EXPLAIN的输出列 EXPLAIN命令用来提供MySQL的执行信息,用来显示SQL语句执行的效率。 平时我们发现某条SQL语句执行慢,可以通过该语句来查看原因,看看是否用到了索引,以及其他优化措施。 EXPLAIN 可以应用在 SELECT, DELETE, INSERT, REPLACE, 和 UPDATE 语句上。 EXPLAIN命令的输出信息如下表所示: 小扎,深吸一口气,怎么...
EXPLAIN ANALYZE: This statement returns the actual SQL execution plan. Compared with the estimated execution plan returned by the EXPLAIN statement, the actual execution plan returned by the EXPLAIN ANALYZE statement contains actual execution information such as the actually executed operators and the acc...
the MySQL optimizer considers many techniques to efficiently performthe lookups involved in an SQL query. A query on a huge table can be performedwithout reading all the rows; a join involving several tables can be performedwithout comparing every combination of rows. The set of operations...
EXPLAIN的输出列 EXPLAIN命令用来提供MySQL的执行信息,用来显示SQL语句执行的效率。 平时我们发现某条SQL语句执行慢,可以通过该语句来查看原因,看看是否用到了索引,以及其他优化措施。 EXPLAIN 可以应用在 SELECT, DELETE, INSERT, REPLACE, 和 UPDATE 语句上。 EXPLAIN命令的输出信息如下表所示: 小扎,深吸一口气,怎么...
摘要: DBA经常会用到的explain来查看SQL语句的执行计划,今天小人斗胆,从MySQL 5.7 Reference Manual中把MySQL EXPLAIN Output Format翻译过来。欢迎拍砖 Explain语句提供了一个select语句执行计划的信息。 Explain为每个用了select语句的表,返回一行信息。它列出了表中的顺序输出,MySQL会读取他们,然后再处理。MySQL解决了所...
eq_ref can be used for indexed columns that are compared using the = operator. The comparison value can be a constant or an expression that uses columns from tables that are read before this table. In the following examples, MySQL can use an eq_ref join to process ref_table: ...
Mysql执行计划(Explain)说明Mysql执行计划(Explain)说明 Dependingonthedetailsofyourtables,columns,indexes,andtheconditionsinyourWHEREclause,theMySQLoptimizerconsidersmanytechniquestoefficientlyperformthelookupsinvolvedinanSQLquery.Aqueryonahugetablecanbeperformedwithoutreadingalltherows;ajoininvolvingseveraltablescanbe...
Always Encrypted with secure enclaves also addresses some of the limitations of Randomized encryption, which enables pattern matching, comparison operations, and indexing on columns using this encryption type. Next unit: Enable encrypted connections ...
For example, these conditions might include comparison operators of the numeric and date type, such as >, <, =, >=, <=, and the character type such as LIKE. Note: In order to use an index, the condition must be sargable. For example, the condition YEAR(date_column) < 1992 cannot ...
http://dev.mysql.com/doc/refman/5.0/en/comparison-operators.html#operator_equal-to It helps to think of NULL as “something i don’t know anything about besides the fact that i don’t know anything about it (although i may know its data type)” For something you don’t know anything...