MySQL has two filesort algorithms for sorting and retrieving results. The original method uses only the ORDER BY columns. The modified method uses not just the ORDER BY columns, but all the columns used in the
The differing behaviors can be demonstrated using the two utf8mb4 binary collations, one of which is PAD SPACE, the other of which is NO PAD. 我们先来看看PAD SPACE与NO PAD的具体意义: PAD SPACE:在排序和比较运算中,忽略字符串尾部空格。 NO PAD:在排序和比较运算中,字符串尾部空格当成普通字符,...
The storage engine can’t optimize accesses with any columns to the right of the first range condition. For example, if your query is WHERE last_name="Smith" AND first_name LIKE 'J%' AND dob='1976-12-23', the index access will use only the first two columns in the index, because ...
(1)两次扫描算法(Two passes)实现方式是先将需要排序的字段和可以直接定位到相关行数据的指针信息取出，然后在设定的内存（通过参数sort_buffer_size设定）中进行排序，完成排序之后再次通过行指针信息取出所需的Columns。注：该算法是4.1之前采用的算法，它需要两次访问数据，尤其是第二次读取操作会导致大量的随机I/O...
When EXPLAIN is used with FOR CONNECTION connection_id rather than an explainable statement, it displays the execution plan for the statement executing in the named connection. See Section 8.8.4, “Obtaining Execution Plan Information for a Named Connection”. 当EXPLAIN与FOR CONNECTION connection_id...
set_filter() : PFS_data_lock_container, PFS_data_lock_wait_container, Relay_log_info, Table_columns_view< ExclusionFilter > set_filter_value() : Sql_cmd_change_repl_filter set_finish_callback() : Parallel_reader set_finish_registration() : Multi_factor_auth_info set_first() : lob::pl...
Backport of fix for Community Bug#24671968: Fixed an issue where a query could produce incorrect results if the WHERE clause contained a dependent subquery, the table had a secondary index on the columns in the select list followed by the columns in the subquery, andGROUP BYorDISTINCTpermitted...
Two columns in anORDER BYcan sort in the same direction (bothASC, or bothDESC) or in opposite directions (oneASC, oneDESC). A condition for index use is that the index must have the same homogeneity, but need not have the same actual direction. ...
and ordering results by two columns: a varchar cast to a decimal, and a plain varchar. Removing the CAST, or either of the columns from the ORDER BY, seems to bypass the issue. I can provide more information about the table / query if this would be helpful to the investigation. Cheers...
How can you identify differences in specific columns between two tables? What MySQL tools are most efficient for large-scale table comparisons? How to find duplicates between two tables in MySQL? What types of comparison reports can be generated by the tool, and in which formats?