Optimizing String and Character Fields
Comparing String Columns
When values from different columns are compared, the comparison runs more quickly when the columns are of the same character set and collation. If they are different, the strings need to be converted while the query runs. So, where possible, declare string columns using the same character set and collation when you may need to compare them.
VARCHAR vs BLOB
ORDER BY and GROUP BY clauses can generate temporary tables in memory (see MEMORY Storage Engine) if the original table doesn't contain any BLOB fields. If a column is less than 8KB, you can make use of a Binary VARCHAR rather than a BLOB.