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List:       pgsql-performance
Subject:    Re: [PERFORM] Which update action quicker?
From:       Emi Lu <emilu () encs ! concordia ! ca>
Date:       2014-09-24 14:13:05
Message-ID: 5422D171.5060801 () encs ! concordia ! ca
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Hello,

> For a big table with more than 10 Million 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, c2...c_N) <> (a.c1, c2... 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.

The situation is:
(t1.c1, c2, ... c_N) <> (a.c1, c2...c_N) won't return too many diff records. So, the \
calculation will only be query most of the case.

But if truncate/delete and copy will cause definitely write all more than 10 million \
data.

If for situation like this, will it still be quicker to delete/insert quicker?
Thank you
Emi



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