Hi
I was using a query previously, that was efficient
select * from table where pred1 and pred2 and pred3;
Later I was asked to introduce new ones, but they were not based on
table columns but variables declared in SP.
select * from table where pred1 and pred2 and pred3 and variable1
='number1 and variable2 =number2;
so in turn this translates to pure mathematical comparison, sometimes
simple like 2=2,
I;ve added only 2 of them in the ex above, but actually there are 13
(does this influence the path, duh!!). Anyways, what happened was,
after they were added, it started doing a table scan, and was very
very inefficient. Later, I converted the additional predicates into
simple if loops, and executed the original query, and SP was very
fast.
Is there any database logic connected to doing a table scan? or any
registry/variable setting I can do so db2 doesnt do the table scan?
Please advise, since my team has done this mistake in many places,
and rolling back is going to be a pain in backside. 4 3885
Arun Srinivasan wrote:
Hi
I was using a query previously, that was efficient
select * from table where pred1 and pred2 and pred3;
Later I was asked to introduce new ones, but they were not based on
table columns but variables declared in SP.
select * from table where pred1 and pred2 and pred3 and variable1
='number1 and variable2 =number2;
so in turn this translates to pure mathematical comparison, sometimes
simple like 2=2,
I;ve added only 2 of them in the ex above, but actually there are 13
(does this influence the path, duh!!). Anyways, what happened was,
after they were added, it started doing a table scan, and was very
very inefficient. Later, I converted the additional predicates into
simple if loops, and executed the original query, and SP was very
fast.
Is there any database logic connected to doing a table scan? or any
registry/variable setting I can do so db2 doesnt do the table scan?
Please advise, since my team has done this mistake in many places,
and rolling back is going to be a pain in backside.
Which version and platform are you on?
New versions of Db2 for LUW typically "push out" "constant predicates".
That is:
SELECT * FROM T WHERE pred(T) AND pred(context)
is turned into:
SELECT * FROM (SELECT 1 FROM VALUES(1) WHERE pred(context)), (SELECT *
FROM T WHERE pred(T))
with the constant pred being the outer of a nested loop join.
You should be able to see this in a db2exfmt output
Cheers
Serge
--
Serge Rielau
DB2 Solutions Development
IBM Toronto Lab
I am really surprised at the following explains.
1. The correct one, with just one constant predicate check.
select ri_indv_id
from elg.artelgf
where client_cd ='300' and segment_type_cd ='003' and carrier_cd =
'60054' and ri_indv_id =11233 and covpf_rf ='MED' and
--- below is the only constant predicate check
value(
'01-01-2009' , CURRENT DATE)CURRENT DATE - 30 days
fetch first 1 rows only
Section Code Page = 1252
Estimated Cost = 38.574055
Estimated Cardinality = 0.047454
( 3) Table Constructor
| 1-Row(s)
( 3) Residual Predicate(s)
| #Predicates = 1
( 2) Nested Loop Join
( 2) | Access Table Name = ELG.ARTELGF ID = 47,4
| | Index Scan: Name = ELG.PK_ARTELGF2 ID = 6
| | | Regular Index (Not Clustered)
| | | Index Columns:
| | | | 1: CLIENT_CD (Ascending)
| | | | 2: SEGMENT_TYPE_CD (Ascending)
| | | | 3: CARRIER_CD (Ascending)
| | | | 4: RI_INDV_ID (Ascending)
| | | | 5: COVPF_RF (Ascending)
| | | | 6: GROUP_NUM (Ascending)
| | | | 7: POLICY_NUM (Ascending)
| | | | 8: POLICY_START_DT (Ascending)
| | | | 9: CARRIER_OFFICE_ CD (Ascending)
| | | | 10: SGSRC_RF (Ascending)
| | | | 11: QHIT_IND (Include Column)
| | #Columns = 0
| | Compressed Table
| | Skip Inserted Rows
| | Skip Deleted Keys
| | Skip Deleted Rows
| | #Key Columns = 5
| | | Start Key: Inclusive Value
| | | | | 1: '300 '
| | | | | 2: '003'
| | | | | 3: '60054'
| | | | | 4: 000000011233
| | | | | 5: 'MED'
| | | Stop Key: Inclusive Value
| | | | | 1: '300 '
| | | | | 2: '003'
| | | | | 3: '60054'
| | | | | 4: 000000011233
| | | | | 5: 'MED'
| | Index-Only Access
| | Index Prefetch: None
| | Lock Intents
| | | Table: Intent Share
| | | Row : Next Key Share
( 1) Return Data to Application
| #Columns = 1
End of section
Optimizer Plan:
RETURN
( 1)
|
NLJOIN
( 2)
/ \
TBSCAN IXSCAN
( 3) ( 2)
| / \
TFunc: Index: Table:
SYSIBM ELG ELG
GENROW PK_ARTELGF2 ARTELGF
2. Here we have the same query but havin 2 const predicate checks
instead of one and see what our db2 engine comes up with.
