MERGE Tables works ?

Hi Everybody! Im facing a problem. We register telecomunications calls. As the volume is so big, we decided to store the data in different tables. The criteria we used for splitting the info was to create a table per each Switch and Day. So we have Sw1_20160601, Sw2_20160602...Sw3_20160620..etc etc Each table has aprox 15MM rows. In order to create some reports we have created a *MERGE TABLE *and added to this table the 90 tables (3 switches x 30 days). BUT, now Im trying to run a very simple query, like SELECT cdr_date FROM xxxx LIMIT 3; To get at least 3 rows (just to see that this schema works). It took 21 mins to give me those 3 rows.... Am I doing something wrong? Rgds Ariel