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MySQL 8.0 Reference Manual  /  ...  /  Row Constructor Expression Optimization

Pre-General Availability Draft: 2017-06-23

8.2.1.18 Row Constructor Expression Optimization

Row constructors permit simultaneous comparisons of multiple values. For example, these two statements are semantically equivalent:

SELECT * FROM t1 WHERE (column1,column2) = (1,1);
SELECT * FROM t1 WHERE column1 = 1 AND column2 = 1;

In addition, the optimizer handles both expressions the same way.

The optimizer is less likely to use available indexes if the row constructor columns do not cover the prefix of an index. Consider the following table, which has a primary key on (c1, c2, c3):

CREATE TABLE t1 (
  c1 INT, c2 INT, c3 INT, c4 CHAR(100),
  PRIMARY KEY(c1,c2,c3)
);

In this query, the WHERE clause uses all columns in the index. However, the row constructor itself does not cover an index prefix, with the result that the optimizer uses only c1 (key_len=4, the size of c1):

mysql> EXPLAIN SELECT * FROM t1
       WHERE c1=1 AND (c2,c3) > (1,1)\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t1
   partitions: NULL
         type: ref
possible_keys: PRIMARY
          key: PRIMARY
      key_len: 4
          ref: const
         rows: 3
     filtered: 100.00
        Extra: Using where

In such cases, rewriting the row constructor expression using an equivalent nonconstructor expression may result in more complete index use. For the given query, the row constructor and equivalent nonconstructor expressions are:

(c2,c3) > (1,1)
c2 > 1 OR ((c2 = 1) AND (c3 > 1))

Rewriting the query to use the nonconstructor expression results in the optimizer using all three columns in the index (key_len=12):

mysql> EXPLAIN SELECT * FROM t1
       WHERE c1 = 1 AND (c2 > 1 OR ((c2 = 1) AND (c3 > 1)))\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t1
   partitions: NULL
         type: range
possible_keys: PRIMARY
          key: PRIMARY
      key_len: 12
          ref: NULL
         rows: 3
     filtered: 100.00
        Extra: Using where

Thus, for better results, avoid mixing row constructors with AND/OR expressions. Use one or the other.

Under certain conditions, the optimizer can apply the range access method to IN() expressions that have row constructor arguments. See Range Optimization of Row Constructor Expressions.


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