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Don Burleson Blog 







How to index with a SQL CASE statement

Oracle Database Tips by Donald BurlesonDecember 29,  2015

Question:  I have a SQL statement using a CASE statement and it does not use an index.  How can I make an index that can be used by a CASE statement?  Here is my SQL statement which causes a full-table scan, but only when the CASE statement is included:

select ...
from TABLE_1
where ....
when 'PO' then PO_ID
when 'VOUCHER' then voucher_id
ELSE journal_id
end = '0000000001'

The table has regular indexes defined, based on PO, VOUCHER & Journal. I've found that, without the CASE statement in the SQL, the SOURCE_TRAN index is used, but the index is not used when I add the CASE statement to the SQL. How can I use a CASE expression to index this query?

Answer:  Using function-based indexes (FBI) you can create an index on any built-in function, including a CASE expression.  Here we use CASE within the create index syntax:

create index
case_index as
when 'PO' then PO_ID
when 'VOUCHER' then voucher_id
ELSE journal_id
end = '0000000001'

Once created, you need to create CBO statistics, but beware that there are numerous bugs and issues when analyzing a function-based index.  See these important notes on statistics and function-based indexes:

EXEC DBMS_STATS.gather_index_stats('OWNER', 'CASE_INDEX');

exec dbms_stats.gather_table_stats(
   tabname=> 'CASE_TAB,
   method_opt=> 'FOR ALL HIDDEN COLUMNS SIZE 1′

exec dbms_stats.gather_table_stats(
ownname => 'OWNER',
tabname => 'CASE_TAB',
cascade => TRUE);

As a final step, run the execution plan for the query and ensure that your SQL with CASE is using the appropriate index.
Followup:  Thanks B, FBI works great!!!
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See these additional notes on using function-based indexes here.
  1. Oracle SQL tips

  2. Dictionary query to display function-based index function

  3. Oracle function based indexes

  4. Oracle - Function Based Indexes (FBI)

  5. Oracle Concepts - Function-based indexes




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