On Mon, Jan 4, 2021 at 5:44 PM Luc Vlaming <l...@swarm64.com> wrote: > On 04-01-2021 12:16, Hou, Zhijie wrote: > >> ================ > >> 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.
Right. We wanted to ignore parallel tuple cost for only the upper Gather path. > I was wondering actually if we need the state machine. Reason is that as > AFAICS the code could be placed in create_gather_path, where you can > also check if it is a top gather node, whether the dest receiver is the > right type, etc? To me that seems like a nicer solution as its makes > that all logic that decides whether or not a parallel CTAS is valid is > in a single place instead of distributed over various places. IMO, we can't determine the fact that we are going to generate the top Gather path in create_gather_path. To decide on whether or not the top Gather path generation, I think it's not only required to check the root->query_level == 1 but we also need to rely on from where generate_useful_gather_paths gets called. For instance, for query_level 1, generate_useful_gather_paths gets called from 2 places in apply_scanjoin_target_to_paths. Likewise, create_gather_path also gets called from many places. IMO, the current way i.e. setting flag it in apply_scanjoin_target_to_paths and ignoring based on that in cost_gather seems safe. I may be wrong. Thoughts? With Regards, Bharath Rupireddy. EnterpriseDB: http://www.enterprisedb.com
