Where a query uses an or clause, the optimizer may choose a more expensive or strategy not only for table scans, but also against covering index scans because the strategy provides a better locking concurrency.
or clauses take one of the following forms:
where column_name1 = <value>
or column_name1 = <value>
...
or:
where column_name1 = <value>
or column_name2 = <value>
...
