On Thu, Apr 16, 2020 at 8:22 AM Richard Guo <guofenglinux@gmail.com> wrote:
>
>
> On Thu, Apr 16, 2020 at 6:35 PM Richard Guo <guofenglinux@gmail.com> wrote:
>>
>>
>> On Wed, Apr 15, 2020 at 10:47 PM Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:
>>>
>>>
>>> Well, yeah. The problem is the Unique simply compares the columns in the
>>> order it sees them, and it does not match the column order desired by
>>> incremental sort. But we don't push down this information at all :-(
>>
>>
>> This is a nice optimization better to have. Since the 'Sort and Unique'
>> would unique-ify the result of a UNION by sorting on all columns, why
>> not we adjust the sort order trying to match parse->sortClause so that
>> we can avoid the final sort node?
>>
>> Doing that we can transform plan from:
>>
>> # explain (costs off) select * from foo union select * from foo order by 1,3;
>> QUERY PLAN
>> -----------------------------------------------
>> Incremental Sort
>> Sort Key: foo.a, foo.c
>> Presorted Key: foo.a
>> -> Unique
>> -> Sort
>> Sort Key: foo.a, foo.b, foo.c
>> -> Append
>> -> Seq Scan on foo
>> -> Seq Scan on foo foo_1
>> (9 rows)
>>
>> To:
>>
>> # explain (costs off) select * from foo union select * from foo order by 1,3;
>> QUERY PLAN
>> -----------------------------------------
>> Unique
>> -> Sort
>> Sort Key: foo.a, foo.c, foo.b
>> -> Append
>> -> Seq Scan on foo
>> -> Seq Scan on foo foo_1
>> (6 rows)
>>
>
> Attached is what I'm thinking about this optimization. Does it make any
> sense?
Shouldn't this go one either a new thread or on the thread for the
patch Tomas was referencing (by Teodor I believe)?
Or are you saying you believe this patch guarantees we never see this
problem in incremental sort costing?
James
