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SQL query taking time

Hello Experts,

I have the below SQL which is taking 12 sec to execute when attached to a web application:

SELECT org_entity_name
       ,org_entity_id
       ,org_hierarchy_name
       ,division
       ,area
       ,organization
   FROM ms_grc_parent_orgs mgpo
  WHERE EXISTS (SELECT 1
           FROM ms_flow_down_org
          WHERE org_entity_id = mgpo.org_entity_id
                AND user_name = 'E131094')
        AND org_hierarchy_name = 'Level 4 - Department';

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Execution plan is as below:

				Plan hash value: 4043587597
 
---------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name               | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |                    |     1 |   141 |   585   (1)| 00:00:08 |
|*  1 |  HASH JOIN SEMI              |                    |     1 |   141 |   585   (1)| 00:00:08 |
|*  2 |   TABLE ACCESS FULL          | MS_GRC_PARENT_ORGS |   161 | 20930 |     5   (0)| 00:00:01 |
|   3 |   TABLE ACCESS BY INDEX ROWID| MS_FLOW_DOWN_ORG   |  3524 | 38764 |   580   (1)| 00:00:07 |
|*  4 |    INDEX RANGE SCAN          | MFDO_USER_NAME_IDX |  3524 |       |    16   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
 
   1 - access("ORG_ENTITY_ID"="MGPO"."ORG_ENTITY_ID")
   2 - filter("ORG_HIERARCHY_NAME"='Level 4 - Department')
   4 - access("USER_NAME"='E131094')
 

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Records present on the tables are as below:

select count(*) from ms_grc_parent_orgs;	 -- 263
			
select count(*) from ms_flow_down_org; -- 243177 

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 indexes are present only on table ms_flow_down_org for columns :  ACTIVITY_NAME, ORG_ENTITY_ID, USER_ID, USER_NAME [No composite index present] but even index creation on "ms_grc_parent_orgs" table doesn't help.
No change in the plan or on time.

Please help on this.
0
Swadhin Ray
Asked:
Swadhin Ray
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5 Solutions
 
Guy Hengel [angelIII / a3]Billing EngineerCommented:
ms_grc_parent_orgs => 1 index on org_hierarchy_name
ms_flow_down_org => 1 composite index on  org_entity_id +  user_name

ensure index stats are updated

next levels to optimize are:
* indexes on dedicated tablespaces
* partitioned table

however, with so few records, the db taking 12 seconds the issue is rather at instance level:
* not enough RAM on the box
* not enough RAM assigned to the instance
* too slow disks
* disks too occupied
* too many waits
0
 
Swadhin RaySenior Technical Engineer Author Commented:
After creating the index the cost was reduced but the CPU % got increased :

create index  MGPO_ORG_HIERARCHY_NAME_IDX on MS_GRC_PARENT_ORGS(ORG_HIERARCHY_NAME);
create index  MFDO__OEI_USER_NAME_IDX on MS_FLOW_DOWN_ORG(ORG_ENTITY_ID,USER_NAME );

---OUTPUT 
index MGPO_ORG_HIERARCHY_NAME_IDX created.
index MFDO__OEI_USER_NAME_IDX created.

explain plan for
 SELECT org_entity_name
       ,org_entity_id
       ,org_hierarchy_name
       ,division
       ,area
       ,organization
   FROM ms_grc_parent_orgs mgpo
  WHERE EXISTS (SELECT 1
           FROM ms_flow_down_org
          WHERE org_entity_id = mgpo.org_entity_id
                AND user_name = 'E131094')
        AND org_hierarchy_name = 'Level 4 - Department';

select * from table(dbms_xplan.display);

