In response to Leo Mannhart : > On Mon, 2009-06-29 at 17:08 +1000, Robert Edwards wrote: > > A. Kretschmer wrote: > > > In response to Robert Edwards : > > >> Can anyone suggest a way that I can impose uniqueness on a and b when > > >> c is NULL? > > > > > > Sure, use a functional index: > > > > > > test=# create table bobtest (a int, b int, c int); > > > CREATE TABLE > > > test=*# create unique index idx_bobtest on > > > bobtest(a,b,coalesce(c::text,'NULL')); > > > CREATE INDEX > > > test=*# insert into bobtest (a, b) values (1, 4); > > > INSERT 0 1 > > > test=*# insert into bobtest (a, b, c) values (1, 4, NULL); > > > ERROR: duplicate key value violates unique constraint "idx_bobtest" > > > test=!# > > > > Why changing the datatype of coulumn "c"? Will this not probably disable > the use of the index? Why not only:
Maybe, depends on the query. > > lem=# create table bobtest (a int, b int, c int); > CREATE TABLE > lem=# create unique index idc_bobtest on bobtest (a, b, coalesce(c, 1)); bad idea if 1 is a valid value for this column. Andreas -- Andreas Kretschmer Kontakt: Heynitz: 035242/47150, D1: 0160/7141639 (mehr: -> Header) GnuPG-ID: 0x3FFF606C, privat 0x7F4584DA http://wwwkeys.de.pgp.net -- Sent via pgsql-sql mailing list (pgsql-sql@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-sql
