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* Which update action quicker?
@ 2014-09-23 20:37 Emi Lu <emilu@encs.concordia.ca>
0 siblings, 1 reply; 3+ messages in thread
From: Emi Lu @ 2014-09-23 20:37 UTC (permalink / raw)
To: pgsql-performance
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<div class="moz-text-flowed" style="font-size: 18px;"
lang="x-western"><small>Hello list,
<br>
<br>
For a big table with more than 1,000,000 records, may I know
which update is quicker please?
<br>
<br>
(1) update t1
<br>
set c1 = a.c1
<br>
from a
<br>
where pk and
<br>
t1.c1 <> a.c1;
<br>
......
<br>
update t1
<br>
set c_N = a.c_N
<br>
from a
<br>
where pk and
<br>
t1.c_N <> a.c_N;
<br>
<br>
<br>
(2) update t1
<br>
set c1 = a.c1 ,
<br>
c2 = a.c2,
<br>
...
<br>
c_N = a.c_N
<br>
from a
<br>
where pk AND
<br>
( t1.c1 <> a.c1 OR t1.c2 <>
a.c2..... t1.c_N <> a.c_N)
<br>
<br>
<br>
Or other quicker way for update action?
<br>
<br>
Thank you
<br>
Emi
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^ permalink raw reply [nested|flat] 3+ messages in thread
* Re: Which update action quicker?
@ 2014-09-24 13:48 Heikki Linnakangas <hlinnakangas@vmware.com>
parent: Emi Lu <emilu@encs.concordia.ca>
0 siblings, 1 reply; 3+ messages in thread
From: Heikki Linnakangas @ 2014-09-24 13:48 UTC (permalink / raw)
To: emilu@encs.concordia.ca; pgsql-performance
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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^ permalink raw reply [nested|flat] 3+ messages in thread
* Re: Which update action quicker?
@ 2014-09-24 14:13 Emi Lu <emilu@encs.concordia.ca>
parent: Heikki Linnakangas <hlinnakangas@vmware.com>
0 siblings, 0 replies; 3+ messages in thread
From: Emi Lu @ 2014-09-24 14:13 UTC (permalink / raw)
To: Heikki Linnakangas <hlinnakangas@vmware.com>; pgsql-performance
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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2014-09-23 20:37 Which update action quicker? Emi Lu <emilu@encs.concordia.ca>
2014-09-24 13:48 ` Heikki Linnakangas <hlinnakangas@vmware.com>
2014-09-24 14:13 ` Emi Lu <emilu@encs.concordia.ca>
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