Re: Exclusion Constraints on Arrays?

On Sun, May 13, 2012 at 12:12 AM, David E. Wheeler
<david@justatheory.com> wrote:
> I need a constraint that ensures that a text[] column has only unique values -- that is, that there is no overlap of
valuesbetween rows. I thought this was a made-to-order for an exclusion constraint. So I tried it: 
>
> david=# create table tags (names text[] primary key, exclude using gist (names WITH &&));NOTICE:  CREATE TABLE /
PRIMARYKEY will create implicit index "tags_pkey" for table "tags" 
> ERROR:  data type text[] has no default operator class for access method "gist"
> HINT:  You must specify an operator class for the index or define a default operator class for the data type.
>
> Rats! It looks like there is only a gin operator family for arrays, not gist, and exclusion constraints support only
gistindexes, and I couldn't find an operator class, either. Have I missed something, in my (likely) ignorance? Or are
thereperhaps some types to consider modifying to support exclusion constraints? 

Hmm, it looks like GIN can't support exclusive constraints because
amgettuple support is required, and unfortunately that got remove for
GIN in this commit:

commit ff301d6e690bb5581502ea3d8591a1600fd87acc
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date:   Tue Mar 24 20:17:18 2009 +0000
   Implement "fastupdate" support for GIN indexes, in which we try to
accumulate   multiple index entries in a holding area before adding them to the
main index   structure.  This helps because bulk insert is (usually) significantly faster   than retail insert for GIN.
   This patch also removes GIN support for amgettuple-style index scans.  The   API defined for amgettuple is difficult
tosupport with fastupdate, and   the previously committed partial-match feature didn't really work with   it either.
Wemight eventually figure a way to put back amgettuple   support, but it won't happen for 8.4. 

Code comments explain the problem in more detail:
       /*        * First, scan the pending list and collect any matching
entries into the        * bitmap.      After we scan a pending item, some other
backend could post it        * into the main index, and so we might visit it a second time
during the        * main scan.  This is okay because we'll just re-set the same
bit in the        * bitmap.      (The possibility of duplicate visits is a
major reason why GIN        * can't support the amgettuple API, however.) Note that it would not do        * to scan
themain index before the pending list, since concurrent        * cleanup could then make us miss entries entirely.
 */       scanPendingInsert(scan, tbm, &ntids); 

It seems like maybe we could work around this by remembering the
contents of the pending list throughout the scan.  Every time we hit a
TID while scanning the main index, we check whether we already
returned it from the pending list; if so, we skip it, but if not, we
return it.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company