Does left join preserve order?

User 2824 | 12/14/2015, 3:24:24 PM


Can I make any assumption regarding the order of left join output? (on a few small tests it seems like the order is preserved, the question is can I count on it?) Thanks, Ella


User 954 | 12/14/2015, 7:42:43 PM


join operation in general is NOT preserving the order.

Basically we chunk input data and do join operation on corresponding chunks and finally merge the partial outputs. Input data ordering may violated depending on how we merge the partial outputs.

Note that in small dataset we only have ONE chunk, so no merge required and ordering is preserved.

Regards, Emad

User 2824 | 12/15/2015, 8:59:45 AM

ok, thanks!