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Query Tuning - Help Needed
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chinnielr

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Joined: 11 Jul 2005
Posts: 87

PostPosted: Wed May 26, 2010 3:38 pm    Post subject: Query Tuning - Help Needed
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Hi,

One of my query is running close to 45 mins. I am trying to fine tune the query and I ran EXPLAIN. Please find below the details from PLAN_TABLE.

Can you please help me in identifying the area(s) for fine tuning based on the below EXPLAIN report.

Code:


QUERYNO   QBLOCKNO   PLANNO   METHOD   TNAME   ACCESSTYPE   

1           1           1         0     ASUK        N
1           1           2         1     TBLM        I
1           1           3         2     ACH         I
1           1           4         3     --          -


MATCHCOLS   INDEXONLY   SORTC_UNIQ   SORTC_ORDERBY   
2            Y           N                    N                 
3            N           N                    N 
2            Y           N                    N
0            N           Y                    Y


TSLOCKMODE   PREFETCH   MIXOPSEQ   CORRELATION_NAME   
IS           --           0                    U
IS           --           0                    C
IS           --           0                    M
--           --           0                    --


PAGE_RANGE   QBLOCK_TYPE
     
 Y            SELECT
 -            SELECT
 -            SELECT
 -            SELECT


Appreciate your help!!!

Thanks in advance,
Chinni.
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dbzTHEdinosauer

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PostPosted: Wed May 26, 2010 5:04 pm    Post subject:
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6.3 Chapter 6-3. Tuning your queries

6.4 Chapter 6-4. Using EXPLAIN to improve SQL performance
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ashimer

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PostPosted: Wed May 26, 2010 6:08 pm    Post subject:
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Where is the query or do i miss something ?
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chinnielr

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PostPosted: Wed May 26, 2010 6:09 pm    Post subject: Reply to: Query Tuning - Help Needed
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Hi dbzTHEdinosauer,

Thanks for your kind details and quick response. I have referred the links given by you and I am not able to see any issues with the SQL. Is this SQL is already an optimized one?

Please find below my SQL. Can you please suggest some hints on how to proceed further in tuning the SQL if I am missing something.

Is the DISTINCT culprit here?

Code:


SELECT DISTINCT(U.CL_NBR) ,                   
                U.CL_ACC_NBR,                 
                U.UH_NBR,                 
                U.COMPANY,                 
                C.CON_NBR,                   
                C.CODE,                         
                C.DDATE,                         
                C.DTIME,                         
                C.NET_CONS,                 
                C.SL_RPC,                   
                C.SUB_TYPE,                     
                C.STATUS,                   
                C.AT_CODE,                         
                C.UNITS,                   
                C.RATE                       
                                                     
FROM  DB06.ASUK    U,                     
      DB06.TBLM    M,                     
      DB06.ACH     C                     
                                                     
WHERE (U.COMPANY  =  'AB' AND             
       U.CODE IN ('RRR','TRE','PRP','CON',     
                        'RRT','RRU','RRV','RRW'))     
AND   U.CL_NBR       >  ''                     
AND   C.COMPANY  =   U.COMPANY
AND   M.CODE          =   U.CODE         
AND   M.CODE          =   C.CODE         
AND   M.UH_NBR  =   U.UH_NBR
AND   C.UH_NBR  =   U.UH_NBR
AND   C.DDATE           >= :S_DATE           
                                                     
ORDER BY CL_NBR, CL_ACC_NBR ;           
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ashimer

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PostPosted: Wed May 26, 2010 6:18 pm    Post subject:
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Even if you remove DISTINCT also, an order by is there for DB2 to do a sort.
Table ASUK data can be minimized first using a inline view and then use that data to join to other two tables.

Code:

SELECT DISTINCT(U.CL_NBR) ,                   
                U.CL_ACC_NBR,                 
                U.UH_NBR,                 
                U.COMPANY,                 
                C.CON_NBR,                   
                C.CODE,                         
                C.DDATE,                         
                C.DTIME,                         
                C.NET_CONS,                 
                C.SL_RPC,                   
                C.SUB_TYPE,                     
                C.STATUS,                   
                C.AT_CODE,                         
                C.UNITS,                   
                C.RATE                       
                                                     
FROM
(SELECT
CL_NBR ,                   
CL_ACC_NBR,                 
UH_NBR,                 
COMPANY,
CODE
FROM
 DB06.ASUK
WHERE
COMPANY  =  'AB' AND
       AND CL_NBR       >  ''                                 
       AND CODE IN ('RRR','TRE','PRP','CON',     
                        'RRT','RRU','RRV','RRW'))   AS U
      DB06.TBLM    M,                     
      DB06.ACH     C                     
                                                     
