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 `video` (
`video_name` varchar(255) NOT NULL PRIMARY KEY default '',
`video_url` varchar(255) NOT NULL default '',
`video_type` varchar(255) default 'unknown',
`video_user_id` int(11) NOT NULL default '0',
`video_user_name` varchar(255) NOT NULL default '',
`video_timestamp` varchar(14) NOT NULL default ''
)ENGINE=InnoDB
Function: Wikimedia\Rdbms\Database::sourceFile( /workspace/src/extensions/Video/includes/../sql/video.sql )
Error: 1071 Specified key was too long; max key length is 767 bytes (localhost:/tmp/quibble-mysql-2zh95mun/socket)
```