SQLContext.sql() caused deadlock in multi-thread env

Doing (the same query) in more than one threads with SQLConext.sql may lead to deadlock. Here is a way to reproduce it (since this is multi-thread issue, the reproduction may or may not be so easy).

1. Register a relatively big table.

2. Create two different classes and in the classes, do the same query in a method and put the results in a set and print out the set size.

3. Create two threads to use an object from each class in the run method. Start the threads. For my tests, it can have a deadlock just in a few runs.