WHERE   

C.COMPANY  =   U.COMPANY
AND   M.CODE          =   U.CODE         
AND   M.CODE          =   C.CODE         
AND   M.UH_NBR  =   U.UH_NBR
AND   C.UH_NBR  =   U.UH_NBR
AND   C.DDATE           >= :S_DATE           
                                                     
ORDER BY CL_NBR, CL_ACC_NBR ; 


This will reduce the number of rows required for joining.
Please try.
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chinnielr

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PostPosted: Wed May 26, 2010 6:30 pm    Post subject: Reply to: Query Tuning - Help Needed
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Hi Ashimer,

Thanks for quick turn around. Will the changed query provided by you reduce the running time of the query?

Also can you please explain me the sub-query written in your query?

Regards,
Chinni
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ashimer

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PostPosted: Wed May 26, 2010 6:38 pm    Post subject:
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Try it out ; it's not a subquery - instead of joining the entire table ASUK, i am just using only those rows which satisfies the conditions.
It is safe to filter rows and then join.
Try to EXPLAIN this query too and see what difference does it make.
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chinnielr

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PostPosted: Wed May 26, 2010 7:43 pm    Post subject: Reply to: Query Tuning - Help Needed
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Hi Ashimer,

I ran the query and unfortunaly there is no improvement in the run time.

When I see the ACCESSTYPE type in EXPLAIN_TABLE ( as below) it shows as 'N' for the table ASUK for other tables it shows as 'Y'. Also METHOD shows as 0 for ASUK table. Are these the culprits?

Please suggest me a solution in tuning the query.

Regards,
Chinni.
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GuyC

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PostPosted: Wed May 26, 2010 8:05 pm    Post subject:
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no, method 0 just means first table : there will always be a first table.

and accesstype N is not bad, it means it uses the values in the IN-list which is good.

you need to give more info on #rows and available indexes
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ashimer

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PostPosted: Wed May 26, 2010 10:01 pm    Post subject:
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Sorry to hear that. Please provide all info.
Of tables ASUK and ACH which table is smaller ? Making the smaller table as outer one in a join will help to reduce the number of times the inner table must be re-accessed. The conditions COMPANY and CODE can be applied to ACH too making it the outer table as selective predicates are available. These are just assumptions without knowing the indexes ; check when was RUNSTATS / REORG done on these tables last.
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dick scherrer

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PostPosted: Thu May 27, 2010 12:01 am    Post subject:
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Hello,

Many times writing the smallest amount of code results in poor performance - regardless of how well the query is written. The fastest possible 1,000,000 database accesses will still take considerable resources.

Depending on the actual processing, it is often possible to reduce processing resource requirements by 90-95%.

Many processes test perfectly with tiny amounts of data. When full volume is used for the first time in Production (which, imho, should never happen), there is an instant panic. . .
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rocky_balboa

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PostPosted: Thu May 27, 2010 12:47 am    Post subject: Reply to: Query Tuning - Help Needed
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ashimer

Quote:
an order by is there for DB2 to do a sort.


Not necessary....
Appropriate indexing may permit DB2 to avoid sorting the data to match the ORDER BY clause. If the data is already sequenced via the index, DB2 may choose to use the index and avoid sorting the data.

Quote:
Making the smaller table as outer one in a join will help to reduce the number of times the inner table must be re-accessed.


DB2's choice of inner and outer table is a delicate trade off. More the no. of inner table rows ,longer the scan...if outer rows are more then no. of opening and closing of internal cursor on inner table are more... DB2 favours the larger table as the outer table in a nested loop join...

chinnielr

Check whether the columns indexed and used by are also clustered....this info you will not get in plan table...You will need to query DB2 catalog.

If INDEXONLY is N then tablespace data pages are also being used... so check of you have appropriate indexes defined....

You might also want to refer to columns PREFETCH,JOIN_DEGREE for more information...

Also you might need to rewrite the query,tweaking some parts...

Query optimization can be daunting...but you need to keep trying...
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ashimer

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PostPosted: Thu May 27, 2010 10:33 am    Post subject:
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Quote:

DB2's choice of inner and outer table is a delicate trade off. More the no. of inner table rows ,longer the scan...if outer rows are more then no. of opening and closing of internal cursor on inner table are more... DB2 favours the larger table as the outer table in a nested loop join...


