When running MySQL/MariaDB with charset utf8mb4, a character is 4 bytes. Hence a key created on a VARCHAR(255) takes 255 characters * 4 bytes/characters = 1020 bytes. However Innodb defaults to a maximum of 767 bytes for an index.
The extension fails to install on Debian Stretch which uses utf8mb4 as the default charset:
Query: CREATE TABLE IF NOT EXISTS `ajaxpoll_vote` ( `poll_id` varchar(32) NOT NULL default '', `poll_user` varchar(255) NOT NULL default '', `poll_ip` varchar(255) default NULL, `poll_answer` int(3) default NULL, `poll_date` datetime default NULL, PRIMARY KEY (`poll_id`,`poll_user`) ) ENGINE=InnoDB Function: Wikimedia\Rdbms\Database::sourceFile( /workspace/src/extensions/AJAXPoll/includes/../sql/create-table--ajaxpoll_vote.sql ) Error: 1071 Specified key was too long; max key length is 767 bytes (localhost:/tmp/quibble-mysql-hkzoeqmk/socket)