Hi > ================ > wrt v18-0002....patch: > > It looks like this introduces a state machine that goes like: > - starts at CTAS_PARALLEL_INS_UNDEF > - possibly moves to CTAS_PARALLEL_INS_SELECT > - CTAS_PARALLEL_INS_TUP_COST_CAN_IGN can be added > - if both were added at some stage, we can go to > CTAS_PARALLEL_INS_TUP_COST_IGNORED and ignore the costs > > what i'm wondering is why you opted to put logic around > generate_useful_gather_paths and in cost_gather when to me it seems more > logical to put it in create_gather_path? i'm probably missing something > there?
IMO, The reason is we want to make sure we only ignore the cost when Gather is the top node. And it seems the generate_useful_gather_paths called in apply_scanjoin_target_to_paths is the right place which can only create top node Gather. So we change the flag in apply_scanjoin_target_to_paths around generate_useful_gather_paths to identify the top node. Best regards, houzj
