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DB2 9 for z/OS 6/14/2009
© YL&A 1999 - 2009 1
Using Explain Tables for Tuning Queries
Susan LawsonYL&A
Yevich, Lawson & Assoc. Inc.2743 S. Veterans Pkwy PMB 226
Springfield, IL 62704
www.ylassoc.comwww db2expert com
© YL&A 1997-2009
IBM is a registered trademark of International Business Machines Corporation.DB2 is a trademark of IBM Corp.
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www.db2expert.com
DB2 9 for z/OS 6/14/2009
© YL&A 1999 - 2009 2
Disclaimer PLEASE READ THE FOLLOWING NOTICE
n The information contained in this presentation is based on techniques, algorithms, and documentation published by the several authors and companies, and in addition is the result of research. It is therefore subject to change at any time without notice or warning.
n The information contained in this presentation has not been submitted to any formal tests or review and is distributed on an “As is” basis without any warranty, either expressed or implied.
n The use of this information or the implementation of any of these techniques is a client responsibility and depends on the client’s ability to evaluate and integrate them into the client’s operational environment.
n While each item may have been reviewed for accuracy in a specific situation, there is no guarantee that the same or similar results will be obtained elsewhere.
© YL&A 1997-2009
n Clients attempting to adapt these techniques to their own environments do so at their own risks.
n Foils, handouts, and additional materials distributed as part of this presentation or seminar should be reviewed in their entirety.
Abstract
DB2 z/OS V8 and 9 have introduced us to many new EXPLAIN tables. This presentation will introduce users some of the new EXPLAIN tables used by these tools for advanced query analysis, and demonstrate how SQL queries can be used to gather and analyze the advanced information.
© YL&A 1997-2009
DB2 9 for z/OS 6/14/2009
© YL&A 1999 - 2009 3
EXPLAIN and EXPLAIN Tools and Queries
n EXPLAIN has been enhanced greatly for DB2 V8 and DB2 9n Several EXPLAIN tools are available
– Visual Explain– Optimization Service Centerp
l Replaces Visual Explainl Works with V8 NFM
– Data Studiol Visual Explain Plug-Ins
n However, DB2 EXPLAIN facility is built-in to DB2– No additional tools are needed to perform an EXPLAIN
© YL&A 1997-2009
– EXPLAIN tables contain access path information– Anyone can read the access path information from the EXPLAIN
tables
Visual Explain Tool (DB2 V8)
n A GUI interface to the information in the EXPLAIN tablesn Provides information about statements in the systemn Can dynamically EXPLAIN statementsn Can get Statistics for accessed objectsn Ability to browse subsystem parametersn Compatible with V7 subsystemsn With V8 subsystems
– More information than the PLAN_TABLE provides– Cardinality and Filter Factor information– Additional of predicate information
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Additional of predicate information– Some rewritten predicates exposed
n Method of transmission of information to IBM– XML File– Service SQL Feature
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Visual Explain – Index Scan
© YL&A 1997-2009
VISUAL EXPLAIN - Fetch
© YL&A 1997-2009
DB2 9 for z/OS 6/14/2009
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Visual Explain – Statistics Advisor
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Visual Explain – Join Predicates and Filter Factor
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DB2 9 for z/OS 6/14/2009
© YL&A 1999 - 2009 6
Optimization Service Center (DB2 9)n A workstation-based tool
– Easy interaction with DB2 EXPLAIN and the explain tables– Allows you to analyze
l SQL statementsl Objectsl Objectsl Statisticsl The statement cachel Workloads
n Replaces Visual Explainn Has Many of the features of Visual Explainn Compatible with V8 Subsystems in New Function Mode
H dditi l f t th t b h d
© YL&A 1997-2009
n Has additional features that can be purchased
Why Not Use These Products for EXPLAIN?
