Processing or queries for Allpages-Locked tables

If the or query uses the OR Strategy (different or clauses might match the same rows), query processing retrieves the row IDs and matching key values from the index and stores them in a worktable, holding shared locks on the index pages containing the rows. When all row IDs have been retrieved, the worktable is sorted to remove duplicate values. Then, the worktable is scanned, and the row IDs are used to retrieve the data rows, acquiring shared locks on the data pages. The index and data page locks are released at the end of the statement (for isolation level 1) or at the end of the transaction (for isolation levels 2 and 3).

If the or query has no possibility of returning duplicate rows, no worktable sort is needed. At isolation level 1, locks on the data pages are released as soon as the scan moves off the page.