Optimize view with CASE and GROUP BY - sql

Question

I'm trying to create a view which will be queried against with a simple WHERE clause. I have the following tables (simplified) :
CREATE TABLE table_1 (
"ID" VARCHAR2(38 BYTE),
"USER" VARCHAR2(255 BYTE),
"FIELD_TYPE_1" VARCHAR2(255 BYTE),
"FIELD_TYPE_2" VARCHAR2(255 BYTE)
);
CREATE TABLE table_2 (
"FK_ID" VARCHAR2(38 BYTE),
"TYPE" NUMBER
);
table_1 have 12M rows and table_2 have 25M rows.
My current view is defined as :
CREATE OR REPLACE VIEW vw_t1 AS
SELECT
CASE
WHEN t2.TYPE = 1 THEN t1.field_type_1
ELSE t1.field_type_2
END FIELD_TYPE,
t1.USER,
COUNT(*)
FROM table_1 t1 JOIN table_2 t2 ON t1.ID = t2.FK_ID
GROUP BY t1.USER,
CASE WHEN t2.TYPE = 1 THEN t1.field_type_1 else t1.field_type_2 end;
and queries against the view perform poorly taking 10s to return:
SELECT * FROM vw_t1 WHERE field_type LIKE 'some text'
-------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 6867K| 491M| | 394K (1)| 00:00:16 |
| 1 | HASH GROUP BY | | 6867K| 491M| 553M| 394K (1)| 00:00:16 |
|* 2 | FILTER | | | | | | |
|* 3 | HASH JOIN RIGHT OUTER | | 6867K| 491M| 57M| 282K (1)| 00:00:12 |
| 4 | TABLE ACCESS STORAGE FULL| table_1 | 2420K| 30M| | 6891 (1)| 00:00:01 |
|* 5 | TABLE ACCESS STORAGE FULL| table_2 | 6835K| 404M| | 248K (1)| 00:00:10 |
-------------------------------------------------------------------------------------------------------
I have thought of returning the fields 'field_type_1', 'field_type_2', and 'type', and have the users do the CASE themselves, but the queries won't return the same results due to the GROUP BY.

