I'm having performance issues executing the following query (Q1):
select
z_out.*,
a_out.id
from orders a_out, test z_out
where a_out.id=z_out.id and a_out.created>trunc(sysdate) and rownum<10
Table orders contains millions of rows; orders.id is the primary key and orders.craeted is indexed.
The view is:
create or replace view test as
select/*+qb_name(q_outer)*/
id,
min(value) keep (dense_rank first order by id) as value
from (
select/*+qb_name(q_inner)*/
id,
case
when substr(id, -1)<'5'
--and exists(select 1 from dual@db2)
then 'YYY'
end as attr_1
from orders a1
) a2, small_table b2
where b2.attr_1 in (nvl(a2.attr_1, '#'), '*')
group by id
where small_table b2 contains about 200 records (all the columns are varchar2).
Executing Q1 has great performances and the following execution plan:
Plan hash value: 2906430222
-----------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
-----------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 274 | 64 (0)| 00:00:01 | | |
|* 1 | COUNT STOPKEY | | | | | | | |
| 2 | NESTED LOOPS | | 1 | 274 | 64 (0)| 00:00:01 | | |
| 3 | PARTITION LIST ALL | | 1 | 22 | 59 (0)| 00:00:01 | 1 | 2 |
| 4 | PARTITION RANGE ALL | | 1 | 22 | 59 (0)| 00:00:01 | 1 | LAST |
| 5 | TABLE ACCESS BY LOCAL INDEX ROWID| ORDERS | 1 | 22 | 59 (0)| 00:00:01 | 1 | 29 |
|* 6 | INDEX RANGE SCAN | IDX_ORDERS_CREATED | 1 | | 57 (0)| 00:00:01 | 1 | 29 |
| 7 | VIEW PUSHED PREDICATE | TEST | 1 | 252 | 5 (0)| 00:00:01 | | |
|* 8 | FILTER | | | | | | | |
| 9 | SORT AGGREGATE | | 1 | 55 | | | | |
| 10 | NESTED LOOPS | | 259 | 14245 | 5 (0)| 00:00:01 | | |
|* 11 | INDEX UNIQUE SCAN | PK_ID | 1 | 14 | 2 (0)| 00:00:01 | | |
|* 12 | INDEX STORAGE FAST FULL SCAN | IDX_MN_AN_AD_ALL | 259 | 10619 | 3 (0)| 00:00:01 | | |
-----------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<10)
6 - access("A_OUT"."CREATED">TRUNC(SYSDATE@!))
8 - filter(COUNT(*)>0)
11 - access("ID"="A_OUT"."ID")
12 - storage("B2"."ATTR_1"=NVL(CASE WHEN SUBSTR("ID",(-1))<'5' THEN 'YYY' END ,'#') OR "B2"."ATTR_1"='*')
filter("B2"."ATTR_1"=NVL(CASE WHEN SUBSTR("ID",(-1))<'5' THEN 'YYY' END ,'#') OR "B2"."ATTR_1"='*')
Q1 performance issues happen when the line --and exists(select 1 from dual@db2) in the view is uncommented.