Plan hash value: 1039081615
 
-------------------------------------------------------------------------------------------------
| Id  | Operation             | Name                    | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |                         |     1 |   141 |   229   (2)| 00:00:03 |
|*  1 |  HASH JOIN SEMI       |                         |     1 |   141 |   229   (2)| 00:00:03 |
|*  2 |   TABLE ACCESS FULL   | MS_GRC_PARENT_ORGS      |   161 | 20930 |     5   (0)| 00:00:01 |
|*  3 |   INDEX FAST FULL SCAN| MFDO__OEI_USER_NAME_IDX |  3524 | 38764 |   223   (1)| 00:00:03 |
-------------------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   1 - access("ORG_ENTITY_ID"="MGPO"."ORG_ENTITY_ID")
   2 - filter("ORG_HIERARCHY_NAME"='Level 4 - Department')
   3 - filter("USER_NAME"='E131094')
 
Note
-----
   - 'PLAN_TABLE' is old version

ALTER INDEX MGPO_ORG_HIERARCHY_NAME_IDX
  REBUILD COMPUTE STATISTICS;

ALTER INDEX MFDO__OEI_USER_NAME_IDX
  REBUILD COMPUTE STATISTICS;	

Still the plan was same: 

Plan hash value: 1039081615
 
-------------------------------------------------------------------------------------------------
| Id  | Operation             | Name                    | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |                         |     1 |   141 |   229   (2)| 00:00:03 |
|*  1 |  HASH JOIN SEMI       |                         |     1 |   141 |   229   (2)| 00:00:03 |
|*  2 |   TABLE ACCESS FULL   | MS_GRC_PARENT_ORGS      |   161 | 20930 |     5   (0)| 00:00:01 |
|*  3 |   INDEX FAST FULL SCAN| MFDO__OEI_USER_NAME_IDX |  3524 | 38764 |   223   (1)| 00:00:03 |
-------------------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   1 - access("ORG_ENTITY_ID"="MGPO"."ORG_ENTITY_ID")
   2 - filter("ORG_HIERARCHY_NAME"='Level 4 - Department')
   3 - filter("USER_NAME"='E131094')
 
Note
-----
   - 'PLAN_TABLE' is old version

Open in new window

0
 
Guy Hengel [angelIII / a3]Billing EngineerCommented:
and the time of the actual query?
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Swadhin RaySenior Technical Engineer Author Commented:
is taking around 9.5-10.5 Secs
0
 
slightwv (䄆 Netminder) Commented:
For the inner select, try only bringing back the 'first' row?

 WHERE EXISTS (SELECT 1
           FROM ms_flow_down_org
          WHERE org_entity_id = mgpo.org_entity_id
                AND user_name = 'E131094'
                AND rownum=1)
        AND org_hierarchy_name = 'Level 4 - Department';

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Maybe use the first rows hint:

...
SELECT /*+ FIRST_ROWS */ 1
...
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Alexander Eßer [Alex140181]Software DeveloperCommented:
SELECT /*+ FIRST_ROWS */ 1

How many "first rows" does the CBO try to fetch?!
I thought this hint needs a certain number of rows like: ?!?

select /*+ first_rows(1) */ 1 from...

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I don't see the parameter for this hint declared as optional in the official docs:
https://docs.oracle.com/cd/E11882_01/server.112/e41084/sql_elements006.htm#BABGCCFG
0
 
johnsoneSenior Oracle DBACommented:
The FIRST_ROWS hint has nothing to do with the number of rows to be returned.  Essentially the default behavior of the CBO is what is the ALL_ROWS hint.  This tends toward full table scans.  A FIRST_ROWS hint tends toward nested loops.  The emphasis is on getting back the first rows the fastest.

Given the size of the tables, it doesn't look to me like you would ever get to use an index on the outer table.  What if you switched the order of the 2 columns in the MFDO__OEI_USER_NAME_IDX index?  That might get you an index range scan rather than a full scan.  That might save some time.
0
 
Alexander Eßer [Alex140181]Software DeveloperCommented:
The FIRST_ROWS hint has nothing to do with the number of rows to be returned.
That is not fully true, as shown here:

https://docs.oracle.com/cd/E11882_01/server.112/e41084/sql_elements006.htm#BABGCCFG

The FIRST_ROWS hint instructs Oracle to optimize an individual SQL statement for fast response, choosing the plan that returns the first n rows most efficiently. For integer, specify the number of rows to return.

