introduction to db2 access path v0.8

7/29/2019 Introduction to DB2 Access Path v0.8

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Copyright1996-2006 NCS Pte Ltd. All Rights Reserved.

Copyright1996-2006 NCS Pte Ltd. All Rights Reserved.

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DBA Presentation

Introduction to DB2 Access Path


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What is Access Path?

1. An access path is an algorithm used by DB2 to satisfy the requirements ofSQL statements..

2. The chosen access path determines the performance of the SQL query.


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How is an SQL query being read in DB2?

1. SQL specifies what data to retrieve or manipulate, but does not specify howthe database accomplishes these tasks.

2. DB2 database management systems have their own sequencing and

optimization, (1) they find the data sources, (2) read the rows or indexes to

rows, and (3) then narrow the columns returned those qualified in a

select list.


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How does DB2 optimize the access on a table?

1. The DB2 optimizer is the heart and soul of DB2. It analyzes SQL statements

and determines the most efficient access path available for satisfying eachstatement.

2. Optimizer chooses an access plan for a SQL statement by modeling the

execution cost of several alternative access plans and choosing the one

with the minimal estimated cost.

Continues.


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3. An access plan can use one of two ways to access data in a table

(a) by directly reading the table - table scan

(b) by reading an index and then retrieving the row in the table - index scan

4. "Cost" is defined as the estimated CPU time it takes (TIMERONS) to run the

request.

5. Optimization is based on timerons, not on resource utilization or real time.

6. Usually the fastest plan is also the most resource efficient plan, but this is not

necessarily true.

7. The goal of the optimizer is to eliminate I/O as early as possible by identifying

the best path to data.

8. The optimizer will "rewrite" the query.


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How do we see the optimized access plan that DB2 selected?

1. EXPLAIN is a monitoring tool that produces access path information aboutplan, package, or SQL statement when it is bound. The output appears in a

table called PLAN_TABLE.

2. The plan table can be populated by two ways.

(a) by executing the SQL statement EXPLAIN.

(b) when you bind or rebind a plan or package using option EXPLAIN(YES).

3. The information in PLAN_TABLE can help you to:

(a) Design tables, indexes, and application programs

(b) Determine when to rebind an application

(c) Determine the access path chosen for a query

Continues..


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4. For example: The following query to a plan table returns the rows for all the

SQL statements in a plan in their logical order:

SELECT QUERYNO, PLANNO, METHOD, CHAR(TNAME,18) TNAME, ACCESSTYPE

FROM CPFU.PLAN_TABLE

WHERE PROGNAME = 'SEM0715'

ORDER BY TIMESTAMP, QUERYNO, QBLOCKNO,PLANNO;

---------+---------+---------+---------+---------+---------+---------

QUERYNO PLANNO METHOD TNAME ACCESSTYPE

---------+---------+---------+---------+---------+---------+---------

87 1 0 B_PERSINFO I

87 2 1 B_SEMPL I

5. There are many information available in the Plan Table. For example.

- tells you whether an index access or table space scan is used.

- tells you how many indexes and index columns are used and what I/Omethods are used to read the pages.

- tells you which join method and type are used, the order in which DB2

joins the tables, and when and why it sorts any rows.


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Plan Table Important columns

The Plan Table containing many columns, related to a particular SQL statement.

PLANNO : The number of the steps in the query. It shows order of those steps.

METHOD: Indicates the join method used for the table.

0 - First table accessed 1 - Nested loop join2 - Merge scan join 3 - Sorts needed by the query

4 - Hybrid join

MATCHCOLS : Number of matching index columns.

INDEXONLY : Whether Table Space is referred or not.

ACCESSNAME : Name of the index used.

ACCESSTYPE : Discussed in the next slide.

Continues..


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Different Access Types available in DB2

1. Is access through an index?

Yes, if ACCESSTYPE is I, I1, N or MX

2. Is access through more than one index?

Yes, if ACCESSTYPE=M

3. How many columns of the index are used in matching?

You may read it from MATCHCOLS


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How do we EXPLAIN an SQL statement?

