Oracle : Using WITH Clause for improving performance

Many times using inline views destroy the execution plans. This is mainly because the execution plan of inline view interfere with the overall execution plan of the query.

To counter this, inline view can be taken to the top inside WITH clause. But this alone will not help. We need to incorporate hint /*+ materialize */ , as only then database treats the view as a physical table. Database creates a temporary table out of the view which is then used in the query. Execution plan is found far improved than before.


Query with following structure showed nested loop join between tables tab1 and tab2. This mainly was influenced by the inline view. Specifying USE_HASH hint also does not work.

SELECT col1,
col2,
Sum(col4)
FROM tab1,
tab2,
(SELECT DISTINCT Last_day(To_date(dates,'DD-MM-YYYY')) end_dt,
Add_months(Last_day(To_date(dates,'DD-MM-YYYY')) + 1,
-1) strt_dt,
To_char(To_date(dates,'DD-MON-YYYY'),'MMYYYY') mon_dt
FROM (SELECT To_char(Add_months(SYSDATE,-10) + (LEVEL - 1),'DD-MON-YYYY') AS dates
FROM dual
CONNECT BY To_number(To_char((Add_months(SYSDATE,-10) + (LEVEL - 1)),'YYYYMMDD')) <= To_number(To_char(SYSDATE,'YYYYMMDD')))) tim
WHERE tab1.assign_dt <= tim.end_dt
AND tab1.assign_dt >= tim.strt_dt
AND tab1.col3 = tab2.col3
GROUP BY col1,
col2

Changing the query to the following using WITH caluse leads to perfect execution plan using HASH join .


WITH TIM as (
(SELECT /*+ materialize */ DISTINCT Last_day(To_date(dates,'DD-MM-YYYY')) end_dt,
Add_months(Last_day(To_date(dates,'DD-MM-YYYY')) + 1,
-1) strt_dt,
To_char(To_date(dates,'DD-MON-YYYY'),'MMYYYY') mon_dt
FROM (SELECT To_char(Add_months(SYSDATE,-10) + (LEVEL - 1),'DD-MON-YYYY') AS dates
FROM dual
CONNECT BY To_number(To_char((Add_months(SYSDATE,-10) + (LEVEL - 1)),'YYYYMMDD')) <= To_number(To_char(SYSDATE,'YYYYMMDD'))))
)
SELECT col1,
col2,
Sum(col4)
FROM tab1,
tab2,
tim
WHERE tab1.assign_dt <= tim.end_dt
AND tab1.assign_dt >= tim.strt_dt
AND tab1.col3 = tab2.col3
GROUP BY col1,
col2

DB2 : collecting detailed statistics

to get the best optimizer execution plan, collecting detailed table statisitics might help:

db2 -x -r "runstats.db2" "select rtrim(concat('runstats on table ',concat(rtrim(tabSchema),concat('.',concat(rtrim(tabname),' on all columns with distribution on all columns and sampled detailed indexes all allow write access'))))) from syscat.tables where type='T'"
db2 -v -f "runstats.db2"

DB2 : Generating SQL Explain plan

This is how explain plan can be generated in DB2 database.

Create a file statements.db2:
select deptno from dept@

db2expln -database U001DB -stmtfile statements.db2 -terminator @ -terminal -user siebel siebel

Section Code Page = 1252

Estimated Cost = 30087412.000000
Estimated Cardinality = 19275950.000000

Access Table Name = SIEBEL.S_ORG_EXT ID = 7,201
| #Columns = 2
| Relation Scan
| | Prefetch: Eligible
| Lock Intents
| | Table: Intent Share
| | Row : Next Key Share
| Sargable Predicate(s)
| | Process Build Table for Hash Join
Left Outer Hash Join
| Early Out: Single Match Per Outer Row
| Estimated Build Size: 7776000
| Estimated Probe Size: 9948879872
| Access Table Name = SIEBEL.S_ORG_EXT ID = 7,201
| | #Columns = 2
| | Relation Scan
| | | Prefetch: Eligible
...
...

