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11.01.2004 at 07:23AM PST, ID: 21190179
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3.6

how to change execution plan from FULL table access to index access to speed up the query?

Asked by sarahchi in Oracle Database

Tags: , ,


experts,

I have a slow query:

select * from (SELECT * FROM (SELECT distinct WELL.ID as the_well_id, data_value, well.column_number as well_column_number, well.row_number as well_row_number, well.compound_id, well.plate_id as well_plate_id FROM data_point, well, data_description, datatype, experiment, plate WHERE data_point.well_id = well.id and well.plate_id = plate.id and data_description.datatype_id = datatype.id and data_description.experiment_id = experiment.id and experiment.screen_id = 196 and data_point.data_description_id = data_description.id)) where rownum <= 25;

this is the explain plan:

Operation             Object
--------------------- ----------------
SELECT STATEMENT ()   NULL
COUNT (STOPKEY)       NULL
VIEW ()               NULL
SORT (UNIQUE STOPKEY) NULL
NESTED LOOPS ()       NULL
NESTED LOOPS ()       NULL
HASH JOIN ()          NULL
HASH JOIN ()          NULL
HASH JOIN ()          NULL
TABLE ACCESS (FULL)   EXPERIMENT
TABLE ACCESS (FULL)   DATA_DESCRIPTION
TABLE ACCESS (FULL)   DATA_POINT
TABLE ACCESS (FULL)   WELL
INDEX (UNIQUE SCAN)   SYS_C001505
INDEX (UNIQUE SCAN)   SYS_C001535

As you can see,  it performs several full table accesses,
including one on DATA_POINT, which I'm going to guess is the killer,
since that table is enormous. I do not know why the query is not making
any use of the indexes attached to DATA_POINT or any of the other
tables here.

I suspect that if I can make Oracle use the indexes -- or perhaps
construct the right indexes -- then this query will drop from 30.0 to
0.30 seconds in runtime. Can you help here?
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Zone: Oracle Database
Tags: oracle, plan, table
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Solution Provided By: oratim
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