◇ Select tables optimized away:当我们使用某些聚合函数来访问存在索引的某个字段的时候,MySQL Query Optimizer 会通过索引而直接一次定位到所需的数据行完成整个查 询。当然,前提是在Query 中不能有GROUP BY 操作。如使用MIN()或者MAX()的时候 ◇ Using filesort:当我们的Query 中包含ORDER BY 操作,而且无法利...
在5.6.10以后的版本里面,是可以直接对dml语句进行explain分析操作的.MySQL 优化器是基于开销来工作的,它并不提供任何的QEP的位置。这意味着QEP 是在每条SQL 语句执行的时候动态地计算出来的。在MySQL 存储过程中的SQL 语句也是在每次执行时计算QEP 的。存储过程缓存仅仅解析查询树。 DESC命令: 查看表信息: desc查看...
1、id的含义 The SELECT identifier. This is the sequential number of the SELECT within the query. The value can be NULL if the row refers to the union result of other rows. In this case, the table column shows a value like <unionM,N> to indicate that the row refers to the union of...
The column information is retrieved from the table using only information in the index tree without having to do an additional seek to read the actual row. This strategy can be used when the query uses only columns that are part of a single index. For InnoDB tables that have a user-define...
<dsql_query>最上層/文件元素。 <sql>回應SQL_statement。 <params>目前不使用此標記。 <materialized_view_candidates> (預覽)包含建議具體化檢視的 CREATE 陳述式,可讓 SQL 陳述式有較好的效能。 <dsql_operations>摘要說明和包含查詢步驟,並包含查詢的成本資訊。 也包含所有<dsql_operation>區塊。 此標記...
可以通过两种方式定位执行效率较低SQL语句: 使用参数:--log-slow-queries [=file_name],MySQL会将long_query_time的SQL语句日志写入文件; 使用参数show processlist:查询MySQL线程状态、是否锁表。 慢查询日志在查询结束以后才记录,在应用反映执行效率问题时查询慢查询慢查询日志并不能定位问题。可以使用show processlist...
These commands exist to help database administrators and developers understand how SQLite is processing SQL commands. They are designed for interactive tuning and adjustments. It is not recommended that applications programmatically query or utilize this data, as both the data and the format may change...
This tutorial describes how to use Explain reports to locate and fix problematic (slow) queries. It uses the DBT-3 database and begins with the following simple query example. SELECT * FROM orders WHERE YEAR(o_orderdate) = 1992 AND MONTH(o_orderdate) = 4 AND o_clerk LIKE '%0223';As...
SQL EXPLAINANALYZESELECT*FROMcategoryORDERBYcat_id; 此简单查询返回以下信息: 分析 此选项执行语句,并返回查询计划。 该语句的输出被丢弃,但仍将执行该语句。 可以组合 ROLLBACK 和 EXPLAIN ANALYZE 语句,以防止通过更改数据库的查询(例如 INSERT、UPDATE 或 DELETE)来应用更改。 例如: ...
verbose The EXPLAIN statement outputs results in the row format, with an additional estCost column for the estimated cost of the query in the results. For more information about how to use this format, see SQL Plan Management. plan_cache The EXPLAIN statement outputs results in the row format...