DB2 UDB for z/OS Version 8 Performance Topics - IBM Redbooks

|On composite (multi-column) indexes, collect correlated key stats |
|on column pairings, as well as cardinality stats, like 2ndkeycard,|
|3rdkeycard, etc., with Runstats. These additional statistics will |
|allow DB2 to make better filter estimates for Equality and Range |
|and In List predicates, thereby selecting proper indexes and more |
|realistic estimates for rows returned from the query (QCARD). |
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|ANL7024I *** GOOD NEWS: |
|Index Screening is now available for List Prefetch processing, |
|which allows Random indexes to "screen" additional predicates |
|after matching index scan, and before data access. |
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|ANL6020I *** NOTE: |
|This index is presently designated as allowing "duplicate values".|
|Make sure this is a nonunique index that does not fall into one of|
|the following "unique" categories: explicitly unique, primary key,|
|non-primary RI parent key, unique where not null, unique column |
|constraint. Unique indexes have definite advantages whenever they |
|can be declared by the user, so be sure it contains duplicates. |
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THIS IS AN "INDEX ONLY" ACCESS: NO DATA PAGES ARE READ FROM THE TABLE
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|ANL7015I *** GUIDELINE: |
|Collecting Non-uniform column statistics greatly enhance the DB2 |
|Optimizer accuracy when processing Equals and In (List) preds. |
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THIS TABLE IS JOINED WITH THE PRIOR TABLE VIA THE "NESTED LOOP" METHOD.
THIS IS THE INNER TABLE IN THE NESTED LOOP JOIN.
THIS IS A CARTESIAN JOIN, ALL ROWS TO ALL ROWS, NO JOIN PREDICATE.
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|ANL6034I *** NOTE: |
|Only the Nested Loop join method supports the use of non-Equijoin |
|predicates (T1.C1