1. The EXPLAIN statement obtains information about access path selection foran explainable statement.

2. A statement is explainable if it is a SELECT or INSERT within SELECT

statement, or the searched form of an UPDATE or DELETE statement.

3. Output from EXPLAIN is one or more rows of data inserted into the plan table.

4. We can EXPLAIN a SQL in two ways.

(a) Using SPUFI execution.

(b) Using Admin Tool.

5. Following Examples would demonstrate both methods.


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How do we list PLAN_TABLE & DSN_STATEMNT_TABLE entries that use

Production statistics?

Invoke EXPLAIN from DB2 Admin Tool, use the following COLL IDs:

CCPFE00 Collection ID for Prod Copy of Packages

CPPFE01 Collection ID for UAT version of programs


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How do we analyze access paths for alternate SQL?

A SQL from a program LIS0119 was taken for this analysis.

Following steps were carried out to illustrate the access path analysis:

1. The original SQL was analyzed and identified its access path.

2. Deliberately the SQL was changed to see the difference in access path

3. The changed SQL was analyzed and identified its access path.4. Both the access path was compared and identified the best access path.


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Good Practices when coding SQLs:

1. Try to avoid using SQL without WHERE predicate.

2. Use WITH UR when committed data is not required, e.g. during investigation

to avoid contention.

3. Try to avoid using UNION ALL, instead use UNION.

4. Try avoid using DISTINCT, GROUP BY, and ORDER BY clauses, because

sorting is needed, especially when the result set is big.

5. Avoid using SELECT *, instead retrieve only the required columns.

6. If you dont need entire set of result, then use FETCH FIRST n ROWS ONLYclause.


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Guidelines for coding WHERE predicate:

1. Ensure at least one matching indexed column is used in the WHERE

predicate.

2. Ensure the most restrictive predicate is indexed.

SELECT LST_UPD_TS FROM T_PERSDTL

WHERE SEX=M AND CTZ=Y

AND EEACN_C=1234 D Most restrictive column, indexed

3. When using the range of data in the WHERE predicate use BETWEEN

clause, instead of > and < operators.

a LBS_TRN_DTE BETWEEN01-01-2009 AND 31-01-2009

O LBS_TRN_DTE >=01-01-2009 AND LBS_TRN_DTE


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Guidelines for coding WHERE predicate:

5. Predicate can be coded in ON clause, WHERE clause, or HAVING clause.

From pgm:BKOA011

SELECT A.LIFE_POL_NUM

FROM T_LBOMSTR A LEFT JOIN T_LBOTRN B

ON A.LIFE_POL_NUM = B.LIFE_POL_NUM

WHERE EEACN_C = :WS-EEACN-C AND LBO_ALLT_STS_CDE = 'A'

From pgm:LISM075SELECT SUM(ANC_PRM_AMT),SUM(LIFE_AVAIL_FUND_AMT) - SUM(LBO_ALLT_AMT)

INTO :WS-TOTAL-LIFE-ANTY-PRM,:WS-RRA-BALANCE

FROM T_POLICY

WHERE EEACN_C = :WS-CPF-ACCT AND LIFE_POL_STS_CDE = 'A' AND

LIFE_FUND_SRC_CDE = '1'

HAVING COUNT(ANC_PRM_AMT) > 0

6. The descending order of efficient predicates OPERATORS are:

(i) = (ii) COLUMN op* Value (iii) IN

(iv) BETWEEN (v) sub-query (vi) NOT

*op can be replaced with Refer to DB2 V9 Performance Monitoring & Tuning Guide for Indexable and Stage 1 predicates.


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Avoid using the following predicates which will not use index

COL valueCOL NOT BETWEEN val1 and val2

COL NOT IN (val1, val2)

COL NOT LIKE val%

COL EXISTS (sub query) You may use IN instead.

