SNAPSHOT ISOLATION Table 7-1: Versioning Database at READ

SNAPSHOT ISOLATION Table 7-1: Versioning Database at READ COMMITTED Isolation Step User 1 User 2 1 BEGIN TRAN SELECT name FROM tab WHERE id = 1 **value is Name 2 BEGIN TRAN UPDATE tab SET name = Newname WHERE id = 1 3 SELECT name FROM tab WHERE id = 1 **value is Name 4 COMMIT 5 SELECT name FROM tab WHERE id = 1 **value is NewName 6 COMMIT 7 SELECT name FROM tab WHERE id = 1 **value is NewName next value until user 1 commits his transaction. He sees the new value only in step 7. In SQL Server this is called transaction-level SNAPSHOT isolation. Both statement- and transaction-level SNAPSHOT isolation require that SNAPSHOT be enabled by using the SNAPSHOT isolation option of the ALTER DATABASE command. The following SQL batch does this for the pubs database. ALTER DATABASE pubs SET ALLOW_SNAPSHOT_ISOLATION ON SNAPSHOTisolation can be turned on or off as needed. Once SNAPSHOT isolation has been enabled, transaction-level isolation is used by specifically setting the transaction isolation level to SNAPSHOT. The following SQL batch does this.

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