I object.

Please read http://publib.boulder.ibm.com/infocenter/dzichelp/v2r2/index.jsp?topic=/com.ibm.db29.doc.perf/db2z_nestedloopjoin.htm

Quote:

Not necessary....
Appropriate indexing may permit DB2 to avoid sorting the data to match the ORDER BY clause. If the data is already sequenced via the index, DB2 may choose to use the index and avoid sorting the data.


Agree on a general case; but here please see columns SORTC_UNIQ and SORTC_ORDERBY of PLAN table.

Quote:

If INDEXONLY is N then tablespace data pages are also being used... so check of you have appropriate indexes defined....


Obviously DB2 has to go for data pages else all the fetched columns must be in the index - what say ?
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rocky_balboa

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PostPosted: Thu May 27, 2010 4:31 pm    Post subject: Reply to: Query Tuning - Help Needed
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Quote:
The outer table is small.


Don't let this statement(in the link) blur your conclusions of the NLJOIN....As you can see my post , I have not made a sweeping statement that NLJOIN will always use a larger table.None of these are hard and fast rules...DB2 will even use the larger table as outer if selective predicates applies to it....similarly if there is no index ...then even if the table is small it is not used as inner table.



Quote:
Obviously DB2 has to go for data pages else all the fetched columns must be in the index - what say ?



having an index defined and being used in the query is a necessary condition..... not sufficient....that is why I used the term "appropriate".
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ashimer

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PostPosted: Thu May 27, 2010 4:58 pm    Post subject:
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I was contradicting your statement "DB2 favours the larger table as the outer table in a nested loop join" - dont worry my visions are not blurred by the link; that was just provided as a documentation.

I was emphasizing on the current SQL where we have indexes , probably a healthy MATCHCOLS , ACCESSTYPE.
ASUK has a method 0 , but not performing well; why dont we change that and make ACH in place of ASUK as the outer table.

We better focus on the issue here rather than generalizing.
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ashimer

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PostPosted: Thu May 27, 2010 5:06 pm    Post subject:
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None of these are "hard and fast rules" ; i am just referring to the o/p of the EXPLAIN posted above and suggesting what can be done here ; not having a discussion on DB2.
Let's leave it to the optimizer to take the final decision icon_biggrin.gif
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rocky_balboa

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PostPosted: Thu May 27, 2010 6:03 pm    Post subject:
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focussuing on just some particular rules can be very dangerous unless you are aware of the general rules ... have a clear macro view only then you can go for a micro focus....which is what I was trying to share with chinnielr...so that he/she could be better informed.

An again a "healthy MATCHCOLS" does not gurantee a great performance.
Such data mask much more information than they reveal.... icon_smile.gif

Obviously we can't help chinnielr further... because we neither know the data nor the # of rows nor the various DB2 subsystem parameters(Sort pool,RID pool,buffer pool....)
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chinnielr

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PostPosted: Fri May 28, 2010 2:04 pm    Post subject:
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Hi All,

Thanks for all your inputs.

BTW when I ran the changed query as mentioned by ashimer it is taking more time than the original one.

I am providing some more info on the indexes for these tables.

Please pardon me if there is an issue with formatting.

I understand that all the columns under where clause have indexes defined. But still I am not sure why there is a performance issue with this query.

Can somebody help me out to resolve this issue.

Code:


