MySQL——通过EXPLAIN分析SQL的执行计划 在MySQL中,我们可以通过EXPLAIN命令获取MySQL如何执行SELECT语句的信息,包括在SELECT语句执行过程中表如何连接和连接的顺序。 下面分别对EXPLAIN命令结果的每一列进行说明: select_type:表示SELECT的类型,常见的取值有: 类型 说明 SIMPLE 简单表,不使用表连接或子查询 ...通过...
SQL - Clone Tables SQL - Temporary Tables SQL - Alter Tables SQL - Drop Table SQL - Delete Table SQL - Constraints SQL Queries SQL - Insert Query SQL - Select Query SQL - Select Into SQL - Insert Into Select SQL - Update Query SQL - Delete Query SQL - Sorting Results SQL Views SQL...
1 row in set (0.00 sec) (7). unique_subquery 该类型替换了下面形式的IN子查询的ref: value IN (SELECT primary_key FROM single_table WHERE some_expr) unique_subquery是一个索引查找函数,可以完全替换子查询,效率更高。 (8).index_subquery 该联接类型类似于unique_subquery。可以替换IN子查询,但只适合...
Sequential ScanscanAmazon Redshift relation scan or table scan operator or step. Scans whole table sequentially from beginning to end; also evaluates query constraints for every row (Filter) if specified with WHERE clause. Also used to run INSERT, UPDATE, and DELETE statements. ...
Learn how to use the SQLite EXPLAIN command to analyze SQL statements and understand their execution plans.
Performance monitoring can also be challenging, as intermittent issues may not be easily isolated using standard tools like EXPLAIN. The complexity increases when procedures interact with other database objects, such as indexes and constraints. This process of analyzing code, testing ...
Allows a large table to be broken up into a number of smaller partitions which can be queried much more quickly than the table as a whole a union all view is built over the top to provide the original functionality Check constraints or where clauses provide partition elimination capabilities ...
The Optimizer can determine this based on UNIQUE and PRIMARY KEY constraints on the tables. When outer joins are used the row preserving table (table without the outer join operator) must come after the other table in the predicate (table with the outer join operator) to ensure all of...
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The population of the Explain tables by the Explain facility will not activate triggers or referential or check constraints. For example, if an insert trigger were defined on the EXPLAIN_INSTANCE table, and an eligible statement were explained, the trigger would not be activated. ...