| From: | "Hou, Zhijie" <houzj(dot)fnst(at)cn(dot)fujitsu(dot)com> |
|---|---|
| To: | Luc Vlaming <luc(at)swarm64(dot)com> |
| Cc: | Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>, Zhihong Yu <zyu(at)yugabyte(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Dilip Kumar <dilipbalaut(at)gmail(dot)com>, vignesh C <vignesh21(at)gmail(dot)com> |
| Subject: | RE: Parallel Inserts in CREATE TABLE AS |
| Date: | 2021-01-04 11:16:02 |
| Message-ID: | b86351d00dfd4e7d89a3877a9fd3daf1@G08CNEXMBPEKD05.g08.fujitsu.local |
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| Lists: | pgsql-hackers |
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
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