Re: Questionable coding in orderedsetaggs.c - Mailing list pgsql-hackers

From Atri Sharma
Subject Re: Questionable coding in orderedsetaggs.c
Date
Msg-id CAOeZVifJdS8=8G69h=EfrSiZWawGvjXfm3UxyztZ5OX+UFwT-A@mail.gmail.com
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In response to Re: Questionable coding in orderedsetaggs.c  (Tom Lane <tgl@sss.pgh.pa.us>)
List pgsql-hackers


On Sunday, January 26, 2014, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Jeremy Harris <jgh@wizmail.org> writes:
> In ordered_set_startup() sorts are initialised in non-randomAccess mode
> (tuplesort_begin_heap() and ~datum(), last argument).

> The use of tuplesort_skip_tuples() feels very like a random access to
> me.  I think it doesn't fail because the only use (and implementation)
> is to skip forwards; if backwards were tried (as the interface permits)
> external sorts would fail because multiple tapes are present for
> FINALMERGE.

Well, we certainly don't want to incur the overhead of randomAccess mode
when we're not actually going to use it, so I'd resist changing the code
in ordered_set_startup().

It's true that if tuplesort_skip_tuples() supported backwards skip, it
would need to insist that randomAccess mode be enabled *when a backwards
skip is used*.  But such a feature is purely hypothetical ATM.

                         

+1

In ordered set functions, we normally don't skip backwards and skip tuples while sorting in,for e.g. Hypothetical set functions in only a forward manner. 


--
Regards,
 
Atri
l'apprenant

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