USI access is a two-AMP operation, because Vantage typically distributes a USI row to a different AMP than the base table row the index references. If the system distributes the USI subtable row to the same AMP the base table row the index references, only one AMP is accessed, but the operation still has two steps.
The following stages are involved in a USI base table row access.
- The requested USI value is accessed by hashing to its subtable.
- The pointer to the base table row is read and used to access the stored row directly.