n PC workstation maybe does not have enough power– These tools use a lot of the machine
n DB2 Connect Enterprise Edition is requiredn Remote connections to mainframe are required
Thi i it– This raises security concernsl Especially for production
n SPUFI or QMF is often preferred– This is a fast and easy method
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DB2 9 for z/OS 6/14/2009
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Additional Explain Tables
n Most available in V8n Now very accessible in V9 via Optimization Service
Center or SQLn Documented in IBM DSN VIEWREF TABLEDB2 9 Performance
Guide
DSN_VIEWREF_TABLEDSN_QUERY_TABLE
DSN_PGRANGE_TABLEDSN_SORTKEY_TABLE
DSN_SORT_TABLEDSN_DETCOST_TABLE
DSN FILTER TABLEPLAN_TABLE
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DSN_FILTER_TABLEDSN_PTASK_TABLE
DSN_PGROUP_TABLEDSN_STRUCT_TABLE
DSN_PREDICAT_TABLE
EXPLAIN Tables Explained
� The DB2 EXPLAIN facility populates the EXPLAIN tables– If any of the EXPLAIN Tables exist for the Userid, then they will
be populated automatically when you EXPLAIN– The EXPLAIN Tables can be queried just like any other table
� Visual Explain and Optimization Service Center define the EXPLAIN tables– However, once created you can use them independent of these
products– You can also migrate or create them yourself
� It is possible to have a “Green Screen” advanced EXPLAIN
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EXPLAIN– Helps those especially without distributed access to the
mainframe
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EXPLAIN Tables and Columns of Primary Interest
� When EXPLAINing there are just a few primary tables and columns that should be initially focused on– There is much more, but this is just for initial analysis
� Your first look at the Access PathMore details are available but the first look simplifies the view and� More details are available, but the first look simplifies the view and can answer many questions
� WARNING!– Most EXPLAIN table analysis should be left in the hands of IBM
of professional EXPLAIN products!– Any “Tweaking” of EXPLAIN is done without IBM support
© YL&A 1997-2009
PLAN_TABLEDSN_FILTER_TABLE
DSN_PREDICAT_TABLEDSN_DETCOST_TABLE
PLAN_TABLE
� This is the base access path information
METHODMERGE_JOIN_COLSCREATOR
Columns for initial analysis First table accessed, join method, or sort
Number of merge scan joined columns
Name of table TNAMECORRELATION_NAMEACCESSTYPEACCESSNAMEINDEXONLYMIXOPSEQMATCHCOLSSORT#### ColsPREFETCH
accessed Table correlation name in queryIndex access or table
space scanIndex name if used
‘Y’ if index-only accessMulti-index step number (if used)Quantity of index
matching columns used Sort reasonsPrefetch type if used ‘Y’ if table space scan and
© YL&A 1997-2009
PREFETCHPAGE_RANGEJOIN_TYPEPARENT_QBLOCKNOTABLE TYPEBIND_TIME
Prefetch type if used ‘Y’ if table space scan and partition elimination
Join type if used Parent query block number (subqueries, etc.)
Table, work file, MQT, subquery, etc.
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Warnings About Using the Other EXPLAIN Tables
