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Oracle STAR Transformation tips


Oracle Tips by Burleson Consulting

Oracle STAR Transformations and SQL

To enable a data warehouse, the following suggested initialization parameter settings might be used:

         cursor_space_for_time=TRUE

         db_cache_size=XXX

         db_block_size=32

         db_file_multiblock_read_count=64

         filesystemio_options=ASYNC

         pga_aggregate_target=XXX

         optimizer_index_cost_adj=XXX

         optimizer_index_caching=XXX

         query_rewrite_enabled=TRUE

         shared_pool_size=150M

        star_transformation_enabled=TRUE

         workarea_size_policy=AUTO

         session_cached_cursors=100

         log_buffer=XXX

         bitmap_merge_area_size=XXX

         create_bitmap_area_size=XXX

For star_transformation join plans, the following parameters must also be considered:

        star_transformation_enabled= TRUE

         No hint STAR: So forcing a star_query excludes star_transformation

         No BIND VARIABLE in SELECT statement

         No CONNECT BY and start with

         For fact table columns involved in EQUIJOIN predicate, there must be bitmap index defined on them.

         More than 2 bitmap index on fact table

         Fact table must have more than 15,000 rows

         Fact table cannot be a view

         Fact table can not be a remote table

         No hint FULL on fact table

Failure to set proper parameters can result in a botched attempt to initiate a star_transformation join as shown by the following example.

Bad Star Transformation Plan

The following is an example of a failed attempt at a star_transformation join:

 

ALTER SESSION SET _always_star_transformation= TRUE;

 

select /*+ star_transformation X "star_transformation"  */ wdate, hour, minute, sum(bytes) from

network_fact nf,

date_dimension ddi,

hour_dimension hdi,

minute_dimension mdi

where

nf.date_key=ddi.date_key

and nf.hour_key=hdi.hour_key

and nf.minute_key=mdi.minute_key

and wdate>=to_date('2004/10/14 21', 'yyyy/mm/dd hh24')

and wdate<=to_date('2004/10/15 21', 'yyyy/mm/dd hh24')

group by wdate, hour, minute;

 

Execution Plan

----------------------------------------------------------

    0      SELECT STATEMENT Optimizer=CHOOSE

    1    0   SORT (GROUP BY)

    2    1     HASH JOIN

    3    2       TABLE ACCESS (FULL) OF 'MINUTE_DIMENSION'

    4    2       HASH JOIN

    5    4         MERGE JOIN (CARTESIAN)

    6    5           TABLE ACCESS (FULL) OF 'DATE_DIMENSION'

    7    5           BUFFER (SORT)

    8    7             TABLE ACCESS (FULL) OF 'HOUR_DIMENSION'

    9    4         TABLE ACCESS (FULL) OF 'NETWORK_FACT'

Good Star Transformation Plan

The following is what a successful star_transformation join looks like:

 

Execution Plan

----------------------------------------------------------

    0      SELECT STATEMENT Optimizer=CHOOSE

    1    0   NESTED LOOPS

    2    1     HASH JOIN

    3    2       HASH JOIN

    4    2       TABLE ACCESS (FULL) OF 'MINUTE_DIMENSION'

    5    2     PARTITION CONCATENATED

    6    2        TABLE ACCESS BY ROWID 

    7    2          BITMAP CONVERSION TO ROWIDS

    8    2        BITMAP AND

    9    2        BITMAP MERGE

   10    2        BITMAP KEY ITERATION

   11    2        SORT BUFFER

   12    2       TABLE ACCESS (FULL) OF 'MINUTE_DIMENSION'

               BITMAP INDEX RANGE SCAN I_C1

               BITMAP MERGE

               BITMAP KEY ITERATION

               SORT BUFFER

             TABLE ACCESS ... D2

              BITMAP INDEX RANGE SCAN I_C2

BITMAP MERGE

BITMAP KEY ITERATION

SORT BUFFER

TABLE ACCESS ... D3

BITMAP INDEX RANGE SCAN I_C3

TABLE ACCESS ... D2

TABLE ACCESS BY ... D3

 

The execution plan looks like:

 

SELECT STATEMENT C=301

NESTED LOOPS

HASH JOIN

HASH JOIN

TABLE ACCESS ... D1

PARTITION CONCATENATED

TABLE ACCESS BY ROWID F

BITMAP CONVERSION TO ROWIDS

BITMAP AND

BITMAP MERGE

BITMAP KEY ITERATION

SORT BUFFER

TABLE ACCESS ... D1

BITMAP INDEX RANGE SCAN I_C1

BITMAP MERGE

BITMAP KEY ITERATION

SORT BUFFER

TABLE ACCESS ... D2

BITMAP INDEX RANGE SCAN I_C2

BITMAP MERGE

BITMAP KEY ITERATION

SORT BUFFER

TABLE ACCESS ... D3

BITMAP INDEX RANGE SCAN I_C3

TABLE ACCESS ... D2

TABLE ACCESS BY ... D3

 

This means that with:

         (select C1_1 from D1 where D1.C1_2 op constant1), a bitmap B1 using I_C1 is generated.

         (select C2_1 from D2 where D1.C2_2 op constant2), a bitmap B2 using I_C2 is generated.

         (select C3_1 from D3 where D1.C3_2 op constant3), a bitmap B3 using I_C3 is generated.

Next, the DBA performs an AND between those bitmaps.  At the end, those rows from F with rowid coming from bitmap merging are taken.

[http://www.dba-oracle.com/include_SQL_Tuning_book.htm]

SEE CODE DEPOT FOR FULL SCRIPTS


   

 


 

 

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