Application Programming and SQL Guide - Kmlinux

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Column correlation . . . . . . . . . . . . . . . . . . . . . . 700
DB2 predicate manipulation . . . . . . . . . . . . . . . . . . . 703
Predicates with encrypted data . . . . . . . . . . . . . . . . . 707
Using host variables efficiently . . . . . . . . . . . . . . . . . . . 707
Changing the access path at run time . . . . . . . . . . . . . . . 707
Rewriting queries to influence access path selection . . . . . . . . . . 710
Writing efficient subqueries . . . . . . . . . . . . . . . . . . . . 713
Correlated subqueries . . . . . . . . . . . . . . . . . . . . . 713
Noncorrelated subqueries . . . . . . . . . . . . . . . . . . . 714
Conditions for DB2 to transform a subquery into a join . . . . . . . . . 716
Subquery tuning . . . . . . . . . . . . . . . . . . . . . . . 717
Using scrollable cursors efficiently . . . . . . . . . . . . . . . . . 718
Writing efficient queries on tables with data-partitioned secondary indexes 719
Special techniques to influence access path selection . . . . . . . . . . 721
Obtaining information about access paths . . . . . . . . . . . . . 722
Fetching a limited number of rows: FETCH FIRST n ROWS ONLY . . . . 722
Minimizing overhead for retrieving few rows: OPTIMIZE FOR n ROWS 723
Favoring index access . . . . . . . . . . . . . . . . . . . . . 725
Using the CARDINALITY clause to improve the performance of queries with
user-defined table function references . . . . . . . . . . . . . . 725
Reducing the number of matching columns . . . . . . . . . . . . . 726
Creating indexes for efficient star-join processing . . . . . . . . . . . 728
Rearranging the order of tables in a FROM clause . . . . . . . . . . 731
Updating catalog statistics . . . . . . . . . . . . . . . . . . . 731
Using a subsystem parameter . . . . . . . . . . . . . . . . . . 732
Chapter 27. Using EXPLAIN to improve SQL performance . . . . . . . 735
Obtaining PLAN_TABLE information from EXPLAIN . . . . . . . . . . . 736
Creating PLAN_TABLE . . . . . . . . . . . . . . . . . . . . 736
Populating and maintaining a plan table . . . . . . . . . . . . . . 744
Reordering rows from a plan table . . . . . . . . . . . . . . . . 745
Asking questions about data access . . . . . . . . . . . . . . . . 746
Is access through an index? (ACCESSTYPE is I, I1, N or MX) . . . . . . 746
Is access through more than one index? (ACCESSTYPE=M) . . . . . . 747
How many columns of the index are used in matching? (MATCHCOLS=n) 748
Is the query satisfied using only the index? (INDEXONLY=Y) . . . . . . 748
Is direct row access possible? (PRIMARY_ACCESSTYPE = D) . . . . . 749
Is a view or nested table expression materialized? . . . . . . . . . . 752
Was a scan limited to certain partitions? (PAGE_RANGE=Y) . . . . . . 752
What kind of prefetching is expected? (PREFETCH = L, S, D, or blank) 753
Is data accessed or processed in parallel? (PARALLELISM_MODE is I, C,
or X) . . . . . . . . . . . . . . . . . . . . . . . . . . 753
Are sorts performed? . . . . . . . . . . . . . . . . . . . . . 753
Is a subquery transformed into a join? . . . . . . . . . . . . . . . 754
When are aggregate functions evaluated? (COLUMN_FN_EVAL) . . . . . 754
How many index screening columns are used? . . . . . . . . . . . 754
Is a complex trigger WHEN clause used? (QBLOCKTYPE=TRIGGR) . . . 755
Interpreting access to a single table . . . . . . . . . . . . . . . . . 755
Table space scans (ACCESSTYPE=R PREFETCH=S) . . . . . . . . . 755
Index access paths . . . . . . . . . . . . . . . . . . . . . . 756
UPDATE using an index . . . . . . . . . . . . . . . . . . . . 761
Interpreting access to two or more tables (join) . . . . . . . . . . . . 761
Definitions and examples of join operations . . . . . . . . . . . . . 761
Nested loop join (METHOD=1) . . . . . . . . . . . . . . . . . 764
Merge scan join (METHOD=2) . . . . . . . . . . . . . . . . . . 766
Hybrid join (METHOD=4) . . . . . . . . . . . . . . . . . . . . 767
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