Thread: Which update action quicker?

<div class="moz-text-flowed" lang="x-western" style="font-size: 18px;"><small>Hello list, <br /><br /> For a big table
withmore 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 />
Orother quicker way for update action? <br /><br /> Thank you <br /> Emi </small><br /></div> 

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

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
mostof 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