Message-ID: <48B6FCF8.3090807@encs.concordia.ca>
Date: Thu, 28 Aug 2008 15:31:04 -0400
From: Emi Lu <emilu@encs.concordia.ca>
Reply-To: emilu@encs.concordia.ca
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To: pgsql-performance@postgresql.org
Subject: update - which way quicker?
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Good morning,

Tried to compare Table1 based on Table2

. update table1.col = false
   if table1.pk_cols not in table2.pk_cols


For the following two ways, (2) always performs better than (1) right,
and I need your inputs.
========================================================================
(1) update table1
     set col = false
     where table1.pk_co1 || table1.pk_col2.... || table1.pk_colN

     NOT IN

     (select pk_co1 || pk_col2.... || pk_colN
      from table2
      )

(2) ResultSet(rs) =
     select pk_col1||pk_col2... || pk_colN
     from table1
     left join table2 using (pk_col1..., pk_colN)
     where table2.pk_col1 is null

     Then for each rs record, do:
     update table1
     set col = false
     where col1||... colN in rs.value

Thanks a lot!