USI retrieval can be a 1 or 2 AMP operation.
In a scenario where the hash value of PI and USI are the same, the SI subatble row as well as the base table row will be on the same amp.
Still it is a 2 step operation, since there is a Lookup on the USI subtable & then use RowId to Lookup the basetable. But it maybe called a "1 AMP" operation
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