FEDERATEDtable does not support indexes in the usual sense; because access to the table data is handled remotely, it is actually the remote table that makes use of indexes. This means that, for a query that cannot use any indexes and so requires a full table scan, the server fetches all rows from the remote table and filters them locally. This occurs regardless of any
LIMITused with this
SELECTstatement; these clauses are applied locally to the returned rows
- Queries that fail to use indexes can thus cause poor performance and network overload. In addition, since returned rows must be stored in memory, such a query can also lead to the local server swapping, or even hanging.
- "a number of limitations and due to network latency are rarely used...not enabled by default and is no longer maintained by Oracle and it's use is discouraged"