where 1=2

To use this when creating an empty table (create table t1 as select * from t2 where 1=2) I have done many times, but now I am looking at a query like this:

select *
from t1
where 1!=1
or c1 = '1';

Back in the days (oracle 7) I know we used this form to try to force a full table scan and I haven't seen it on many years.

The query uses primary key index when executed on 11.2.0.3 but on 11.2.0.4 it does a full table scan (c1 is unique primary key).

Even when I used this myself many years ago I didn't know why it worked, it mostly just did. What happens when the optimizer encounter this? Why could this sometimes force a full table scan?

/johan

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