� Can quite easily query these tables via SQL � Should limit what is viewed in the tables� These tables exist in support of the IBM query analysis
products–IBM can change/eliminate them at any time
� IBM does NOT support your use of these tables beyond the authorized tools–Will not respond to issues about these tables–You are using them on your own and at your own risk
© YL&A 1997-2009
DSN_FILTER_TABLE
� Contains information about how predicates are used during query processing– One row per simple and compound predicate
COLLIDPROGNAMEQUERYNOEXPLAIN_TIME
PREDNOSTAGE
These columns used for join to PLAN_TABLE
The relative predicate number used to join to
DSN_PREDICAT_TABLE
Th di t t ( h
Columns for initial analysis
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The predicate stage (where processed within DB2).
Matching, Screening, Stage 1, or Stage 2
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DSN_PREDICAT_TABLE
� Contains information about the predicates in a query– One row per simple and compound predicate
COLLID These columns used for join to PLAN TABLE
Columns for initial analysis
PROGNAMEQUERYNOEXPLAIN_TIMEPREDNO
FILTER_FACTORBOOLEAN_TERMTEXT
The relative predicate number used to join to DSN_FILTER_TABLE
join to PLAN_TABLE
The predicate filter factor (percent of rows to be returned based on this
predicate alone)
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predicate alone)
Whether or not the predicate is Boolean Term(when it evaluates to false does it make the
entire WHERE clause false)
The actual predicate text including rewritten or generated predicates
DSN_DETCOST_TABLE
� Provides detailed cost information about the mini-plans in a query– There can be multiple rows per query block and plan step
� Take the one with the smallest value
COLLIDPROGNAMEQUERYNOEXPLAIN_TIMEQBLOCKNOPLANNO
These columns used for join to PLAN_TABLE
The number of rows the optimizer thinks will qualify
after only local predicates are
Columns for initial analysis
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ONECOMPROWS applied (take the lowest value)
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Basic PLAN_TABLE Query SELECT SUBSTR(DIGITS(QUERYNO),5) CONCAT 'SELECT SUBSTR(DIGITS(QUERYNO),5) CONCAT '--' CONCAT ' CONCAT SUBSTR(DIGITS(QBLOCKNO),4) CONCAT 'SUBSTR(DIGITS(QBLOCKNO),4) CONCAT '--' CONCAT SUBSTR(DIGITS(PLANNO),4) AS ' CONCAT SUBSTR(DIGITS(PLANNO),4) AS Q_QB_PL ,PROGNAME AS PNAME ,SUBSTR(CHAR(METHOD),1,1) AS MT Q_QB_PL ,PROGNAME AS PNAME ,SUBSTR(CHAR(METHOD),1,1) AS MT ,SUBSTR(TNAME,1,18) AS TNAME ,CHAR(TABNO) AS T_NO ,ACCESSTYPE AS AT ,SUBSTR(TNAME,1,18) AS TNAME ,CHAR(TABNO) AS T_NO ,ACCESSTYPE AS AT ,CHAR(MATCHCOLS) AS MC ,SUBSTR(ACCESSNAME,1,8) AS ACC_NM ,INDEXONLY AS IX ,CHAR(MATCHCOLS) AS MC ,SUBSTR(ACCESSNAME,1,8) AS ACC_NM ,INDEXONLY AS IX ,SORTN_JOIN CONCAT SORTC_UNIQ CONCAT SORTC_JOIN CONCAT SORTC_ORDERBY ,SORTN_JOIN CONCAT SORTC_UNIQ CONCAT SORTC_JOIN CONCAT SORTC_ORDERBY CONCAT SORTC GROUPBY AS NJ CUJOG PREFETCH AS PF COLUMN FN EVAL ASCONCAT SORTC GROUPBY AS NJ CUJOG PREFETCH