mysql - 当表达式不涉及列时，查询优化器是否将表达式转换为常量

Question

我有类似的查询:

-- MySQL Query
SELECT * FROM table1 WHERE start_date >= CURRENT_TIMESTAMP - INTERVAL 7 DAY

-- MSSQL Query
SELECT * FROM table1 WHERE start_date >= GETDATE() - 7

由于右侧的表达式不依赖于列值，我是否应该期望查询优化器将其优化为常量值，例如:

SELECT * FROM table1 WHERE start_date >= '2012-04-28 14:54:31'

或者我应该计算常量并使用代码构建查询。

Answer 1

根据 MySQL 的文档 NOW()函数(CURRENT_TIMESTAMP 是同义词):

NOW() returns a constant time that indicates the time at which the statement began to execute. (Within a stored function or trigger, NOW() returns the time at which the function or triggering statement began to execute.) This differs from the behavior for SYSDATE(), which returns the exact time at which it executes.

因此，查询优化器会根据需要将其视为常量。一 jar see this在 EXPLAIN 输出中。

我不能代表 MSSQL，尽管也许 this blog值得一读吗？