MySQL has server variables named
new variable can be set per-session and globally, and is dynamic. The
old variable is not dynamic, and only global in scope. Both default to FALSE in MySQL 5.1.
According to the manual, the
was used in MySQL 4.0 to turn on some 4.1 behaviors, and is retained for backward compatibility.
That same page notes the following about the
when old is enabled, it changes the default scope of index hints to that used prior to MySQL 5.1.17. That is, index hints with no FOR clause apply only to how indexes are used for row retrieval and not to resolution of ORDER BY or GROUP BY clauses.
That’s right — the
old variable changes some index hint behavior to before 5.1.17, and the
new variable is provided for backwards compatibility of a 4.0 function.
Perhaps now that they have already used the vague terms, MySQL will stick to descriptive names, such as
new_behavior (or, even better, specifying what that new behavior is. If not in the variable itself, at least in the manual!)