191 is a magic number for VARCHAR columns in MySQL when innodb_large_prefix is disabled, which was the default for MySQL <= 5.7.6. Here is a short list of MySQ

191 is a magic number for VARCHAR columns in MySQL when innodb_large_prefix is disabled, which was the default for MySQL <= 5.7.6. Here is a short list of MySQL magic numbers for column types. Space efficiency and performance: - VARBINARY(255) / VARCHAR(255) with single-byte character sets: 255 is the biggest size that requires only 1 byte of storage overhead. - VARCHAR(63) with utf8mb4: 63 is the biggest size that requires only 1 byte of storage overhead because 64 × 4 > 255. - VARBINARY(512) / VARCHAR(512): the largest columns of their types that are compatible with in-memory temporary tables when UNION or UNION ALL is used. Larger columns will use on-disk tables instead. Limitations of 3072-byte index keys (innodb_large_prefix enabled; default for MySQL >= 5.7.7): - VARBINARY(3072) / VARCHAR(3072) with single-byte character sets: 3072 is the biggest size that can be fully indexed. - VARCHAR(768) with utf8mb4: 768 is the biggest size that can be fully indexed because 769 × 4 > 3072. Limitations of 767-byte index keys (innodb_large_prefix disabled; default for MySQL <= 5.7.6): - VARBINARY(767) / VARCHAR(767) with single-byte character sets: 767 is the biggest size that can be fully indexed without enabling innodb_large_prefix. - VARCHAR(191) with utf8mb4: 191 is the biggest size that can be fully indexed without enabling innodb_large_prefix because 192 × 4 > 767. This is what the OP is about. VARCHAR(768), mentioned above, should be more relevant nowadays. Different magic numbers come in if you are using VARCHAR with utf8 (the MySQL character set, not the real UTF-8). I'll let you work out the details, but the magic numbers are 85, 255, 1024.