MySQL 8.0

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select version();

version()

8.0.36

create table mytable (id int primary key auto_increment , myjs json);
insert into mytable (myjs) values ('[ { "foo": 1, "bar": 2 }, { "foo": 3, "bar": 3 } ]');
CREATE INDEX myidx 
ON mytable ( (CAST(myjs AS UNSIGNED ARRAY)) );

Cannot store an array or an object in a scalar key part of the index 'myidx'.