MyISAM indexes are limited to 1000 bytes per index. Recently, Linux distros (e.g. Debian 9) started to default MariaDB installations to utf8mb4, which can use up to 4 bytes per character. Hence, two varchar columns indexed in a single key have a total maximum length of 250. Instead, we use some lower, round numbers. Change-Id: I4b53bc217912bc7070102a0af4938763e61b041d