select ri_indv_id
from elg.artelgf
where client_cd ='300' and segment_type_cd ='003' and carrier_cd =
'60054' and ri_indv_id =11233 and covpf_rf ='MED' and
--below are two const predicate checks instead of 1
value(
'01-01-2009' , CURRENT DATE)CURRENT DATE - 30 days and ( (
'01-01-1999' between '01-01-2000' and '01-01-2009' AND days(
value('01-01-2009' , current date))- days('01-01-1999' )>=
30 ))
fetch first 1 rows only
Section Code Page = 1252
Estimated Cost = 0.000101
Estimated Cardinality = 0.000000
( 2) Table Constructor
| N-Rows
( 2) Residual Predicate(s)
| #Predicates = 1
( 1) Return Data to Application
| #Columns = 1
End of section
Optimizer Plan:
RETURN
( 1)
|
TBSCAN
( 2)
|
TFunc:
SYSIBM
GENROW
On Nov 14, 2:17*pm, Arun Srinivasan <arunro...@gmai l.comwrote:
I am really surprised at the following explains.
1. The correct one, with just one constant predicate check.
select ri_indv_id
* from elg.artelgf
* where client_cd ='300' and segment_type_cd ='003' and carrier_cd =
* * * * * '60054' and ri_indv_id =11233 and covpf_rf ='MED'and
--- below is the only constant predicate check
* * * * * value(
* * * * * '01-01-2009' , CURRENT DATE)CURRENT DATE - 30 days
* fetch first 1 rows only
Section Code Page = 1252
Estimated Cost = 38.574055
Estimated Cardinality = 0.047454
( * *3) Table Constructor
* * * * | *1-Row(s)
( * *3) Residual Predicate(s)
* * * * | *#Predicates = 1
( * *2) Nested Loop Join
( * *2) | *Access Table Name = ELG.ARTELGF *ID = 47,4
* * * * | *| *Index Scan: *Name = ELG.PK_ARTELGF2 *ID = 6
* * * * | *| *| *Regular Index (Not Clustered)
* * * * | *| *| *Index Columns:
* * * * | *| *| *| *1: CLIENT_CD (Ascending)
* * * * | *| *| *| *2: SEGMENT_TYPE_CD (Ascending)
* * * * | *| *| *| *3: CARRIER_CD (Ascending)
* * * * | *| *| *| *4: RI_INDV_ID (Ascending)
* * * * | *| *| *| *5: COVPF_RF (Ascending)
* * * * | *| *| *| *6: GROUP_NUM (Ascending)
* * * * | *| *| *| *7: POLICY_NUM (Ascending)
* * * * | *| *| *| *8: POLICY_START_DT (Ascending)
* * * * | *| *| *| *9: CARRIER_OFFICE_ CD (Ascending)
* * * * | *| *| *| *10: SGSRC_RF (Ascending)
* * * * | *| *| *| *11: QHIT_IND (Include Column)
* * * * | *| *#Columns = 0
* * * * | *| *Compressed Table
* * * * | *| *Skip Inserted Rows
* * * * | *| *Skip Deleted Keys
* * * * | *| *Skip Deleted Rows
* * * * | *| *#Key Columns = 5
* * * * | *| *| *Start Key: Inclusive Value
* * * * | *| *| *| *| *1: '300 *'
* * * * | *| *| *| *| *2: '003'
* * * * | *| *| *| *| *3: '60054'
* * * * | *| *| *| *| *4: 000000011233
* * * * | *| *| *| *| *5: 'MED'
