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SET @start_global_value = @@global.optimizer_switch;
SELECT @start_global_value;
#
# exists as global and session
#
select @@global.optimizer_switch;
select @@session.optimizer_switch;
show global variables like 'optimizer_switch';
show session variables like 'optimizer_switch';
select * from information_schema.global_variables where variable_name='optimizer_switch';
select * from information_schema.session_variables where variable_name='optimizer_switch';
#
# show that it's writable
#
set global optimizer_switch=10;
select @@global.optimizer_switch;
set session optimizer_switch=5;
select @@session.optimizer_switch;
set global optimizer_switch="index_merge_sort_union=on";
select @@global.optimizer_switch;
set session optimizer_switch="index_merge=off";
select @@session.optimizer_switch;
set session optimizer_switch="default";
select @@session.optimizer_switch;
#
# incorrect assignments
#
--error ER_WRONG_TYPE_FOR_VAR
set global optimizer_switch=1.1;
--error ER_WRONG_TYPE_FOR_VAR
set global optimizer_switch=1e1;
--error ER_WRONG_VALUE_FOR_VAR
set session optimizer_switch="index_merge";
--error ER_WRONG_VALUE_FOR_VAR
set session optimizer_switch="foobar";
SET @@global.optimizer_switch = @start_global_value;
SELECT @@global.optimizer_switch;
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