MySQL Range Optimization

8.2.1.3 Range Optimization

MYSQL的Range Optimization的目的還是儘可能的使用索引

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 descriptions of conditions under which the optimizer uses range access.

8.2.1.3.1 The Range Access Method for Single-Part Indexes

針對單一索引

For a single-part index, index value intervals can be conveniently represented by corresponding conditions in theWHERE 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.

    對於BTREE索引和HASH索引來說，索引的範圍優化基本上只適用於等值查詢。譬如=, <=>, IN(), IS NULL, IS NOT NULL操作符。

    相對於HASH索引，BTREE索引同樣支持非等值查詢，譬如>, <, >=, <=, BETWEEN, !=, <>和LIKE（註意，like的常量值不能以通配符開頭）

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

• A constant from the query string

• A column of a const or system table from the same join

• The result of an uncorrelated subquery

Any expression composed entirely from subexpressions of the preceding types

常量值一般指三種：查詢條件為常量，const表或system表，非關聯子查詢的結果

其中，const表指的是最多只有一個匹配行，譬如基於主鍵的查詢：

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 WHERE clause:

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.

以下是MySQL提取範圍條件的思路，並不是等價變換，目的還是在於儘可能的使用索引的範圍查詢，包括後面的紅色部分也提到，這樣變換後的條件會沒有原來的條件嚴格，MySQL這樣做的目的在於利用索引過濾掉很大一部分記錄，然後再對剩下的記錄進行額外的篩選。

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:

1. 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')
   

2. 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')
   

3. 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')
   

4. 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.

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.

8.2.1.3.2 The Range Access Method for Multiple-Part Indexes

針對複合索引

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(key_part1, key_part2, key_part3), and the following set of key tuples listed in key order:

key_part1  key_part2  key_part3
  NULL       1          'abc'
  NULL       1          'xyz'
  NULL       2          'foo'
   1         1          'abc'
   1         1          'xyz'
   1         2          'abc'
   2         1          'aaa'

The condition key_part1 = 1 defines this interval:

(1,-inf,-inf) <= (key_part1,key_part2,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 key_part3 = 'abc' does not define a single interval and cannot be used by the range access method.

如果使用前導列key_part1，則可以使用索引，如果直接使用key_part3，則不能使用索引。

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_part1cmpconst1
  AND key_part2cmpconst2
  AND ...
  AND key_partNcmpconstN;
  

  Here, const1, const2, … are constants, cmp is one of the =, <=>, or 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 HASH index:

  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 8.2.1.3.1, “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 'pattern' (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 <>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:

  key_part1 = 'foo' AND key_part2 >= 10 AND key_part3 > 10
  

  The single interval is:

  ('foo',10,-inf) < (key_part1,key_part2,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.

       對於BTREE的複合索引來說，一旦其中的一個索引列使用了非等值查詢，則在其後的索引列將無法繼續使用索引。

       譬如：key_part1 = 'foo' AND key_part2 >= 10 AND key_part3 > 10

       因為第二個索引列key_part2使用了非等值查詢，則第三個索引列key_part3無法使用索引。

       所以，它的變換形式為('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 withAND, 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_part1,key_part2) < (1,2)
  (5,-inf) < (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 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_part1 and 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 8.2.1.3.1, “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.

8.2.1.3.3 Equality Range Optimization of Many-Valued Comparisons

Consider these expressions, where col_name is an indexed column:

col_name IN(val1, ..., valN)
col_name = val1 OR ... OR col_name = valN

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，則每一個range的對應的row為1。如果不是唯一索引，優化器有兩個方式來評估每個range對應的行數：index dives和index statistics。其中，index dives能提供更精確的估計，但是成本會比較高，index statistics速度較快，但精度沒有index dives高，選擇哪種方式由eq_range_index_dive_limit決定，5.7.3之前預設值為10，指的是當range的個數小於或等於9時，MySQL預設會選擇index dives，超過9個，則選擇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 col_name IN (10, 20, 30) 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.

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.

The 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 eq_range_index_dive_limit to N + 1.

To update table index statistics for best estimates, use ANALYZE TABLE.

8.2.1.3.4 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:

• Only IN predicates can be used, not NOT IN.

• 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 theHandler_read_first, Handler_read_key, and Handler_read_next status variables.