When an application specifies a value that can be used to access a table using its USI, a 2-AMP operation results. USI access can be a single-AMP operation if the USI value for a row happens to hash to a subtable on the same AMP as the primary index for the same row, but is never more than a 2-AMP operation.
In the following query, the column s_name is defined as a USI on the original supplier table having s_suppkey as its UPI:
EXPLAIN SELECT s_suppkey, s_acctbal FROM supplier WHERE s_name = 'Supplier#000038729';
Explanation
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1) First, we do a two-AMP RETRIEVE step from CAB.supplier by way
of unique index # 8 "CAB.supplier.S_NAME = 'Supplier#000038729'"
with no residual conditions. The estimated time for this step is
0.07 seconds.