Default refresh on snapshot performed by Oracle - Oracle Replication

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• Nirja Shah   -Posted on 18 Sep 15
  - Complete Refreshes
  - To perform a complete refresh of a snapshot, the server that manages the snapshot executes the snapshot's defining query.
  - The result set of the query replaces the existing snapshot data to refresh the snapshot.
  - Oracle can perform a complete refresh for any snapshot.
  
  - Fast Refreshes
  - To perform a fast refresh, the server that manages the snapshot first identifies the changes that occurred in the master since the most recent refresh of the snapshot and then applies them to the snapshot.
  - Fast refreshes are more efficient than complete refreshes when there are few changes to the master because participating servers and networks replicate less data.
  - Fast refreshes are available for snapshots only when the master table has a snapshot log.
  
  - Automatic Refresh Intervals
  - When you create a snapshot refresh group, you can specify an automatic refresh interval for the group.
  - When setting a group's refresh interval, consider the following characteristics:
  
  1. The dates or date expressions specifying the refresh interval must evaluate to a future point in time.
  2. The refresh interval must be greater than the length of time necessary to perform a refresh.
  3. Relative date expressions evaluate to a point in time relative to the most recent refresh date. If a network or system failure interferes with a scheduled group refresh, the evaluation of a relative date expression could change accordingly.
  4. Explicit date expressions evaluate to specific points in time, regardless of the most recent refresh date.
  
  - Refresh Types
  - By default, Oracle attempts to perform a fast refresh of each snapshot in a refresh group.
  - If Oracle cannot perform a fast refresh for an individual snapshot, for example when a master table has no snapshot log, the server performs a complete refresh for the snapshot.

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