Unlike base tables, procedure calls executed in the FROM clause do not have column statistics. Therefore, the optimizer uses defaults or guesses for all selectivity estimates on data coming from a procedure call. The execution time of a procedure call, and the total number of rows in its result set, are estimated using statistics collected from previous calls. These statistics are maintained in the stats column of the ISYSPROCEDURE system table by the ProCall algorithm. See SYSPROCEDURE system view, and ProcCall algorithm (PC).
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