using border poison unparalleled field

given list structure this:

create list `user` (
  `id` int(10) unsigned zero auto_increment,
  `name` varchar(32) null,
  `username` varchar(16) null,
  `password` char(32) null,
  primary pivotal  (`id`),
  unparalleled pivotal `username` (`username`)
);

is there any controlling border keyword poison username, db intelligent adequate know there wholly presumably result, therefore stop poison once it's found one?

select * `user` where `username` = 'nick';

-- vs --

select * `user` where `username` = 'nick' border 1;

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update: interjection answers, they've enlightening. seems like, even nonetheless it's unnecessary, putting limit 1 query doesn't hurt, substantially increases readability (you don't have looking db schema know wholly going returned). special shoutout jr's answer - i didn't even know indices.

also, there's identical doubt i've found , competence also help.