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Re: Reverse-sort indexes and NULLS FIRST/LAST sorting

From: Heikki Linnakangas <heikki(at)enterprisedb(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgreSQL(dot)org
Subject: Re: Reverse-sort indexes and NULLS FIRST/LAST sorting
Date: 2007-01-02 10:35:34
Message-ID: 459A3576.3040609@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-hackers
I'd like to see this implemented with more general collation support in 
mind.

In general, each index column can be ordered by one collation. A query 
matching the index collation can use the index directly, a query asking 
for another collation needs to convert. The trivial way to convert from 
one collation to another is to sort according to the new collation. For 
other conversions, there can be cheaper ways. I'd like to have explicit 
support for collations and converting between them, perhaps by 
introducing a new ConvertCollation node type. The default implementation 
would be to sort, but for example to convert from NULLS FIRST to NULLS 
LAST could be implemented by buffering the null columns to temporary 
storage and returning all non-null rows first. There's similar tricks 
that could be used to convert between many Western European collations, 
without resorting to full sort.

The NULLS FIRST/LAST support, as well as ascending and descending 
orderings would be special cases of the general collation and collation 
conversion machinery.

I don't know how much work that would be to implement, compared to what 
you're proposing. It would require adding an extra collation concept to 
all the places that care about sort ordering, but you're proposing to 
add nulls-first-or-last flag to all those places anyway.

Tom Lane wrote:
> The SQL2003 spec adds optional "NULLS FIRST" and "NULLS LAST" modifiers
> for ORDER BY clauses.  Teodor proposed an implementation here:
> http://archives.postgresql.org/pgsql-patches/2006-12/msg00019.php
> which I didn't care for at all:
> http://archives.postgresql.org/pgsql-hackers/2006-12/msg00133.php
> 
> Doing this right is going to require introducing the nulls-first-or-last
> concept into all the system's handling of sort ordering.  Messy as that
> sounds, I think it will end up logically cleaner than what we have now,
> because it will let us fix some issues involving descending-order index
> opclasses and backwards-sort mergejoins.  Neither of those can really work
> correctly right now, the reason being exactly that we lack a framework for
> dealing with variable sort positioning of NULLs.
> 
> I'm hoping to fix this as a consequence of the work I'm doing with
> operator families for 8.3.  What I'd like to come out of it is support
> for both NULLS FIRST/LAST and reverse-sort index columns.  Reverse-sort
> indexes are already in the TODO list, the application being to create
> an index whose sort order matches a query like "ORDER BY x ASC, y DESC".
> There are some user-visible decisions to be made first, so this message
> is to start a discussion about what we want.
> 
> One way we could handle this is to say that reverse-sort indexes are
> implemented by adding explicit catalog entries for reverse-sort opclasses,
> with no additions to the underlying btree index mechanisms.  So you
> might make an index using a command like
> 
> 	CREATE INDEX fooi ON foo (x, y reverse_int4_ops);
> 
> btree indexes would always sort by the given opclass with NULLS LAST.
> So the two possible orderings that could be derived from this index
> (using forward or backward scan respectively) are
> 
> 	ORDER BY x ASC NULLS LAST, y DESC NULLS LAST
> 	ORDER BY x DESC NULLS FIRST, y ASC NULLS FIRST
> 
> The other way that seems like it could win acceptance is to make REVERSE
> an explicit optional property of an index column; and if we do that we
> might as well allow NULLS FIRST/LAST to be an optional property as well.
> Then you could say something like
> 
> 	CREATE INDEX fooi ON foo (x, y REVERSE NULLS FIRST);
> 
> (Or maybe use DESC instead of REVERSE as the keyword --- not very
> important at this point.)  This index would support scans with these
> two sort orderings:
> 
> 	ORDER BY x ASC NULLS LAST, y DESC NULLS FIRST
> 	ORDER BY x DESC NULLS FIRST, y ASC NULLS LAST
> 
> This second way is more flexible in that it allows indexes to support
> mixed null orderings; another attraction is that we'd not have to create
> explicit reverse-sort opclasses, which would be a tedious bit of extra
> work for every datatype.  On the other hand, adding these extra flag bits
> to indexes seems like a horribly ugly wart, mainly because they're
> irrelevant to anything except a btree index.  (Or at least irrelevant to
> anything that doesn't support ordered scans, but in practice that's only
> btree for the foreseeable future.)  Also, having to account for these
> options in the btree code would make it more complex and perhaps slower.
> 
> Comments?  I've got mixed feelings about which way to jump myself.


-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

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