When performing joins, the PROC SQL optimiser determines which of the available algorithms are used to perform the join. These include:

It can be helpful from an optimisation perspective to know which join type was used by the optimiser. This is where the (undocumented) _method option is helpful:

PROC SQL _METHOD;
QUIT;

Kirk.jpg

Full credit to Kirk Laflerfor this tip. For more information see his paper here: http://support.sas.com/resources/papers/proceedings13/200-2013.pdf