COL NOT EXISTS (sub query)

Using functions on a column will deter index being used

a LST_UPD_TS BETWEEN2009-01-01-00.00.00.000000

AND2009-01-31-00.00.00.000000

ODATE(LST_UPD_TS) BETWEEN 01-01-2009 AND 31-01-2009

a POL_LTTR_TP_CDE LIKEPLCLB%

OSUBSTR(POL_LTTR_TP_CDE,1,4) = PLCLB


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Access Path Exception Report

Purpose: For all programs moved to UAT*, this report will display queries which

may have potential performance implications. Access path details aregenerated using production statistics loaded in development (CPFE schema).

Only queries which have the following access types will be reported:

Table Scan (Type = R)

Multiple Index Scan (Access Type = M/MI/MX/MU)

Index Scan with 0 MATCHCOL (Access Type = I, MATCHCOLS = 0)

Frequency : Report is generated daily at 4PM. Programs checked out to UAT

after 4PM will be included in next days report.

Report Inputs : CPFE.PLAN_TABLE, CPFE.DSN_STATEMNT_TABLE,

SYSIBM.SYSTABLES

Report File : TST8.PMC.D001A.EXPLNRPT.GD

*NOTE: UAT Checkout after 4 pm and if moved to production on same day or

before 4 pm the next day will not be captured by this report


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Sample Access Path Exception Report and Description of Details

PROG NAME - The name of the program or package containing the statement

being explained

QUERYNO - A number intended to identify the statement being explained. For arow produced by an EXPLAIN statement, specify the number in the

QUERYNO clause. For a row produced by non-EXPLAIN statements, specify

the number using the QUERYNO clause, which is an optional part of the

SELECT, INSERT, UPDATE and DELETE statement syntax. Otherwise, DB2

assigns a number based on the line number of the SQL statement in the

source program. When the values of QUERYNO are based on the statementnumber in the source program, values greater than 32767 are reported as 0.

However, in a very long program, the value is not guaranteed to be unique. If

QUERYNO is not unique, the value of TIMESTAMP is unique.


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QBLOCKNO - A number that identifies each query block within a query.

The value of the numbers are not in any particular order, nor are theynecessarily consecutive.

PLAN NO. - The number of the step in which the query indicated in

QBLOCKNO was processed. This column indicates the order in which the

steps were executed.

TABLE NAME - The name of a table, materialized query table, created or

declared temporary table, materialized view, or materialized table

expression. The value is blank if METHOD is 3. The column can also

contain the name of a table in the form DSNWFQB(qblockno).

DSNWFQB(qblockno) is used to represent the intermediate result of a

UNION ALL or an outer join that is materialized. If a view is merged, thename of the view does not appear.


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ACCESS TYPE - Latest access type for QUERYNO. This is method of

accessing the table, and may have the following values:

I - By an index (and MATCHCOL = 0; which means index scan with no

matching index key)M - By a multiple index scan (followed by MX, MI, or MU)

MI - By an intersection of multiple indexes

MU - By a union of multiple indexes

MX - By an index scan on the index named in ACCESSNAME

R - By a table space scan

PROCMS estimated processor cost in milliseconds for the revised

query

MATCHOLS For access type I or MX, this refers to the number of

index keys used in an index scan. Otherwise, 0.

TABLE ROW COUNT Row count for the table referenced in the

query.

REMARKS Describes the access type


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What information is needed when we escalate to DBA team (in relation to

Access Path) ?

1. The owner of the issue should have completed their preliminary analysis.

2. The following information need to be furnished when escalating to DBA

(a) Job Name, Job Type, and environment.

(b) Priority of this issue or any expected time limitation.

(c) Affected SQL code.

(d) The access path information and any finding of improvements or

suggestions.

(d) Any special condition of the situation or resource e.g. was the job was

rescheduled or was the SQL changed recently, etc.


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Together We can Make the

Difference

Thank You

Mayur & Norman

introduction to db2 access path v0.8

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