BINARY is one of the datatypes you can use with dynamic columns. If you try saving binary string and then retrieve it with COLUMN_JSON() then you get an invalid JSON document back. All the JSON decoders I tested reject such documents.

It doesn't make sense to me that COLUMN_JSON() would return a document that's invalid according to the JSON spec and can't be decoded. But I don't know what it should do. It seems that BINARY in dynamic columns means that COLUMN_JSON() is in an impossible position.

[On a related note, if you save a valid utf8 string that includes control characters (e.g. "I went down to the river,\nI set down on the bank.") then COLUMN_JSON() fails to escape them. While similar, this I think is a bug.]

Tom