The following statement

SET optimizer_switch = REPLACE( @@optimizer_switch, '=off', '=on' )

as well as many other similar constructions, fails with ER_WRONG_VALUE_FOR_VAR: 1231: Variable 'optimizer_switch' can't be set to the value of 'table_elimination=on'.

In 5.2 and 5.3 it works all right.

It does not seem to be about the value, since this way it works:

SET @a = REPLACE( @@optimizer_switch, '=off', '=on' );
SET optimizer_switch = @a;

But not this way

SET @b = @@optimizer_switch;
SET optimizer_switch = REPLACE( @b, '=off', '=on' );

It also does not seem to be about the result of REPLACE being too long, since this one works:

SET optimizer_switch = REPLACE( @@optimizer_switch, '=on', '=off' )

REPLACE statements are a convenient way to set optimizer_switch to a needed value in tests, and keep it version-independent; otherwise, it's not an important problem in itself, but it might signify an underlying issue, either with the REPLACE function, or with the optimizer_switch, so I find it worrisome.

Test case:

SET @saved_switch = @@optimizer_switch;

 # This works

SET optimizer_switch = REPLACE( @@optimizer_switch, '=on', '=off' );

SET optimizer_switch = @saved_switch;

 # This doesn't

SET optimizer_switch = REPLACE( @@optimizer_switch, '=off', '=on' );

SET optimizer_switch = @saved_switch;

 # This works

SET @a = REPLACE( @@optimizer_switch, '=off', '=on' );

SET optimizer_switch = @a;

SET optimizer_switch = @saved_switch;

 # This doesn't

SET @b = @@optimizer_switch;

SET optimizer_switch = REPLACE( @b, '=off', '=on' );