Answer 1

You should test your view's explain plan with changed context. For me it is not possible to test it (with just a few rows) but here are 3 different options and explain plans for the view and sql selecting from the view.
No indexes on any table
Primary key on TABLE_1
Primary key on TABLE_1 and NonUnique Index on TABLE_2
-- V i e w ------------------------------------
SELECT
CASE WHEN t2.TYPE = 1 THEN t1.field_type_1
ELSE t1.field_type_2
END "FIELD_TYPE",
--
t1.USER_ID,
--
COUNT(*) "CNT"
FROM table_1 t1 JOIN table_2 t2 ON t1.ID = t2.FK_ID
GROUP BY t1.USER_ID, CASE WHEN t2.TYPE = 1 THEN t1.field_type_1 else t1.field_type_2 END;
1. ******************************************************************************
Table_1 - none
Table_2 - none
View
----------------------------------------------------------
| Id | Operation | Name | Rows | Time |
----------------------------------------------------------
| 0 | SELECT STATEMENT | | 8 | 00:00:01 |
| 1 | HASH GROUP BY | | 8 | 00:00:01 |
|* 2 | HASH JOIN | | 8 | 00:00:01 |
| 3 | TABLE ACCESS FULL| TABLE_1 | 3 | 00:00:01 |
| 4 | TABLE ACCESS FULL| TABLE_2 | 8 | 00:00:01 |
----------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("T1"."ID"="T2"."FK_ID")
=== Query ==================================================================================
SELECT * FROM a__a WHERE field_type LIKE('%FLD%');
----------------------------------------------------------
| Id | Operation | Name | Rows | Time |
----------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 00:00:01 |
| 1 | HASH GROUP BY | | 1 | 00:00:01 |
|* 2 | HASH JOIN | | 1 | 00:00:01 |
| 3 | TABLE ACCESS FULL| TABLE_1 | 3 | 00:00:01 |
| 4 | TABLE ACCESS FULL| TABLE_2 | 8 | 00:00:01 |
----------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("T1"."ID"="T2"."FK_ID")
filter(CASE "T2"."TYPE" WHEN 1 THEN "T1"."FIELD_TYPE_1" ELSE
"T1"."FIELD_TYPE_2" END LIKE '%FLD%')
...
2. ******************************************************************************
Table_1 - PK (ID)
Table_2 - None
View
| Id | Operation | Name | Rows | Time |
|-----|-------------------------------|------------|-------|----------|
| 0 | SELECT STATEMENT | | 8 | 00:00:01 |
| 1 | HASH GROUP BY | | 8 | 00:00:01 |
| 2 | MERGE JOIN | | 8 | 00:00:01 |
| 3 | TABLE ACCESS BY INDEX ROWID| TABLE_1 | 3 | 00:00:01 |
| 4 | INDEX FULL SCAN | TABLE_1_PK | 3 | 00:00:01 |
|* 5 | SORT JOIN | | 8 | 00:00:01 |
| 6 | TABLE ACCESS FULL | TABLE_2 | 8 | 00:00:01 |
-----------------------------------------------------------------------
=== Query ==================================================================================
SELECT * FROM a__a WHERE field_type LIKE('%FLD%');
-----------------------------------------------------------------------
| Id | Operation | Name | Rows | Time |
-----------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 00:00:01 |
| 1 | HASH GROUP BY | | 1 | 00:00:01 |
| 2 | MERGE JOIN | | 1 | 00:00:01 |
| 3 | TABLE ACCESS BY INDEX ROWID| TABLE_1 | 3 | 00:00:01 |
| 4 | INDEX FULL SCAN | TABLE_1_PK | 3 | 00:00:01 |
|* 5 | FILTER | | | |
|* 6 | SORT JOIN | | 8 | 00:00:01 |
| 7 | TABLE ACCESS FULL | TABLE_2 | 8 | 00:00:01 |
-----------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - filter(CASE "T2"."TYPE" WHEN 1 THEN "T1"."FIELD_TYPE_1" ELSE
"T1"."FIELD_TYPE_2" END LIKE '%FLD%')
6 - access("T1"."ID"="T2"."FK_ID")
filter("T1"."ID"="T2"."FK_ID")
...
3. ******************************************************************************
Table_1 - PK (ID)
Table_2 - INDEX NonUnique (FK_ID, TYPE)
VIEW
-------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Time |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 8 | 00:00:01 |
| 1 | HASH GROUP BY | | 8 | 00:00:01 |
| 2 | MERGE JOIN | | 8 | 00:00:01 |
| 3 | TABLE ACCESS BY INDEX ROWID| TABLE_1 | 3 | 00:00:01 |
| 4 | INDEX FULL SCAN | TABLE_1_PK | 3 | 00:00:01 |
|* 5 | SORT JOIN | | 8 | 00:00:01 |
| 6 | INDEX FULL SCAN | TABLE_2_INDEX_FK_ID_TYPE | 8 | 00:00:01 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - access("T1"."ID"="T2"."FK_ID")
filter("T1"."ID"="T2"."FK_ID")
=== Query ==================================================================================
SELECT * FROM a__a WHERE field_type LIKE('%FLDt%');
-------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Time |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 00:00:01 |
| 1 | HASH GROUP BY | | 1 | 00:00:01 |
| 2 | MERGE JOIN | | 1 | 00:00:01 |
| 3 | TABLE ACCESS BY INDEX ROWID| TABLE_1 | 3 | 00:00:01 |
| 4 | INDEX FULL SCAN | TABLE_1_PK | 3 | 00:00:01 |
|* 5 | FILTER | | | |
|* 6 | SORT JOIN | | 8 | 00:00:01 |
| 7 | INDEX FULL SCAN | TABLE_2_INDEX_FK_ID_TYPE | 8 | 00:00:01 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - filter(CASE "T2"."TYPE" WHEN 1 THEN "T1"."FIELD_TYPE_1" ELSE
"T1"."FIELD_TYPE_2" END LIKE '%FLD%')
6 - access("T1"."ID"="T2"."FK_ID")
filter("T1"."ID"="T2"."FK_ID")