Question

I have queries that are similar to:

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

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

Since the expression on the right hand side does not depend on the column values, should I expect the query optimizer to optimize it into a constant value such as:

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

Or should I calculate the constants and build a query using code.

Answer 1

According to the documentation for MySQL’s NOW() function (for which CURRENT_TIMESTAMP is a synonym):

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.

As such, the query optimiser will treat it as a constant, as desired. One can see this in the EXPLAIN output.

I can’t speak for MSSQL, although perhaps this blog is worth a read?