INFORMATICA : Truncate Logs

There was an unusual behavior when i proceeded with repository manager to truncate the existing session logs. Option "Truncate Log" under "Edit" menu was disabled. Selecting/expanding the required folder also did not help.

Since my informatica server was on AIX with client on windows, i did the following to truncate the logs:

$ pmrep
pmrep>connect -r Powercenter -d Domain_server1 -n Administrator -x Administrator

pmrep>truncatelog -t all
truncatelog completed successfully.

DB2 : Establish remote connection on Unix/AIX

On server hosting DB2 Database:
db2set DB2COMM=tcpip
update database manager configuration using svcename 50000
db2stop
db2start

On client server (with DB2 client installed)
db2 UNCATALOG DATABASE U001DB
db2 UNCATALOG DCS DATABASE U001DB
db2 UNCATALOG NODE SERVER1

db2 CATALOG TCPIP NODE SERVER1 REMOTE SERVER1 SERVER 50000 REMOTE_INSTANCE DB2 SYSTEM SERVER1 OSTYPE AIX
db2 CATALOG DCS DATABASE U001DB AS U001DB
db2 CATALOG DATABASE U001DB AS U001DB AT NODE SERVER1 AUTHENTICATION DCS

Where, SERVER1 is the server where db2 server resides. U001DB is the db2 database name.

Test connection:

On client server:
db2 connect to U001DB user siebel using siebel

Database Connection Information

Database server = DB2/AIX64 9.1.6
SQL authorization ID = SIEBEL
Local database alias = U001DB

Oracle :Outer Join pitfalls

Starting Oracle 10G, Optimizer processes outer joins in much efficient way. Execution plan now has "HASH JOIN RIGHT OUTER", which means that the table with scant records will be scanned first. Table which it is outer joined to will be scanned in the end. This is especially use full for Datawarehouse queries where Fact table is outer joined to lot of dimension tables. So, DB now does not have to temp read and write the fact table records to join them later to the dimension tables.

But, before one jumps into concluding that this new method is almost sure way to win, there are few things one should keep in mind.
1. Ensure NOPARALLEL query execution. This new execution plan for outer joins is severely affected by parallel query i.e. if dimension tables are scanned in parallel and then joined to the fact tables, query exhibits worst performance. To avoid parallel execution, check that tables or any participating indexes have degree set to "1". Alternatively, one an also disable parallel query execution at instance level by setting init parameter "max_parallel_servers" to "0". But this is no advisable as there could be other kind of queries which might want to leverage the advantages of parallel query execution.

2. Keep huge dimension tables out of the outer joins. If Dimension tables (the one which is outer joined) is huge, lets say...1 Million records (for pga 4 gig), outer join using new execution plan may not help. This is because now hash tables for all the joining dimension could not be kept in memory and DB anyways have to do multipass hash join. "HASH JOIN RIGHT OUTER" based execution plan is effective only if the outer joined tables/dimensions are smaller in size.

3. Ensure that all the participating tables has the statistics collected. Missing statistics on tables would cause optimizer to pick up execution plan as it existed before 10G and you may not see the new execution plan for outer joined query.

Informatica 8.6 : issue with pmcmd

On AIX 5.3 pmcmd command fails with following error:

$ ./pmcmd

Could not load program ./pmcmd:
Could not load module /u01/Informatica/PowerCenter8.6/server/bin/libpmser.a.
Dependent module /lib/libz.a could not be loaded.
The module has an invalid magic number.
Could not load module pmcmd.
Dependent module /u01/Informatica/PowerCenter8.6/server/bin/libpmser.a could not be loaded.
Could not load module .

This issue has been fixed on one of the latest hotfixes.

To workaround the issue, rename /lib/libz.a so that libz.a which is shipped with Informatica is picked up.