Index details for TBLM
TABLE                                                                                                                       INDEX
  NAME                    NO  NAME                DATATYPE                 NULL      PK   FK   REFERENTAB          INDEXED  NAME
  ------------------  ------  ------------------  -----------------------  --------  ---  ---  ------------------  -------  ------------------
TBLM                     1        CODE              CHAR(3)                NOT NULL  YES                           YES       TTYNUHA1
TBLM                     1        CODE              CHAR(3)                NOT NULL  YES                           YES       TTYNUHA2
TBLM                     2        UH_NBR            CHAR(8)                NOT NULL  YES                           YES      TTYNUHA1
TBLM                     2        UH_NBR            CHAR(8)                NOT NULL  YES                           YES      TTYNUHA2
TBLM                     3        REG_DATE          DATE                   NOT NULL  YES                           YES      TTYNUHA1
TBLM                     3        REG_DATE          DATE                   NOT NULL  YES                           YES      TTYNUHA2
TBLM                     4        TRN_SL_NBR        CHAR(7)                NOT NULL  YES                           YES      TTYNUHA1
TBLM                     5        TRN_TYPE          CHAR(2)                NOT NULL  YES                           YES      TTYNUHA1
TBLM                     5        TRN_TYPE          CHAR(2)                NOT NULL  YES                           YES      TTYNUHA2
TBLM                     6        TRN_SL_NO_PRE     CHAR(3)                NOT NULL
TBLM                     7        EFF_DATE          DATE                   NOT NULL
TBLM                     8        UNITS             DECIMAL(15,4)          NOT NULL
TBLM                     9        DIST_NBR          DECIMAL(3,0)           NULL
TBLM                     10       TRSFR_UH_NBR      CHAR(8)                NULL
TBLM                     11       REG_DEREG_IND     CHAR(1)                NOT NULL                                 YES     TTYNUHA2
TBLM                     12       PRC               DECIMAL(9,4)           NOT NULL
TBLM                     13       REN_COM_AG        CHAR(7)                NULL
TBLM                     14       AQU_DATE          DATE                   NULL
TBLM                     15       CERT_LOST_IND     CHAR(1)                NOT NULL
TBLM                     16       XD_IND            CHAR(1)                NOT NULL
TBLM                     17       UNITS_REN         DECIMAL(15,4)          NOT NULL
TBLM                     18       REPLACE_REQ_FLAG  CHAR(2)                NOT NULL
TBLM                     19       REPURC_CON_NO     CHAR(10)               NOT NULL
TBLM                     20       DISP_PRC          DECIMAL(9,4)           NOT NULL
TBLM                     21       REPLACEMENT_DATE  DATE                   NULL
TBLM                     22       SHR_CL_SW         CHAR(1)                NULL
TBLM                     23       SCS               CHAR(3)                NOT NULL


Code:


index table for ACH 
TABLE                                                                                                                     INDEX
  NAME                    NO  NAME                DATATYPE                 NULL      PK   FK   REFERENTAB          INDEXED  NAME
  ------------------  ------  ------------------  -----------------------  --------  ---  ---  ------------------  -------  ------------------
  ACH                 1     CODE              CHAR(3)                  NOT NULL  YES  YES     TRUST               YES      TTYNCO1
  ACH                 1     CODE               CHAR(3)                  NOT NULL  YES  YES     TRUST            YES      TTYNCO2
  ACH                1     CODE              CHAR(3)                  NOT NULL  YES  YES     TRUST            YES      TTYNCO3
  ACH                 1     CODE               CHAR(3)                  NOT NULL  YES  YES     TRUST            YES      TTYNCO4
  ACH                 1     CODE               CHAR(3)                  NOT NULL  YES  YES     TRUST            YES      TTYNCO5
  ACH                 1     CODE               CHAR(3)                  NOT NULL  YES  YES     TRUST            YES      TTYNCO6
  ACH                 2     CON_NBR            CHAR(7)                  NOT NULL  YES                           YES      TTYNCO1
  ACH                 2     CON_NBR            CHAR(7)                  NOT NULL  YES                           YES      TTYNCO3
  ACH                 2     CON_NBR            CHAR(7)                  NOT NULL  YES                           YES      TTYNCO4
  ACH                 2     CON_NBR            CHAR(7)                  NOT NULL  YES                           YES      TTYNCO6
  ACH                 3     SL_RPC             CHAR(1)                  NOT NULL                                YES      TTYNCO2
  ACH                 3     SL_RPC             CHAR(1)                  NOT NULL                                YES      TTYNCO3
  ACH                 3     SL_RPC             CHAR(1)                  NOT NULL                                YES      TTYNCO5
  ACH                 3     SL_RPC             CHAR(1)                  NOT NULL                                YES      TTYNCO8
  ACH                 4     SUB_TYPE           CHAR(1)                  NOT NULL
  ACH                 5     C_DATE             DATE                     NOT NULL                                YES      TTYNCO2
  ACH                 5    C_DATE             DATE                     NOT NULL                                YES      TTYNCO3
  ACH                 5     C_DATE             DATE                     NOT NULL                                YES      TTYNCO5
  ACH                 6     STATUS             CHAR(1)                  NOT NULL                                YES      TTYNCO2
  ACH                 6     STATUS             CHAR(1)                  NOT NULL                                YES      TTYNCO3
  ACH                 6     STATUS             CHAR(1)                  NOT NULL                                YES      TTYNCO5
  ACH                25     COMPANY            CHAR(2)                  NOT NULL                                YES      TTYNCO2
  ACH                25     COMPANY            CHAR(2)                  NOT NULL                                YES      TTYNCO3
  ACH                25     COMPANY            CHAR(2)                  NOT NULL                                YES      TTYNCO4
  ACH                25     COMPANY            CHAR(2)                  NOT NULL                                YES      TTYNCO5
  ACH                25     COMPANY            CHAR(2)                  NOT NULL                                YES      TTYNCO6
  ACH                26     AT_CODE            CHAR(7)                  NULL                                    YES      TTYNCO2
  ACH                26     AT_CODE            CHAR(7)                  NULL                                    YES      TTYNCO3
  ACH                27     CAMP_CODE          CHAR(2)                  NULL                                    YES      TTYNCO5
  ACH                28     UH_NBR             CHAR(8)                  NULL                                    YES      TTYNCO4
  ACH                28     UH_NBR             CHAR(8)                  NULL                                    YES      TTYNCO6
  ACH                29     INFO_CODE_1        CHAR(10)                 NULL
  ACH                30     INFO_CODE_2        CHAR(10)                 NULL
  ACH                31     INV_TYPE           CHAR(2)                  NULL                                    YES      TTYNCO8
  ACH                32     EXT_CON            CHAR(10)                 NULL
  ACH                44     EFF_DATE_VERSION_1 DATE                     NOT NULL                                YES      TTYNCO8
  ACH                51     DSTATION           CHAR(2)                  NULL                                    YES      TTYNCO7
  ACH                52     DNUMBER            CHAR(6)                  NULL                                    YES      TTYNCO7



