limit size better than 255.txt

// 2020, 3/08, by Queenie Chen
// DBA, Data Type

when a complex SELECT needs to create temporary table 
it then use the memory engine, putting data in RAM.
VARCHAR then turn into CHAR in the process,
it makes VARCHAR(255) SET utf8mb4 take 1020 bytes.
leads to spill to disk, lower than RAM.

in certain situation, InnoDB will look at the potential size 
of the column may be too big, then it aborts to create a table.