-
-
-
The data provider sets
the default value. OLE DB supports the S, RR, RC, and RU isolation
levels that are defined in this table.
Isolation Levels for OLE DB
|
|
|
Does not allow dirty
Reads, nonrepeatable Reads, or phantom Reads.
|
|
Does not allow dirty
Reads or nonrepeatable Reads; does allow phantom Reads.
|
|
Does not allow dirty
Reads or nonrepeatable Reads; does allow phantom Reads.
|
|
Allows dirty Reads,
nonrepeatable Reads, and phantom Reads.
|
Here is how the terms
in the table are defined.
A transaction that
exhibits this phenomenon has very minimal isolation from concurrent
transactions. In fact, it can see changes that are made by those concurrent
transactions even before they commit.
For example, suppose
that transaction T1 performs an update on a row, transaction T2 then
retrieves that row, and transaction T1 then terminates with rollback.
Transaction T2 has then seen a row that no longer exists.
If a transaction exhibits
this phenomenon, it might read a row once. Then, if the same transaction
attempts to read that row again, the row might have been changed or
even deleted by another concurrent transaction. Therefore, the Read
is not (necessarily) repeatable.
For example, suppose
that transaction T1 retrieves a row, transaction T2 then updates that
row, and transaction T1 then retrieves the same row again. Transaction
T1 has now retrieved the same row twice but has seen two different
values for it.
When a transaction
exhibits this phenomenon, a set of rows that it reads once might be
a different set of rows if the transaction attempts to read them again.
For example, suppose
that transaction T1 retrieves the set of all rows that satisfy some
condition. Suppose that transaction T2 then inserts a new row that
satisfies that same condition. If transaction T1 now repeats its retrieval
request, it sees a row that did not previously exist, a phantom.
-
The default value is
set by the data provider. OLE DB supports the S, RR, and RC isolation
levels defined in the preceding table. The RU isolation level is not
allowed with this option.