The new execution plan is:
Plan hash value: 3271081243
----------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop | Inst |IN-OUT|
----------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 288 | 5273K (1)| 00:03:27 | | | | |
|* 1 | COUNT STOPKEY | | | | | | | | | |
|* 2 | HASH JOIN | | 1 | 288 | 5273K (1)| 00:03:27 | | | | |
| 3 | JOIN FILTER CREATE | :BF0000 | 1 | 22 | 59 (0)| 00:00:01 | | | | |
| 4 | PARTITION LIST ALL | | 1 | 22 | 59 (0)| 00:00:01 | 1 | 2 | | |
| 5 | PARTITION RANGE ALL | | 1 | 22 | 59 (0)| 00:00:01 | 1 | LAST | | |
| 6 | TABLE ACCESS BY LOCAL INDEX ROWID| ORDERS | 1 | 22 | 59 (0)| 00:00:01 | 1 | 29 | | |
|* 7 | INDEX RANGE SCAN | IDX_ORDERS_CREATED | 1 | | 57 (0)| 00:00:01 | 1 | 29 | | |
| 8 | VIEW | TEST | 3840K| 974M| 5273K (1)| 00:03:27 | | | | |
| 9 | SORT GROUP BY | | 3840K| 201M| 5273K (1)| 00:03:27 | | | | |
| 10 | JOIN FILTER USE | :BF0000 | 994M| 50G| 5273K (1)| 00:03:27 | | | | |
| 11 | NESTED LOOPS | | 994M| 50G| 5273K (1)| 00:03:27 | | | | |
| 12 | INDEX FULL SCAN | PK_ID | 3840K| 51M| 66212 (1)| 00:00:03 | | | | |
|* 13 | INDEX STORAGE FAST FULL SCAN | IDX_MN_AN_AD_ALL | 259 | 10619 | 1 (0)| 00:00:01 | | | | |
| 14 | REMOTE | | | | | | | | DB2 | R->S |
----------------------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<10)
2 - access("A_OUT"."ID"="Z_OUT"."ID")
7 - access("A_OUT"."CREATED">TRUNC(SYSDATE@!))
13 - filter("B2"."ATTR_1"=NVL(CASE WHEN (SUBSTR("ID",(-1))<'5' AND EXISTS (SELECT 0 FROM "A1")) THEN 'YYY' END ,'#') OR
"B2"."ATTR_1"='*')
Remote SQL Information (identified by operation id):
----------------------------------------------------
14 - EXPLAIN PLAN INTO PLAN_TABLE@! FOR SELECT 0 FROM "DUAL" "A1" (accessing 'DB2' )
I would like the view to be accessed n times, like in the first scenario.
I tried using hints but didn't succeed.
May be useful to say that even with the line and exists(select 1 from dual@db2) uncommented in the view, the following query has great performances (I know that is different from Q1).
select
(select value from test z_out where a_out.id=z_out.id) as value,
a_out.id
from orders a_out
where a_out.created>trunc(sysdate) and rownum<10
So, I guess the view works fine when it's accessed n times even if the line and exists(select 1 from dual@db2) is uncommented. But I'm not being able to force the execution plan in that direction.
If hints are necessary, I'd like to add them inside the view DDL only (if possible) so that who uses the view won't have to worry about it.
================================================================
Edit: the following were executed:
alter session set statistics_level = 'ALL';
-- Q1 (the query I'm having problems with)
select * from table (dbms_xplan.display_cursor (format=>'ALLSTATS LAST'));
Plan hash value: 3271081243
------------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
------------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 0 |00:00:00.01 | 0 | 0 | | | |
|* 1 | COUNT STOPKEY | | 1 | | 0 |00:00:00.01 | 0 | 0 | | | |
|* 2 | HASH JOIN | | 1 | 1 | 0 |00:00:00.01 | 0 | 0 | 3789K| 3789K| 1078K (0)|
| 3 | JOIN FILTER CREATE | :BF0000 | 1 | 1 | 25602 |00:00:00.22 | 23345 | 161 | | | |
| 4 | PARTITION LIST ALL | | 1 | 1 | 25602 |00:00:00.21 | 23345 | 161 | | | |
| 5 | PARTITION RANGE ALL | | 2 | 1 | 25602 |00:00:00.21 | 23345 | 161 | | | |
| 6 | TABLE ACCESS BY LOCAL INDEX ROWID| ORDERS | 29 | 1 | 25602 |00:00:00.20 | 23345 | 161 | | | |
|* 7 | INDEX RANGE SCAN | IDX_CREATED | 13 | 1 | 25602 |00:00:00.12 | 474 | 161 | 1025K| 1025K| |
| 8 | VIEW | TEST | 1 | 3820K| 0 |00:00:00.01 | 0 | 0 | | | |
| 9 | SORT GROUP BY | | 1 | 3820K| 0 |00:00:00.01 | 0 | 0 | 73728 | 73728 | |
| 10 | JOIN FILTER USE | :BF0000 | 1 | 989M| 106M|00:03:38.87 | 60M| 52960 | | | |
| 11 | NESTED LOOPS | | 1 | 989M| 328M|00:03:04.11 | 60M| 52960 | | | |
| 12 | INDEX FULL SCAN | PK_ID | 1 | 3820K| 1245K|00:00:21.04 | 200K| 52959 | 1025K| 1025K| |
|* 13 | INDEX STORAGE FAST FULL SCAN | IDX_MN_AN_AD_ALL | 1245K| 259 | 328M|00:02:12.09 | 60M| 1 | 1025K| 1025K| |
| 14 | REMOTE | | 1 | | 1 |00:00:00.01 | 0 | 0 | | | |
------------------------------------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<10)
2 - access("A_OUT"."ID"="Z_OUT"."ID")
7 - access("A_OUT"."CREATED">TRUNC(SYSDATE@!))
