Locking for select queries at isolation Level 1

When a select query on an allpages-locked table performs a table scan at isolation level 1, it first acquires a shared intent lock on the table and then acquires a shared lock on the first data page. It locks the next data page, and drops the lock on the first page, so that the locks “walk through” the result set. As soon as the query completes, the lock on the last data page is released, and then the table-level lock is released. Similarly, during index scans on an allpages-locked table, overlapping locks are held as the scan descends from the index root page to the data page. Locks are also held on the outer table of a join while matching rows from inner table are scanned.

select queries on data-only-locked tables first acquire a shared intent table lock. Locking behavior on the data pages and data rows is configurable with the parameter read committed with lock, as follows:

Cursors must be declared as read-only to avoid holding locks during scans when read committed with lock is set to 0. Any implicitly or explicitly up datable cursor on a data-only-locked table holds locks on the current page or row until the cursor moves off the row or page. When read committed with lock is set to 1, read-only cursors hold a shared page or row lock on the row at the cursor position.

read committed with lock does not affect locking behavior on allpages-locked tables. For information on setting the configuration parameter, see in the System Administration Guide.