Which DB type are you using? Let's assuming you are using Mysql, select records from the two table and do inner join on tMap base on the primary key, the job looks like:
tMysqlInput_1(select records from table a)-----main---tMap---main---tLogRow
tMysqlInput_2(select records from table b)
Maybe there exists space in some value, go to check it and trim the value before doing the join on tMap.