AS PF COLUMN FN EVAL ASCONCAT SORTC_GROUPBY AS NJ_CUJOG ,PREFETCH AS PF ,COLUMN_FN_EVAL AS CONCAT SORTC_GROUPBY AS NJ_CUJOG ,PREFETCH AS PF ,COLUMN_FN_EVAL AS CFE ,CHAR(MIXOPSEQ) AS MIX ,ACCESS_DEGREE AS A_DEG ,JOIN_DEGREE AS J_DEG CFE ,CHAR(MIXOPSEQ) AS MIX ,ACCESS_DEGREE AS A_DEG ,JOIN_DEGREE AS J_DEG ,PARALLELISM_MODE AS P_MODE ,MERGE_JOIN_COLS AS MJC ,CORRELATION_NAME ,PARALLELISM_MODE AS P_MODE ,MERGE_JOIN_COLS AS MJC ,CORRELATION_NAME AS COR_NM ,PAGE_RANGE AS PG_RG ,JOIN_TYPE AS JT ,WHEN_OPTIMIZE AS WH_OP AS COR_NM ,PAGE_RANGE AS PG_RG ,JOIN_TYPE AS JT ,WHEN_OPTIMIZE AS WH_OP ,QBLOCK_TYPE AS QB_TYP ,BIND_TIME AS B_TM ,HINT_USED AS HINT ,QBLOCK_TYPE AS QB_TYP ,BIND_TIME AS B_TM ,HINT_USED AS HINT ,PRIMARY_ACCESSTYPE AS PR_ACC ,PRIMARY_ACCESSTYPE AS PR_ACC
FROM PLAN_TABLE A FROM PLAN_TABLE A
----WHERE PROGNAME = 'YLAPROG1' <WHERE PROGNAME = 'YLAPROG1' <---- TARGET A SPECIFIC PROGRAM HERE TARGET A SPECIFIC PROGRAM HERE
WHERE BIND TIME = (SELECT MAX(BIND TIME) FROM PLAN TABLE B WHEREWHERE BIND TIME = (SELECT MAX(BIND TIME) FROM PLAN TABLE B WHERE
© YL&A 1997-2009
WHERE BIND_TIME = (SELECT MAX(BIND_TIME) FROM PLAN_TABLE B WHERE WHERE BIND_TIME = (SELECT MAX(BIND_TIME) FROM PLAN_TABLE B WHERE A.PROGNAME = B.PROGNAME AND A.COLLID = B.COLLID) A.PROGNAME = B.PROGNAME AND A.COLLID = B.COLLID)
ORDER BY PROGNAME, BIND_TIME, Q_QB_PL, MIX; ORDER BY PROGNAME, BIND_TIME, Q_QB_PL, MIX;
Advanced EXPLAIN QueryWITH MAXTIME (COLLID, PROGNAME, QUERYNO, BIND_TIME) AS (SELECT COLLID, PROGNAME, QUERYNO, WITH MAXTIME (COLLID, PROGNAME, QUERYNO, BIND_TIME) AS (SELECT COLLID, PROGNAME, QUERYNO, MAX(BIND_TIME) FROM PLAN_TABLE WHERE COLLID = 'DSNESPCS' AND PROGNAME = 'DSNESM68' GROUP BY MAX(BIND_TIME) FROM PLAN_TABLE WHERE COLLID = 'DSNESPCS' AND PROGNAME = 'DSNESM68' GROUP BY COLLID, PROGNAME, QUERYNO) COLLID, PROGNAME, QUERYNO) SELECT SUBSTR(P.PROGNAME, 1, 8) AS PROGNAME,SUBSTR(DIGITS(P.QUERYNO), 6) CONCAT 'SELECT SUBSTR(P.PROGNAME, 1, 8) AS PROGNAME,SUBSTR(DIGITS(P.QUERYNO), 6) CONCAT '--' CONCAT ' CONCAT SUBSTR(DIGITS(P.QBLOCKNO), 4) CONCAT 'SUBSTR(DIGITS(P.QBLOCKNO), 4) CONCAT '--' CONCAT SUBSTR(DIGITS(P.PLANNO), 4) AS ' CONCAT SUBSTR(DIGITS(P.PLANNO), 4) AS QQP,SUBSTR(CHAR(P.METHOD), 1, 3) AS MTH ,SUBSTR(CHAR(F.PREDNO), 1, 3) AS QQP,SUBSTR(CHAR(P.METHOD), 1, 3) AS MTH ,SUBSTR(CHAR(F.PREDNO), 1, 3) AS P_NO,SUBSTR(CHAR(P.MERGE_JOIN_COLS), 1, 3) AS MJC,SUBSTR(P.CREATOR, 1, 8) AS P_NO,SUBSTR(CHAR(P.MERGE_JOIN_COLS), 1, 3) AS MJC,SUBSTR(P.CREATOR, 1, 8) AS TBCREATOR,SUBSTR(P.TNAME, 1, 18) AS TBNAME,SUBSTR(P.CORRELATION_NAME, 1, 8) AS TBCREATOR,SUBSTR(P.TNAME, 1, 18) AS TBNAME,SUBSTR(P.CORRELATION_NAME, 1, 8) AS CORR_NM,DEC(D.ONECOMPROWS, 10, 1) AS ROWS_POST_FILTER,P.ACCESSTYPE AS CORR_NM,DEC(D.ONECOMPROWS, 10, 1) AS ROWS_POST_FILTER,P.ACCESSTYPE AS ATYP,SUBSTR(P.ACCESSNAME, 1, 15) AS A_NM,P.INDEXONLY AS IXO,CHAR(P.MIXOPSEQ) AS MIX ATYP,SUBSTR(P.ACCESSNAME, 1, 15) AS A_NM,P.INDEXONLY AS IXO,CHAR(P.MIXOPSEQ) AS MIX ,CHAR(P.MATCHCOLS) MCOL ,F.STAGE AS STAGE,DEC(E.FILTER_FACTOR, 11, 10) AS FF,E.BOOLEAN_TERM AS ,CHAR(P.MATCHCOLS) MCOL ,F.STAGE AS STAGE,DEC(E.FILTER_FACTOR, 