Optimize this query getting exceed recourse limit - sql

Question

SELECT DISTINCT
A.IDPRE
,A.IDARTB
,A.TIREGDAT
,B.IDDATE
,B.IDINFO
,C.TIINTRO
FROM
GLHAZQ A
,PRTINFO B
,PRTCON C
WHERE
B.IDARTB = A.IDARTB
AND B.IDPRE = A.IDPRE
AND C.IDPRE = A.IDPRE
AND C.IDARTB = A.IDARTB
AND C.TIINTRO = (
SELECT MIN(TIINTRO)
FROM
PRTCON D
WHERE D.IDPRE = A.IDPRE
AND D.IDARTB = A.IDARTB)
ORDER BY C.TIINTRO
I get below error when I run this query(DB2)
SQL0495N Estimated processor cost of "000000012093" processor seconds
("000575872000" service units) in cost category "A" exceeds a resource limit error
threshold of "000007000005" service units. SQLSTATE=57051
Please help me to fix this problem

Answer 1

Apparently, the workload manager is doing its job in preventing you from using too many resources. You'll need to tune your query so that its estimated cost is lower than the threshold set by your DBA. You would start by examining the query explain plan as produced by db2exfmt. If you want help, publish the plan here, along with the table and index definitions.
To produce the explain plan, perform the following 3 steps:
Create explain tables by executing db2 -tf $INSTANCE_HOME/sqllib/misc/EXPLAIN.DDL
Generate the plan by executing the explain statement: db2 explain plan for select ...<the rest of your query>
Format the plan: db2exfmt -d <your db name> -1 (note the second parameter is the digit "1", not the letter "l").
To generate the table DDL statements use the db2look utility:
db2look -d <your db name> -o tables.sql -e -t GLHAZQ PRTINFO PRTCON

Answer 2

Although not a db2 person, but I would suspect query syntax is the same. In your query, you are doing a sub-select based on the C.TIINTRO which can kill performance. You are also querying for all records.
I would start the query by pre-querying the MIN() value and since you are not even using any other value field from the "C" alias, leave it out.
SELECT DISTINCT
A.IDPRE,
A.IDARTB,
A.TIREGDAT,
B.IDDATE,
B.IDINFO,
PreQuery.TIINTRO
FROM
( SELECT D.IDPRE,
D.IDARTB,
MIN(D.TIINTRO) TIINTRO
from
PRTCON D
group by
D.IDPRE,
D.IDARTB ) PreQuery
JOIN GLHAZQ A
ON PreQuery.IDPre = A.IDPRE
AND PreQuery.IDArtB = A.IDArtB
JOIN PRTINFO B
ON PreQuery.IDPre = B.IDPRE
AND PreQuery.IDArtB = B.IDArtB
ORDER BY
PreQuery.TIINTRO
I would ensure you have indexes on
table Index keys
PRTCON (IDPRE, IDARTB, TIINTRO)
GLHAZQ (IDPRE, IDARTB)
PRTINFO (IDPRE, IDARTB)
If you really DO need your "C" table, you could just add as another JOIN such as
JOIN PRTCON C
ON PreQuery.IDArtB = C.IDArtB
AND PreQuery.TIIntro = C.TIIntro
With such time, you might be better having a "covering index" with
GLHAZQ table key ( IDPRE, IDARTB, TIREGDAT )
PRTINFO (IDPRE, IDARTB, IDDATE, IDINFO)
this way, the index has all the elements you are returning in the query vs having to go back to all the actual pages of data. It can get the values from the index directly