So, again: Obviously this "integer" parameter is not optional. What does Oracle / the CBO do if you ommit this value?!
0
 
slightwv (䄆 Netminder) Commented:
>>I thought this hint needs a certain number of rows like: ?!?

Never knew about the number parameter.  I try to avoid hints and let the optimizer do its magic.  The number is optional.  Not sure what the benefit to specifying the number is.

You would just need to experiment to see any possible benefits.
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Alexander Eßer [Alex140181]Software DeveloperCommented:
Never knew about the number parameter.  I try to avoid hints and let the optimizer do its magic.
I totally agree with you ;-)
But, nevertheless, if this parameter is optional then the official docs are wrong about this one...
0
 
johnsoneSenior Oracle DBACommented:
I never used the integer hint.  It is a optimizer path hint.  It tells the optimizer which join method to try to use.  ALL_ROWS will tend to use hash joins and FIRST_ROWS will tend to use nested loops.
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Alexander Eßer [Alex140181]Software DeveloperCommented:
Got it:
https://docs.oracle.com/cd/B19306_01/server.102/b14200/sql_elements006.htm

The FIRST_ROWS hint specified without an argument, which optimizes for the best plan to return the first single row, is retained for backward compatibility and plan stability only.

Obviously deprecated...
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johnsoneSenior Oracle DBACommented:
We tend to not use hints.  The optimizer has gotten much better so we don't find that we need to use them that often.

FIRST_ROWS is a blanket hint, we tried not to use it.  Hinting the optimizer to use specific indexes was a much better way to go than using this hint.
0
 
slightwv (䄆 Netminder) Commented:
Getting way off topic.

I think the more important part of my post was:  AND rownum=1
0
 
sdstuberCommented:
Try using this hint

/*+ OPT_PARAM('optimizer_dynamic_sampling' 6) */

you can make the sampling higher (but not higher than 9)
if it changes the plan, try gathering extended statistics and then remove the hint

I would not use the ROWNUM=1 method
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sdstuberCommented:
>>> I have the below SQL which is taking 12 sec to execute when attached to a web application:


What is the execution time of the query by itself - without the web app?
0
 
Swadhin RaySenior Technical Engineer Author Commented:
after making the changes as suggested :

 SELECT org_entity_name
       ,org_entity_id
       ,org_hierarchy_name
       ,division
       ,area
       ,organization
   FROM ms_grc_parent_orgs mgpo
  WHERE EXISTS (SELECT /*+ OPT_PARAM('optimizer_dynamic_sampling' 6) */ 1
           FROM ms_flow_down_org
          WHERE org_entity_id = mgpo.org_entity_id
                AND user_name = 'E131094'
                and rownum =1)
        AND org_hierarchy_name = 'Level 4 - Department';

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Now it is taking 0.7 sec
0
 
sdstuberCommented:
you should not need rownum=1 that's just a wasted step in the plan

the HASH JOIN SEMI was already doing the same thing (or should have anyway - that's what the "SEMI" means)

after your remove the rownum, get full plan with the predicate clauses
0
 
sdstuberCommented:
On the assumption that the improvement is because the dynamic sampling found correlation between org_entity_id and user_name, gathering extended statistics would look something like this...

DECLARE
    v_dummy VARCHAR2(100);
BEGIN
    v_dummy :=
        DBMS_STATS.create_extended_stats(
            'YOUR_SCHEMA',
            'MS_FLOW_DOWN_ORG',
            '(ORG_ENTITY_ID,USER_NAME)'
        );
    DBMS_STATS.gather_table_stats('YOUR_SCHEMA', 'MS_FLOW_DOWN_ORG');
END;

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and after that you should hopefully be able to remove the hint but still gain the benefit.
0
 
Swadhin RaySenior Technical Engineer Author Commented:
This question was really a learning for me. Thanks a lot experts for all your comments.


Thanks,
Sloba
0
 
Swadhin RaySenior Technical Engineer Author Commented:
Awesome inputs provided on this question...
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