|From:||Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>|
|To:||Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>|
|Cc:||Daniel Gustafsson <daniel(at)yesql(dot)se>, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>, Alexander Kuzmenkov <a(dot)kuzmenkov(at)postgrespro(dot)ru>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>|
|Subject:||Re: Avoid extra Sort nodes between WindowAggs when sorting can be reused|
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Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk> writes:
> "Tom" == Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
> Tom> * I'm almost thinking that changing to list_union is a bad idea,
> A fair point. Though it looks like list_union is used in only about 3
> distinct places, and two of those are list_union(NIL, blah) to simply
> remove dups from a single list. The third place is the cartesian-product
> expansion of grouping sets, which uses list_union_int to remove
> duplicates - changing the order there will give slightly user-surprising
> but not actually incorrect results.
> Presumably list_concat_unique should be considered to guarantee that it
> preserves the relative order of the two lists and of the non-duplicate
> items in the second list?
I'm thinking that whichever coding we use, the patch should include
comment additions in list.c documenting that some callers have assumptions
thus-and-so about list order preservation. Then at least anybody who
got the idea to try to improve performance of those functions would be on
notice about the risks.
I see that list_union is currently documented like this:
* Generate the union of two lists. This is calculated by copying
* list1 via list_copy(), then adding to it all the members of list2
* that aren't already in list1.
so as long as it stays like that, it's not unreasonable to use it in
this patch. I just want the potential landmine to be obvious at that
regards, tom lane
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