for example: group_concat(distinct a,b order by a seporator '|') dist: true groupExpr: a,b orderByExpr: a separator: "|".

+------+------+------+ | a | b | c | +------+------+------+ | 1 | 2 | 3 | | 4 | 5 | 6 | +------+------+------+ select group_concat(a,b,c separator "|") from t; res[0] = "123|456" inserts = "encode(1,2,3)|encode(4,5,6) we need inserts to store the source keys, so we can use then where merge.