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 *
