- When the FALLBACK option is specified for the table, because the rows in the secondary copy of the table must also be updated.
- When a column on which one or more indexes (secondary or primary) are defined is updated.
You can shorten the processing time for an UPDATE operation by using an indexed column in the WHERE clause of the UPDATE request.
Processing time can also vary between different syntaxes used to perform the identical update operation. Use the EXPLAIN modifier to determine which syntax form produces optimal processing time.