Is it possible to use a case statement when joining different tables based on input parameters?

Question

Hi,
I have a scenario where my stored procedure takes 5 parameters and the users can pass NULL or some value to these parameters and based on the parameters, I need to pull data from various tables.
Is it possible to use a case statement in the join, similar the one in the below example. I'm getting error when I use the below type of statement.
select a.*
from a
case
when parameter1=1 then
inner join a on a.id = b.id
when parameter1=2 then
inner join a on a.id = c.id
end;
Please let me know, if this type of statement works, and if it works will it create any performance issues?. If the above doesn't work, could you please give me some alternate solutions?
Thanks.

Answer 1

Here's a technique for joining A to B or C depending on the input parameters. In theory, you are joining to both tables but the execution plan includes filters to skip whichever join is not appropriate. The drawback is that you have to do outer joins, not inner ones.
CREATE TABLE A AS SELECT LEVEL ak FROM dual CONNECT BY LEVEL <= 100;
CREATE TABLE b AS SELECT ak, bk
FROM A, (SELECT LEVEL bk FROM dual CONNECT BY LEVEL <= 10);
CREATE TABLE c(ak, ck) AS SELECT ak, bk*10 FROM b;
variable p1 NUMBER;
variable p2 NUMBER;
exec :p1 := 1;
exec :p2 := 20;
SELECT /*+ gather_plan_statistics */ A.ak, nvl(b.bk, c.ck) otherk FROM A
LEFT JOIN b ON A.ak = b.ak AND :p1 IS NOT NULL AND b.bk = :p1
LEFT JOIN c ON A.ak = c.ak AND :p1 is null and :p2 IS NOT NULL and c.ck = :p2
WHERE A.ak <= 9;
SELECT * FROM TABLE(dbms_xplan.display_cursor(NULL,NULL,'IOSTATS LAST'));
| Id  | Operation             | Name            | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
|   0 | SELECT STATEMENT      |                 |      1 |        |      9 |00:00:00.01 |       7 |
|*  1 |  HASH JOIN OUTER      |                 |      1 |      9 |      9 |00:00:00.01 |       7 |
|*  2 |   HASH JOIN OUTER     |                 |      1 |      9 |      9 |00:00:00.01 |       7 |
|*  3 |    TABLE ACCESS FULL  | A               |      1 |      9 |      9 |00:00:00.01 |       3 |
|   4 |    VIEW               | VW_DCL_5532A50F |      1 |      9 |      9 |00:00:00.01 |       4 |
|*  5 |     FILTER            |                 |      1 |        |      9 |00:00:00.01 |       4 |
|*  6 |      TABLE ACCESS FULL| B               |      1 |      9 |      9 |00:00:00.01 |       4 |
|   7 |   VIEW                | VW_DCL_5532A50F |      1 |      9 |      0 |00:00:00.01 |       0 |
|*  8 |    FILTER             |                 |      1 |        |      0 |00:00:00.01 |       0 |
|*  9 |     TABLE ACCESS FULL | C               |      0 |      9 |      0 |00:00:00.01 |       0 |
Predicate Information (identified by operation id):
   1 - access("A"."AK"="ITEM_0")
   2 - access("A"."AK"="ITEM_1")
   3 - filter("A"."AK"<=9)
  5 - filter(:P1 IS NOT NULL)
   6 - filter(("B"."AK"<=9 AND "B"."BK"=:P1))
   8 - filter((:P2 IS NOT NULL AND :P1 IS NULL))
   9 - filter(("C"."AK"<=9 AND "C"."CK"=:P2))
You can see that table C was not really accessed: the buffer count is 0.
exec :p1 := NULL;
SELECT /*+ gather_plan_statistics */ A.ak, nvl(b.bk, c.ck) otherk FROM A
LEFT JOIN b ON A.ak = b.ak AND :p1 IS NOT NULL AND b.bk = :p1
LEFT JOIN c ON A.ak = c.ak AND :p1 is null and :p2 IS NOT NULL and c.ck = :p2
WHERE A.ak <= 9;
SELECT * FROM TABLE(dbms_xplan.display_cursor(NULL,NULL,'IOSTATS LAST'));
Now table B is not accessed.
| Id  | Operation             | Name            | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads  |
|   0 | SELECT STATEMENT      |                 |      1 |        |      9 |00:00:00.02 |       7 |      2 |
|*  1 |  HASH JOIN OUTER      |                 |      1 |      9 |      9 |00:00:00.02 |       7 |      2 |
|*  2 |   HASH JOIN OUTER     |                 |      1 |      9 |      9 |00:00:00.01 |       3 |      0 |
|*  3 |    TABLE ACCESS FULL  | A               |      1 |      9 |      9 |00:00:00.01 |       3 |      0 |
|   4 |    VIEW               | VW_DCL_5532A50F |      1 |      9 |      0 |00:00:00.01 |       0 |      0 |
|*  5 |     FILTER            |                 |      1 |        |      0 |00:00:00.01 |       0 |      0 |
|*  6 |      TABLE ACCESS FULL| B               |      0 |      9 |      0 |00:00:00.01 |       0 |      0 |
|   7 |   VIEW                | VW_DCL_5532A50F |      1 |      9 |      9 |00:00:00.01 |       4 |      2 |
|*  8 |    FILTER             |                 |      1 |        |      9 |00:00:00.01 |       4 |      2 |
|*  9 |     TABLE ACCESS FULL | C               |      1 |      9 |      9 |00:00:00.01 |       4 |      2 |