A snapshot that has been restored from an export file has reverted to a previous state. On import, the time of the last refresh is imported as part of the snapshot table definition. The function that calculates the next refresh time is also imported.
Each refresh leaves a signature. A fast refresh uses the log entries that date from the time of that signature to bring the snapshot up to date. When the fast refresh is complete, the signature is deleted and a new signature is created. Any log entries that are not needed to refresh other snapshots are also deleted (all log entries with times before the earliest remaining signature).
Assume that a snapshot is refreshed at time A, exported at time B, and refreshed again at time C. Then, because of corruption or other problems, the snapshot needs to be restored by dropping the snapshot and importing it again. The newly imported version has the last refresh time recorded as time A. However, log entries needed for a fast refresh may no longer exist. If the log entries do exist (because they are needed for another snapshot that has yet to be refreshed), then they are used, and the fast refresh completes successfully. Otherwise, the fast refresh fails, generating an error that says a complete refresh is required.
Snapshots and related items are exported with the schema name given in the DDL statements. To import them into a different schema, use the
TOUSER parameters. This does not apply to snapshot logs, which cannot be imported into a different schema.
Schema names that appear inside function-based indexes, functions, procedures, triggers, type bodies, views, and so on, are not affected by
TOUSER processing. Only the name of the object is affected. After the import has completed, items in any
TOUSER schema should be manually checked for references to old (
FROMUSER) schemas, and corrected if necessary.