,2)(5,-inf) < (
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
EXPLAINoutput 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 that
key_part2 can be
NULL. Then the
key_len column displays two key part lengths for the following condition:
key_part1 >= 1 AND
key_part2 < 2
But, in fact, the condition is converted to this:
key_part1 >= 1 AND
key_part2 IS NOT NULL
Section 184.108.40.206.1, “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.
220.127.116.11.3 Equality Range Optimization of Many-Valued Comparisons
Consider these expressions, where
col_name is an indexed column:
val1 OR ... OR
Each expression is true if
col_name is equal to any of several values. These comparisons are equality range comparisons (where the “range” is a single value). The optimizer estimates the cost of reading qualifying rows for equality range comparisons as follows:
If there is a unique index on
col_name, the row estimate for each range is 1 because at most one row can have the given value.
Otherwise, the optimizer can estimate the row count for each range using dives into the index or index statistics.
如果是唯一索引，则每一个range的对应的row为1。如果不是唯一索引，优化器有两个方式来评估每个range对应的行数：index dives和index statistics。其中，index dives能提供更精确的估计，但是成本会比较高，index statistics速度较快，但精度没有index dives高，选择哪种方式由eq_range_index_dive_limit决定，5.7.3之前默认值为10，指的是当range的个数小于或等于9时，MySQL默认会选择index dives，超过9个，则选择index statistics。
With index dives, the optimizer makes a dive at each end of a range and uses the number of rows in the range as the estimate. For example, the expression
has three equality ranges and the optimizer makes two dives per range to generate a row estimate. Each pair of dives yields an estimate of the number of rows that have the given value.
col_name IN (10, 20, 30)
Index dives provide accurate row estimates, but as the number of comparison values in the expression increases, the optimizer takes longer to generate a row estimate. Use of index statistics is less accurate than index dives but permits faster row estimation for large value lists.
eq_range_index_dive_limit system variable enables you to configure the number of values at which the optimizer switches from one row estimation strategy to the other. To disable use of statistics and always use index dives, set
eq_range_index_dive_limit to 0. To permit use of index dives for comparisons of up to
Nequality ranges, set
N + 1.
To update table index statistics for best estimates, use
18.104.22.168.4 Range Optimization of Row Constructor Expressions
As of MySQL 5.7.3, the optimizer is able to apply the range scan access method to queries of this form:
SELECT ... FROM t1 WHERE ( col_1, col_2 ) IN (( 'a', 'b' ), ( 'c', 'd' ));
Previously, for range scans to be used it was necessary for the query to be written as:
SELECT ... FROM t1 WHERE ( col_1 = 'a' AND col_2 = 'b' )OR ( col_1 = 'c' AND col_2 = 'd' );
For the optimizer to use a range scan, queries must satisfy these conditions:
IN predicates can be used, not
There may only be column references in the row constructor on the
IN predicate's left hand side.
There must be more than one row constructor on the
IN predicate's right hand side.
Row constructors on the
IN predicate's right hand side must contain only runtime constants, which are either literals or local column references that are bound to constants during execution.
Compared to similar queries executed before MySQL 5.7.3,
EXPLAIN output for applicable queries changes from full table or index scan to range scan. Changes are also visible by checking the values of the
Handler_read_next status variables.