As noted elsewhere, JSON columns cannot be indexed directly. 正如其他地方所指出的，JSON列不能直接索引。To create an index that references such a column indirectly, you can define a generated column that extracts the information that should be indexed, then create an index on the generated column, as shown in this example:要创建间接引用此类列的索引，可以定义一个生成的列，该列提取应编制索引的信息，然后在生成的列上创建索引，如本例所示：

mysql> CREATE TABLE jemp (
    ->     c JSON,
    ->     g INT GENERATED ALWAYS AS (c->"$.id"),
    ->     INDEX i (g)
    -> );
Query OK, 0 rows affected (0.28 sec)

mysql> INSERT INTO jemp (c) VALUES
     >   ('{"id": "1", "name": "Fred"}'), ('{"id": "2", "name": "Wilma"}'),
     >   ('{"id": "3", "name": "Barney"}'), ('{"id": "4", "name": "Betty"}');
Query OK, 4 rows affected (0.04 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> SELECT c->>"$.name" AS name
     >     FROM jemp WHERE g > 2;
+--------+
| name   |
+--------+
| Barney |
| Betty  |
+--------+
2 rows in set (0.00 sec)

mysql> EXPLAIN SELECT c->>"$.name" AS name
     >    FROM jemp WHERE g > 2\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: jemp
   partitions: NULL
         type: range
possible_keys: i
          key: i
      key_len: 5
          ref: NULL
         rows: 2
     filtered: 100.00
        Extra: Using where
1 row in set, 1 warning (0.00 sec)

mysql> SHOW WARNINGS\G
*************************** 1. row ***************************
  Level: Note
   Code: 1003
Message: /* select#1 */ select json_unquote(json_extract(`test`.`jemp`.`c`,'$.name'))
AS `name` from `test`.`jemp` where (`test`.`jemp`.`g` > 2)
1 row in set (0.00 sec)

(We have wrapped the output from the last statement in this example to fit the viewing area.)（我们已经包装了本例中最后一条语句的输出，以适合查看区域。）

When you use EXPLAIN on a SELECT or other SQL statement containing one or more expressions that use the -> or ->> operator, these expressions are translated into their equivalents using JSON_EXTRACT() and (if needed) JSON_UNQUOTE() instead, as shown here in the output from SHOW WARNINGS immediately following this EXPLAIN statement:对SELECT或包含一个或多个使用->运算符或->>运算符的表达式的其他SQL语句使用EXPLAIN时，这些表达式将使用JSON_EXTRACT()和（如果需要）JSON_UNQUOTE()转换为等效表达式，如下面EXPLAIN语句后面的SHOW WARNINGS输出所示：

mysql> EXPLAIN SELECT c->>"$.name"
     > FROM jemp WHERE g > 2 ORDER BY c->"$.name"\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: jemp
   partitions: NULL
         type: range
possible_keys: i
          key: i
      key_len: 5
          ref: NULL
         rows: 2
     filtered: 100.00
        Extra: Using where; Using filesort
1 row in set, 1 warning (0.00 sec)

mysql> SHOW WARNINGS\G
*************************** 1. row ***************************
  Level: Note
   Code: 1003
Message: /* select#1 */ select json_unquote(json_extract(`test`.`jemp`.`c`,'$.name')) AS
`c->>"$.name"` from `test`.`jemp` where (`test`.`jemp`.`g` > 2) order by
json_extract(`test`.`jemp`.`c`,'$.name')
1 row in set (0.00 sec)

See the descriptions of the -> and ->> operators, as well as those of the JSON_EXTRACT() and JSON_UNQUOTE() functions, for additional information and examples.请参阅->和->>运算符的说明，以及JSON_EXTRACT()和JSON_UNQUOTE()函数的说明，以获取更多信息和示例。

This technique also can be used to provide indexes that indirectly reference columns of other types that cannot be indexed directly, such as GEOMETRY columns.此技术还可用于提供间接引用无法直接索引的其他类型的列（如GEOMETRY列）的索引。

In MySQL 8.0.21 and later, it is also possible to create an index on a JSON column using the JSON_VALUE() function with an expression that can be used to optimize queries employing the expression. 在MySQL 8.0.21及更高版本中，还可以使用JSON_VALUE()函数和表达式在JSON列上创建索引，该表达式可用于优化使用该表达式的查询。See the description of that function for more information and examples.有关更多信息和示例，请参阅该函数的说明。

JSON columns and indirect indexing in NDB ClusterNDB集群中的JSON列和间接索引

It is also possible to use indirect indexing of JSON columns in MySQL NDB Cluster, subject to the following conditions:也可以在MySQL NDB集群中使用JSON列的间接索引，但需满足以下条件：

The CREATE TABLE statement used to create the table jempn shown here is a version of the jemp table shown previously, with modifications making it compatible with NDB:用于创建此处显示的表jempn的CREATE TABLE语句是前面显示的jemp表的一个版本，经过修改使其与NDB兼容：

CREATE TABLE jempn (
  a BIGINT(20) NOT NULL AUTO_INCREMENT PRIMARY KEY,
  c JSON DEFAULT NULL,
  g INT GENERATED ALWAYS AS (c->"$.name") STORED,
  INDEX i (g)
) ENGINE=NDB;

We can populate this table using the following INSERT statement:我们可以使用以下INSERT语句填充此表：

INSERT INTO jempn (a, c) VALUES
  (NULL, '{"id": "1", "name": "Fred"}'),
  (NULL, '{"id": "2", "name": "Wilma"}'),
  (NULL, '{"id": "3", "name": "Barney"}'),
  (NULL, '{"id": "4", "name": "Betty"}');

Now NDB can use index i, as shown here:现在NDB可以使用索引i，如下所示：

mysql> EXPLAIN SELECT c->>"$.name" AS name
          FROM jempn WHERE g > 2\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: jempn
   partitions: p0,p1
         type: range
possible_keys: i
          key: i
      key_len: 5
          ref: NULL
         rows: 3
     filtered: 100.00
        Extra: Using where with pushed condition (`test`.`jempn`.`g` > 2)
1 row in set, 1 warning (0.00 sec)

mysql> SHOW WARNINGS\G
*************************** 1. row ***************************
  Level: Note
   Code: 1003
Message: /* select#1 */ select
json_unquote(json_extract(`test`.`jempn`.`c`,'$.name')) AS `name` from
`test`.`jempn` where (`test`.`jempn`.`g` > 2)
1 row in set (0.00 sec)

You should keep in mind that a stored generated column uses DataMemory, and that an index on such a column uses IndexMemory.您应该记住，存储的生成列使用DataMemory，而此类列上的索引使用IndexMemory。