a.SIMPLE:查询中不包含子查询或者UNION b.查询中若包含任何复杂的子部分,最外层查询则被...一次就找到了; key=primary的话,表示使用了主键; type=all,表示为全表扫描; key=null表示没用到索引。 type=ref,因为这时认为是多个匹配行,在联合查询中,一般为REF智能推荐mysql性能分析——explain 语法:explain
(10) UNSIGNED NOT NULL, `name` CHAR(64) NOT NULL, `help_category_id` SMALLINT(5) UNSIGNED NOT NULL, `description` TEXT NOT NULL, `example` TEXT NOT NULL, `url` TEXT NOT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=21495809 DEFAULT CHARSET=utf8 1 row in set (0.00 ...
|1| PRIMARY |customer| ALL |NULL| NULL |NULL| NULL |599| Using where || 2 |DEPENDENT SUBQUERY| a |const| PRIMARY |PRIMARY| 2 |const| 1 |Using index| |3| DEPENDENT UNION |b| const |PRIMARY| PRIMARY |2| const |1| Using index || NULL |UNION RESULT| <union2,3> |ALL| NULL ...
mysql官网相关参数解读 一:id SELECT标识符 1.id越大越先执行 2.相同id,从从往下执行 二:select_type 1.SIMPLE :最简单的查询(没有关联查询没有子查询没有union的查询语句) 2:PRIMARY:子查询最外层的查询语句 3.SUBQUERY:子查询内层查询语句 4.DERIVED:派生表查......
当使用主键索引(PRIMARY KEY)或不为空的唯一索引(UNIQUE NOT NULL index) const 该表最多具有一个匹配行,该行在查询开始时读取。因为只有一行,所以优化器的其余部分可以将这一行中列的值视为常量。 const表非常快,因为只读取一次。 system const的一种特殊情况,表仅有一行满足条件 5.1 index index时存在两种情况...
Example: Query data from a row-oriented table. BEGIN; CREATE TABLE row_test_1 ( id bigint not null, name text not null, class text , PRIMARY KEY (id) ); CALL set_table_property('row_test_1', 'orientation', 'row'); CALL set_table_property('row_test_1', 'clustering_key', '...
Hello there,Primary key is basically a field which cannot be same and its unique. For example a students table will have id,name,roll_no fields and so the id field can be primary key .It can be used to perform operations using php for e.g "DELETE FROM 'students' WHERE id=" . $_...
key_column_part2=1; ref All rows with matching index values are read from this table for each combination of rows from the previous tables. ref is used if the join uses only a leftmost prefix of the key or if the key is not a PRIMARY KEY or UNIQUE index (in other words, if the...
In this example only the rows from the sales tables need to be sorted (ID 5), the rows from the products table are already sorted on the join column coming from the primary key index access (ID 4). Figure 19: Example plan output using SORT MERGE JOIN Beginning with Oracle Database ...
Organizations can also be donors (for example, Workplace Giving), sponsors, partners, vendors, or suppliers. Households A household allows you to group individuals together so that you can view their donation history and activities as a group. In a household, each contact is either the primary...