Re: which Update quicker

On Tue, Sep 23, 2014 at 8:35 PM, Emi Lu <emilu(at)encs(dot)concordia(dot)ca> 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)

Definitely the second, and it produces less bloat too.

> Or other quicker way for update action?

You may express the comparison as (t1.c1, t1.c2, ... t1.cN) <> (t2.c1,
t2.c2, ... t2.cN)
It's not going to be faster but maybe it's more readable.

-- Daniele