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 ....
and SOURCE_TRAN = "PO"
and
case SOURCE_TRAN
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
(case SOURCE_TRAN
when 'PO' then PO_ID
when 'VOUCHER' then voucher_id
ELSE journal_id
end = '0000000001'
END);

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(
   ownname=>null,
   tabname=> 'CASE_TAB,
   estimate_percent=>null,
   cascade=>true,
   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.