improve mysql query in large tables
To enhance the performance:


First of all, add an index on calldate. It will avoid you scanning the whole table.


In the query:


Not sure what is unique in the query.
LIMIT 39780720,39790720 means, "Retrieve 39790720 rows starting from the row number 39790720 in the result". I think you want to revise it. 


About the table structure:


It is recommended to have primary key