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Status Not under consideration
Workspace Db2
Components Other/Unknown
Created by Guest
Created on Apr 18, 2023

adjust card estimates on predicates using both COL = '?' and COL > <value> similar to accounting for NUMNULLS

Component: DB2 optimizer

When a column is NULLable, a predicate COL = ? will account for numnulls to estimate cardinality. The problem comes when column is NOT NULL, and "the NULL value" is a real value.

Peoplesoft has been designed to use a majority of NOT NULL columns, therefore PROCESS_INSTANCE is set to 0 instead of NULL. This can be more that 90% of the table, and is not accounted by DB2 for cardinality estimation. So a predicate like PROCESS_INSTANCE = ? does not account for real cardinality, and PROCESS_INSTANCE = ? AND PROCESS_INSTANCE > 0 cannot allow the optimizer to adjust card estimates (similar to excluding numnulls for predicate PROCESS_INSTANCE > 0).

If query would be REOPTimized, stats would not reflect the reality, because Peoplesoft does a "relative mass update" to switch many lines from 0 to a VALUE, and stats would take a long time (> 30 min) to be correct before using PROCESS_INTANCE = <value> predicate.

Needed By Quarter