13 - filter(("B2"."ATTR_1"=NVL(CASE WHEN (SUBSTR("ID",(-1))<'5' AND IS NOT NULL) THEN 'YYY' END ,'#') OR "B2"."ATTR_1"='*'))
Note: Q1 performances prevent the query to complete if and exists(select 1 from dual@db2) in the view is uncommented. To get the previous execution plan I had to alter the session, run Q1, stop Q1 (after about 4 minutes) and then calculate the plan.
The following execution plan was generated the same way, but the view had the line --and exists(select 1 from dual@db2) commented (performances were good).
Plan hash value: 2906430222
-----------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-----------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 9 |00:00:00.01 | 223 |
|* 1 | COUNT STOPKEY | | 1 | | 9 |00:00:00.01 | 223 |
| 2 | NESTED LOOPS | | 1 | 1 | 9 |00:00:00.01 | 223 |
| 3 | PARTITION LIST ALL | | 1 | 1 | 9 |00:00:00.01 | 41 |
| 4 | PARTITION RANGE ALL | | 1 | 1 | 9 |00:00:00.01 | 41 |
| 5 | TABLE ACCESS BY LOCAL INDEX ROWID| ORDERS | 14 | 1 | 9 |00:00:00.01 | 41 |
|* 6 | INDEX RANGE SCAN | IDX_CREATED | 12 | 1 | 9 |00:00:00.01 | 33 |
| 7 | VIEW PUSHED PREDICATE | TEST | 9 | 1 | 9 |00:00:00.01 | 182 |
|* 8 | FILTER | | 9 | | 9 |00:00:00.01 | 182 |
| 9 | SORT AGGREGATE | | 9 | 1 | 9 |00:00:00.01 | 182 |
| 10 | NESTED LOOPS | | 9 | 259 | 2376 |00:00:00.01 | 182 |
|* 11 | INDEX UNIQUE SCAN | PK_ID | 9 | 1 | 9 |00:00:00.01 | 20 |
|* 12 | INDEX STORAGE FAST FULL SCAN | IDX_MN_AN_AD_ALL | 9 | 259 | 2376 |00:00:00.01 | 162 |
-----------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<10)
6 - access("A_OUT"."CREATED">TRUNC(SYSDATE@!))
8 - filter(COUNT(*)>0)
11 - access("ID"="A_OUT"."ID")
12 - storage(("B2"."ATTR_1"=NVL(CASE WHEN SUBSTR("ID",(-1))<'5' THEN 'YYY' END ,'#') OR
"B2"."ATTR_1"='*'))
filter(("B2"."ATTR_1"=NVL(CASE WHEN SUBSTR("ID",(-1))<'5' THEN 'YYY' END ,'#') OR
"B2"."ATTR_1"='*'))
