You maybe want to show all the products that have been added to your database in the last 7 days on your high-traffic website. You are using MySQL and your query probably looks like this:
WHERE created >= NOW() - INTERVAL 7 DAY
Although being correct, this query is a bad query, because it is using NOW(). NOW() returns the current date and time to the split second. In other words: This query will never land in the cache query!
The better way would be to read the current day in PHP and pass it on to the query:
$date = date('Y-m-d');
$sql = '
WHERE created >= "%s" - INTERVAL 7 DAY
$sql = sprintf($sql, $date);
Now we have a SQL query that perfectly lands in the cache query.