Application A holds an Update lock on a single row and application B is trying to read that row. If
both applications are using isolation level UR, which of the following will occur?
A.
Application B will read the row.
B.
Applications A and B will cause a deadlock situation.
C.
Application B will wait until applicationA releases the Update lock.
D.
Application A will be terminated so that application B can read the row.
The uncommitted read isolation level allows an application to access the uncommitted changes of other transactions. Moreover, UR does not prevent another application from accessing a row that is being read, unless that application is attempting to alter or drop the table.
Under UR, access to uncommitted data, non-repeatable reads, and phantom reads are possible. This isolation level is suitable if you run queries against read-only tables, or if you issue SELECT statements only, and seeing data that has not been committed by other applications is not a problem.
UR works differently for read-only and updatable cursors.
Read-only cursors can access most of the uncommitted changes of other transactions.
Tables, views, and indexes that are being created or dropped by other transactions are not available while the transaction is processing. Any other changes by other transactions can be read before they are committed or rolled back. Updatable cursors operating under UR behave as though the isolation level were CS.
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The ISOLATION (UR) option
The ISOLATION (UR) or uncommitted read option allows an application to read while acquiring few locks, at the risk of reading uncommitted data. UR isolation applies only to the following read-only operations: SELECT, SELECT INTO, or FETCH from a read-only result table.
Reading uncommitted data introduces an element of uncertainty.
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