Message-ID: <5422CBB4.3000301@vmware.com>
Date: Wed, 24 Sep 2014 16:48:36 +0300
From: Heikki Linnakangas <hlinnakangas@vmware.com>
Organization: VMware
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To: <emilu@encs.concordia.ca>, "pgsql-performance@postgresql.org"
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Subject: Re: Which update action quicker?
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On 09/23/2014 11:37 PM, Emi Lu wrote:
> Hello list,
>
> For a big table with more than 1,000,000 records, may I know which update is
> quicker please?
>
> (1) update t1
>         set c1 = a.c1
>         from a
>         where pk and
>                    t1.c1       <> a.c1;
>    ......
>         update t1
>         set c_N = a.c_N
>         from a
>         where pk and
>                    t1.c_N       <> a.c_N;
>
>
> (2)  update t1
>         set c1 = a.c1 ,
>               c2  = a.c2,
>               ...
>               c_N = a.c_N
>        from a
>        where pk AND
>                  (  t1.c1 <> a.c1 OR t1.c2 <> a.c2..... t1.c_N <> a.c_N)

Probably (2). <> is not indexable, so each update will have to perform a 
sequential scan of the table. With (2), you only need to scan it once, 
with (1) you have to scan it N times. Also, method (1) will update the 
same row multiple times, if it needs to have more than one column updated.

> Or other quicker way for update action?

If a large percentage of the table needs to be updated, it can be faster 
to create a new table, insert all the rows with the right values, drop 
the old table and rename the new one in its place. All in one transaction.

- Heikki


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