* * * * | *| *| *Stop Key: Inclusive Value
* * * * | *| *| *| *| *1: '300 *'
* * * * | *| *| *| *| *2: '003'
* * * * | *| *| *| *| *3: '60054'
* * * * | *| *| *| *| *4: 000000011233
* * * * | *| *| *| *| *5: 'MED'
* * * * | *| *Index-Only Access
* * * * | *| *Index Prefetch: None
* * * * | *| *Lock Intents
* * * * | *| *| *Table: Intent Share
* * * * | *| *| *Row *: Next Key Share
( * *1) Return Data to Application
* * * * | *#Columns = 1
End of section
Optimizer Plan:
* * * * *RETURN
* * * * *( * 1)
* * * * * *|
* * * * *NLJOIN
* * * * *( * 2)
* * * * / * * *\
*TBSCAN * * * * IXSCAN
*( * 3) * * * * ( * 2)
* *| * * * * * / * * *\
*TFunc: *Index: * * * Table:
*SYSIBM *ELG * * * * *ELG
*GENROW *PK_ARTELGF2 *ARTELGF
2. Here we have the same query but havin 2 const predicate checks
instead of one and see what our db2 engine comes up with.
*select ri_indv_id
* from elg.artelgf
* where client_cd ='300' and segment_type_cd ='003' and carrier_cd =
* * * * * '60054' and ri_indv_id =11233 and covpf_rf ='MED'and
--below are two const predicate checks instead of 1
*value(
* * * * * '01-01-2009' , CURRENT DATE)CURRENT DATE - 30 days and ( (
* * * * * '01-01-1999' between '01-01-2000' and '01-01-2009' AND days(
* * * * * value('01-01-2009' , current date))- days('01-01-1999' )>=
* * * * * 30 ))
* fetch first 1 rows only
Section Code Page = 1252
Estimated Cost = 0.000101
Estimated Cardinality = 0.000000
( * *2) Table Constructor
* * * * | *N-Rows
( * *2) Residual Predicate(s)
* * * * | *#Predicates = 1
( * *1) Return Data to Application
* * * * | *#Columns = 1
End of section
Optimizer Plan:
*RETURN
*( * 1)
* *|
*TBSCAN
*( * 2)
* *|
*TFunc:
*SYSIBM
*GENROW
I got it, actually db2 is far intelligent than what I thought :-p the
reason for this behaviour being, since one of the
constant predicates check was false, it didnt even go to the real
query.
Hail DB2,
Arun Srinivasan wrote:
>Optimizer Plan:
RETURN
( 1)
|
NLJOIN
( 2)
/ \
TBSCAN IXSCAN
( 3) ( 2)
| / \
TFunc: Index: Table:
SYSIBM ELG ELG
GENROW PK_ARTELGF2 ARTELGF
That's the push out I described (GENROW == VALUES).
>Optimizer Plan:
RETURN
( 1)
|
TBSCAN
( 2)
|
TFunc:
SYSIBM
GENROW
I got it, actually db2 is far intelligent than what I thought :-p the
reason for this behaviour being, since one of the
constant predicates check was false, it didnt even go to the real
query.
Hail DB2,
Indeed.
Cheers
Serge
--
Serge Rielau
DB2 Solutions Development
IBM Toronto Lab This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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