Hi,
(this is not a real world problem, just something I'm playing around with).
Lately I had some queries of the form:
select t.*
from some_table t
where t.id not in (select some_id from some_other_table);
I could improve the performance of them drastically by changing the NOT NULL
into an outer join:
select t.*
from some_table t
left join some_other_table ot on ot.id = t.id
where ot.id is null;
Now I was wondering if a DELETE statement could be rewritten with the same
"strategy":
Something like:
delete from some_table
where id not in (select min(id)
from some_table
group by col1, col2
having count(*) > 1);
(It's the usual - at least for me - "get rid of duplicates" statement)
The DELETE .. USING seems to only allow inner joins because it requires the
join to be done in the WHERE clause.
So I can't think of a way to turn that NOT IN from the DELETE into an outer
join with a derived table.
Am I right that this kind of transformation is not possible or am I missing
something?
Regards
Thomas
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