Collecting Statistics on the Columns and Indexes for Join Indexes
Keeping fresh statistics on join indexes is just as critical for join indexes as it is for base tables for optimizing the query plans generated by the Optimizer.
This is particularly true if you have created a single‑table join index whose definition includes a complex expression in its select list and whose statistics are to be used to make more accurate single‑table cardinality estimates when a query that specifies a matching expression in its predicate is run against the table on which the join index is defined. The Optimizer can also use expression mapping when it detects an expression in a query predicate within a non‑matching predicate. In this case, the Optimizer maps to the join index column that is defined using the matched expression. See SQL Request and Transaction Processing for details.
You cannot collect statistics on the PARTITION column of a PPI join index.