All update and delete commands on an allpages-locked table first acquire an update lock on the data page and then change to an exclusive lock if the row meets the qualifications in the query.
Updates and delete commands on data-only-locked tables do not first acquire update locks when:
The query includes search arguments for every key in the index chosen by the query, so that the index unambiguously qualifies the row, and
The query does not contain an or clause.
Updates and deletes that meet these requirements immediately acquire an exclusive lock on the data page or data row. This reduces lock overhead.
