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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