Code:

index tabole for ASUK   

TABLE                                                                                                                     INDEX
  NAME                    NO  NAME                DATATYPE                 NULL      PK   FK   REFERENTAB          INDEXED  NAME
  ------------------  ------  ------------------  -----------------------  --------  ---  ---  ------------------  -------  ------------------
             ASUK          1        CODE          CHAR(3)                  NOT NULL  YES  YES     TRUST            YES      TTYNFBC1
             ASUK          1        CODE          CHAR(3)                  NOT NULL  YES  YES     TRUST            YES      TTYNFBC2
             ASUK          1        CODE          CHAR(3)                  NOT NULL  YES  YES     TRUST            YES      TTYNFBC3
             ASUK          2        UH_NBR        CHAR(8)                  NOT NULL  YES                           YES      TTYNFBC1
             ASUK          2        UH_NBR        CHAR(8)                  NOT NULL  YES                           YES      TTYNFBC2
             ASUK          2        UH_NBR        CHAR(8)                  NOT NULL  YES                           YES      TTYNFBC3
             ASUK          3        COMPANY       CHAR(2)                  NOT NULL       YES  CLIENT_ACC          YES      TTYNFBC2
             ASUK          3        COMPANY       CHAR(2)                  NOT NULL       YES  CLIENT_ACC          YES      TTYNFBC3
             ASUK          4        UNITS         DECIMAL(15,4)            NOT NULL                                YES      TTYNFBC2
             ASUK          4        UNITS         DECIMAL(15,4)            NOT NULL                                YES      TTYNFBC3
             ASUK          5        MTYPE         CHAR(1)                  NOT NULL
             ASUK          6        PMANDATE      CHAR(1)                  NOT NULL
             ASUK          7        CL_ACC_NBR    DECIMAL(9,0)             NOT NULL       YES  CLIENT_ACC          YES      TTYNFBC2
             ASUK          7        CL_ACC_NBR    DECIMAL(9,0)             NOT NULL       YES  CLIENT_ACC          YES      TTYNFBC3
             ASUK          8        CL_NBR        CHAR(8)                  NOT NULL       YES  CLIENT_ACC          YES      TTYNFBC2
             ASUK          8        CL_NBR        CHAR(8)                  NOT NULL       YES  CLIENT_ACC          YES      TTYNFBC3


Regards,
Chinni.
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chinnielr

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Joined: 11 Jul 2005
Posts: 87

PostPosted: Fri May 28, 2010 2:12 pm    Post subject: Reply to: Query Tuning - Help Needed
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Also please let me know how to find out when RUNSTATS & REORG ran last on these tables?

Regards,
Chinni
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ashimer

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Joined: 13 Feb 2004
Posts: 551
Location: Bangalore

PostPosted: Fri May 28, 2010 3:09 pm    Post subject:
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Which one did you try ? post the details (definition) of the index which are being used as shown in the EXPLAIN ; also a rough estimate of data in each table - how many ?
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