请教MySQL创建函数的soname关键字使用

Posted 2023-05-10

参考技术A 找到这么段东西：

mysql> help soname
Name: 'CREATE FUNCTION'
Description:
Syntax:
CREATE [AGGREGATE] FUNCTION function_name RETURNS STRING|INTEGER|REAL|DECIMAL
SONAME shared_library_name

A user-defined function (UDF) is a way to extend MySQL with a new
function that works like a native (built-in) MySQL function such as
ABS() or CONCAT().

function_name is the name that should be used in SQL statements to
invoke the function. The RETURNS clause indicates the type of the
function's return value. As of MySQL 5.0.3, DECIMAL is a legal value
after RETURNS, but currently DECIMAL functions return string values and
should be written like STRING functions.

shared_library_name is the basename of the shared object file that
contains the code that implements the function. The file must be
located in a directory that is searched by your system's dynamic
linker.

To create a function, you must have the INSERT and privilege for the
mysql database. This is necessary because CREATE FUNCTION adds a row to
the mysql.func system table that records the function's name, type, and
shared library name. If you do not have this table, you should run the
mysql_upgrade command to create it.

请教mysql模糊查询两边都用%为啥用不到索引？

问题在这个地方，去掉索引长度 （191），就可以使用到索引。

这里使用的两边%，同时字段符合覆盖索引，但是设置了前缀索引，因此查询用不到索引。

查到的一个说法：

MySQL 不能在 ORDER BY 或 GROUP BY 中使用前缀索引，也不能把它们用作覆盖索引(Covering Index)。

去掉索引长度后的执行结果

参考技术A 模糊查询和使用数据库自带的聚合函数都不会走索引。