Le vendredi 6 juillet 2012 15:41:01, Bruce Momjian a écrit : > On Fri, Jun 29, 2012 at 08:10:03AM +0200, Csaba Nagy wrote: > > Hi all, > > > > > On Thu, Jun 28, 2012 at 5:16 AM, David E. Wheeler > > > <da...@justatheory.com> wrote: I don't see the virtue of this in this > > > case. Since the index is not unique, why not just put the index on > > > (a,b,c,d) and be done with it? Is there some advantage to be had in > > > inventing a way to store c and d in the index without having them > > > usable for indexing? > > > > Why not restrict it to UNIQUE indexes ? > > > > For not unique indexes it has no advantages (it could be in fact indexed > > on all columns anyway as an implementation detail). > > > > For the unique case the problem of many identical entries mentioned by > > Tom is not relevant, so the additional data will only bloat the index > > but not otherwise affect the index performance. > > > > Would this get close enough to index-covered table ? _That_ would be > > interesting - I have a really big table (table/index size: 370G/320G, > > ~8*10^9 rows) which is basically using double space because it's primary > > key is covering all columns of the table. > > I was wondering if there was some way to specify an expression index > that did a unique index check on some columns but included more columns > not part of the unique index.
I haven't tryed it, but I suppose that Exclusion Constraint should allow that. -- Cédric Villemain +33 (0)6 20 30 22 52 http://2ndQuadrant.fr/ PostgreSQL: Support 24x7 - Développement, Expertise et Formation
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