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-owner@xxxxxxxxxxxxxx
[mailto:pgsql-general-owner@xxxxxxxxxxxxxx] On Behalf Of Marc Mamin
Sent: December-12-14 06:41
To: Daniel Begin; 'Tom Lane'; 'Scott Marlowe'
Cc: 'Andy Colson'; pgsql-general@xxxxxxxxxxxxxx
Subject: Re: 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-owner@xxxxxxxxxxxxxx
>[mailto:pgsql-general-owner@xxxxxxxxxxxxxx] On Behalf Of Tom Lane
>Sent: December-08-14 21:52
>To: Scott Marlowe
>Cc: Andy Colson; Daniel Begin; pgsql-general@xxxxxxxxxxxxxx
>Subject: Re: Removing duplicate records from a bulk upload
>(rationale behind selecting a method)
>
>Scott Marlowe <scott.marlowe@xxxxxxxxx> 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
>
>
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