Yes,
The idea was to commit after each deletion. If required wait some time or
put it into a job to spread the load from a one-time punch to a smooth
distribution.
Martin
^∆x
Am 25.08.2016 09:09 schrieb "Jonathan Lewis" <jonathan@xxxxxxxxxxxxxxxxxx>:
Are you going to commit between each delete ?
If not there's the read-consistency problem - maybe addressable by
modifying the code and using AS OF SCN; if so then that's what
dbms_parallel_execute can do for you.
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
@jloracle
------------------------------
*From:* Martin Berger [martin.a.berger@xxxxxxxxx]
*Sent:* 25 August 2016 08:04
*To:* Andrew Kerber
*Cc:* Jonathan Lewis; JDunn@xxxxxxxxx; Chris Taylor;
oracle-l@xxxxxxxxxxxxx
*Subject:* Re: Deletion from large table
In case the single transaction is "to big" for whatever reason, and it
needs to spit into chunks (WHERE ROWNUM < 1000000) has anyone experience if
it makes sense to "cluster" by blocks?
delete from big_table where big_table.rowid in
(select rowid from
(select rowid, rownum rn
from big_tablenot where exists (select 1 from small_table s where
s.id = b.id)
order by rowid)
where rn < 1000000)
The idea is to modify few table blocks per transaction, and as the
available space is clustered as well, new inserts will (hopefully) be in
same or adjacent blocks.
I don't recommend doing so, it's more a question about findings,
side-effects, ...
Martin
2016-08-23 16:57 GMT+02:00 Andrew Kerber <andrew.kerber@xxxxxxxxx>:
I have generally had good performance with syntax like this:
delete from big_table where id in (select big_table.id from small_table,
big_table where small_table.id=big_table.id (+) and small_table.id is
null)
On Tue, Aug 23, 2016 at 9:38 AM, Jonathan Lewis <
jonathan@xxxxxxxxxxxxxxxxxx> wrote:
Best access path does vary with circumstances.
If you're expecting lots of inserts while doing the deletes you may find
that as the delete progresses the rate slows down and the volume of undo
applied for read-consistency climbs.
If you see that as a problem it may be that finding an index that lets
you walk the big table in reverse order of data arrival may (slightly
counter-intuitively) improve performance.
Under any circumstances deleting by tablescan and deleting by index
range scan behave differently with respect to index maintenance (this note
on big updates also applies to big deletes:
http://jonathanlewis.wordpress.com/2006/11/22/tuning-updates/
<https://jonathanlewis.wordpress.com/2006/11/22/tuning-updates/> ).
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
@jloracle
------------------------------
*From:* oracle-l-bounce@xxxxxxxxxxxxx [oracle-l-bounce@xxxxxxxxxxxxx]
on behalf of John Dunn [JDunn@xxxxxxxxx]
*Sent:* 23 August 2016 14:39
*To:* Chris Taylor
*Cc:* oracle-l@xxxxxxxxxxxxx
*Subject:* RE: Deletion from large table
Unfortunately it’s a nightly thing….whilst updates are still going on….
*John *
*From:* Chris Taylor [mailto:christopherdtaylor1994@xxxxxxxxx]
*Sent:* 23 August 2016 14:38
*To:* John Dunn
*Cc:* oracle-l@xxxxxxxxxxxxx
*Subject:* Re: Deletion from large table
Is this a one time thing, or a regularly occurring thing? (A one time
data cleanup versus a nightly routine)
If it's a one time data cleanup (or rarely needed), I'd recommend saving
off the rows you want to keep into another table, truncate the big_table
and reload the rows from the temporary table you created to save the rows
you wanted.
Delete is one of the (if not THE) single most expensive operation you
can run in a database (but I'm sure you're aware of that but wanted to
mention it).
Chris
On Tue, Aug 23, 2016 at 5:17 AM, John Dunn <JDunn@xxxxxxxxx> wrote:
I need to delete large numbers of rows from a large table based upon
whether a record exists in a small table.
I am currently using :
delete from big_table where not exists (select 1 from
small_table s where s.id = b.id)"
big_table may have up to 100,000 rows for the same id value.
small_table will only have one row per id value
Is there a better way to code this?
*John *
--
Andrew W. Kerber
'If at first you dont succeed, dont take up skydiving.'
--
Martin Berger martin.a.berger@xxxxxxxxx
+43 660 2978929 <+436602978929>
@martinberx <https://twitter.com/martinberx>
http://berxblog.blogspot.com
