On Fri, Dec 12, 2014 at 9:57 AM, Daniel Begin <jfd...@hotmail.com> wrote:
> Seems promising but could you provide me a reference to PostgreSQL > documentation regarding this "a%8=*" feature? > Best > % is the modulus operator. Assuming "a" is an integer (I don't remember), then doing 8 selects of "a modulus 8" = for each of the possible results (0..7) will each select about 1/8 of the entire table (I would guess) and the end result put together, they will end up selecting all of the original table. I don't know, myself, why this would be faster. But I'm not any kind of a PostgreSQL expert either. > > Daniel > > -----Original Message----- > From: pgsql-general-ow...@postgresql.org > [mailto:pgsql-general-ow...@postgresql.org] On Behalf Of Marc Mamin > Sent: December-12-14 06:41 > To: Daniel Begin; 'Tom Lane'; 'Scott Marlowe' > Cc: 'Andy Colson'; pgsql-general@postgresql.org > Subject: Re: [GENERAL] Removing duplicate records from a bulk upload > (rationale behind selecting a method) > > > >Thank Tom, > >I understand that the rationale behind choosing to create a new table > >from distinct records is that, since both approaches need full table > >scans, selecting distinct records is faster (and seems more straight > >forward) than finding/deleting duplicates; > > Hi, > on a large table you may get it faster while using more than one thread. > e.g.: > > select a,b,c into newtable from oldtable where a%8 =0 group by a,b,c; > select > a,b,c into newtable from oldtable where a%8 =1 group by a,b,c; ... > select a,b,c into newtable from oldtable where a%8 =7 group by a,b,c; > > This will/should use a shared full table scan on oldtable. > > HTH > > Marc Mamin > > > > > >Best regards, > >Daniel > > > >-----Original Message----- > >From: pgsql-general-ow...@postgresql.org > >[mailto:pgsql-general-ow...@postgresql.org] On Behalf Of Tom Lane > >Sent: December-08-14 21:52 > >To: Scott Marlowe > >Cc: Andy Colson; Daniel Begin; pgsql-general@postgresql.org > >Subject: Re: [GENERAL] Removing duplicate records from a bulk upload > >(rationale behind selecting a method) > > > >Scott Marlowe <scott.marl...@gmail.com> writes: > >> If you're de-duping a whole table, no need to create indexes, as it's > >> gonna have to hit every row anyway. Fastest way I've found has been: > > > >> select a,b,c into newtable from oldtable group by a,b,c; > > > >> On pass, done. > > > >> If you want to use less than the whole row, you can use select > >> distinct on (col1, col2) * into newtable from oldtable; > > > >Also, the DISTINCT ON method can be refined to control which of a set > >of duplicate keys is retained, if you can identify additional columns > >that constitute a preference order for retaining/discarding dupes. See > >the "latest weather reports" example in the SELECT reference page. > > > >In any case, it's advisable to crank up work_mem while performing this > >operation. > > > > regards, tom lane > > > > > >-- > >Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To > >make changes to your subscription: > >http://www.postgresql.org/mailpref/pgsql-general > > > > > > > >-- > >Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To > >make changes to your subscription: > >http://www.postgresql.org/mailpref/pgsql-general > > > > -- > Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make > changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-general > > > > -- > Sent via pgsql-general mailing list (pgsql-general@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-general > -- While a transcendent vocabulary is laudable, one must be eternally careful so that the calculated objective of communication does not become ensconced in obscurity. In other words, eschew obfuscation. Maranatha! <>< John McKown
