RE: Parallel Inserts in CREATE TABLE AS

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
onlycreate 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