binary data in database, blob vs compressed base64
It seems that I have to answer my own question. Most of the time, storing compressed base64 into database is not a good idea. It is more complex than storing blob. And most of the time binary is smaller than base64 string.
I only find one case that compressed base64 is useful: you can't alter the table schema, and there are only text columns, thus you have to store binary data into that table. The only possible way is to convert binary to base64 string.