11, 10) AS FF,E.BOOLEAN_TERM AS BT ,SUBSTR(E.TEXT, 1, 30) AS PRED_TEXT30,P.SORTN_JOIN CONCAT P.SORTC_UNIQ CONCAT P.SORTC_JOIN BT ,SUBSTR(E.TEXT, 1, 30) AS PRED_TEXT30,P.SORTN_JOIN CONCAT P.SORTC_UNIQ CONCAT P.SORTC_JOIN CONCAT P.SORTC_ORDERBY CONCAT P.SORTC_GROUPBY AS NJ_CUJOG,P.PREFETCH AS PF,P.COLUMN_FN_EVAL CONCAT P.SORTC_ORDERBY CONCAT P.SORTC_GROUPBY AS NJ_CUJOG,P.PREFETCH AS PF,P.COLUMN_FN_EVAL AS CFE,P.PAGE_RANGE AS PGRNG,P.JOIN_TYPE AS JT AS CFE,P.PAGE_RANGE AS PGRNG,P.JOIN_TYPE AS JT ,P.QBLOCK_TYPE AS QB_TYP,P.PARENT_QBLOCKNO AS P_QB,P.TABLE_TYPE AS TB_TYP,P.BIND_TIME AS B_TM,P.QBLOCK_TYPE AS QB_TYP,P.PARENT_QBLOCKNO AS P_QB,P.TABLE_TYPE AS TB_TYP,P.BIND_TIME AS B_TMFROM PLAN_TABLE P INNER JOIN MAXTIME M ON M.COLLID = P.COLLID AND M.PROGNAME=P.PROGNAME FROM PLAN_TABLE P INNER JOIN MAXTIME M ON M.COLLID = P.COLLID AND M.PROGNAME=P.PROGNAME AND M.QUERYNO=P.QUERYNO AND M.BIND_TIME=P.BIND_TIME AND M.QUERYNO=P.QUERYNO AND M.BIND_TIME=P.BIND_TIME LEFT JOIN DSN_FILTER_TABLE F LEFT JOIN DSN_FILTER_TABLE F ON M.COLLID = F.COLLID AND M.PROGNAME=F.PROGNAME AND M.QUERYNO=F.QUERYNO AND ON M.COLLID = F.COLLID AND M.PROGNAME=F.PROGNAME AND M.QUERYNO=F.QUERYNO AND P.QBLOCKNO=F.QBLOCKNO P.QBLOCKNO=F.QBLOCKNO AND P.PLANNO = F.PLANNO AND M.BIND_TIME=F.EXPLAIN_TIME AND P.ACCESSTYPE Not IN ('MX', 'MI', 'MU') AND P.PLANNO = F.PLANNO AND M.BIND_TIME=F.EXPLAIN_TIME AND P.ACCESSTYPE Not IN ('MX', 'MI', 'MU')
© YL&A 1997-2009
LEFT JOIN DSN_PREDICAT_TABLE E LEFT JOIN DSN_PREDICAT_TABLE E ON F.PROGNAME = E.PROGNAME AND F.QUERYNO=E.QUERYNO AND F.QBLOCKNO=E.QBLOCKNO ON F.PROGNAME = E.PROGNAME AND F.QUERYNO=E.QUERYNO AND F.QBLOCKNO=E.QBLOCKNO AND F.PREDNO = E.PREDNO AND M.BIND_TIME=E.EXPLAIN_TIME AND F.PREDNO = E.PREDNO AND M.BIND_TIME=E.EXPLAIN_TIME LEFT JOIN TABLE (SELECT MIN(X.ONECOMPROWS) AS ONECOMPROWS FROM DSN_DETCOST_TABLE X LEFT JOIN TABLE (SELECT MIN(X.ONECOMPROWS) AS ONECOMPROWS FROM DSN_DETCOST_TABLE X
WHERE M.PROGNAME = X.PROGNAME AND M.QUERYNO = X.QUERYNO AND P.QBLOCKNO = X.QBLOCKNO WHERE M.PROGNAME = X.PROGNAME AND M.QUERYNO = X.QUERYNO AND P.QBLOCKNO = X.QBLOCKNO AND P.PLANNO = X.PLANNO AND M.BIND_TIME = X.EXPLAIN_TIME) AS D AND P.PLANNO = X.PLANNO AND M.BIND_TIME = X.EXPLAIN_TIME) AS D
ON 1=1 ORDER BY PROGNAME, B_TM, QQP, MIX, F.PREDNO; ON 1=1 ORDER BY PROGNAME, B_TM, QQP, MIX, F.PREDNO;
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Sample EXPLAIN Statement
� Running this statement will populate the EXPLAIN tables–Then we can run the two EXPLAIN reporting queries
EXPLAIN PLAN SET QUERYNO = 3 FOREXPLAIN PLAN SET QUERYNO = 3 FORSELECT A.FIRSTNME, A.LASTNAME, B.DEPTNAMEFROM DSN8710.EMP AINNER JOIN
DSN8710.DEPT BON A.WORKDEPT = B.DEPTNOWHERE A EMPNO = '000010';
© YL&A 1997-2009
WHERE A.EMPNO 000010 ;
EXPLAIN Results
PROGNAME QQP MTH MJC TBCREATOR TBNAME CORR_NM ATYP A_NM IXO MIX MCOL-------- ----------- --- --- --------- ------------------ -------- ---- --------------- --- ------ ------DSNESM68 00003-01-01 0 ---- DSN8710 EMP A I XEMP1 N 0 1DSNESM68 00003-01-02 1 ---- DSN8710 DEPT B I XDEPT1 N 0 1
NJ_CUJOG PF CFE PGRNG JT QB_TYP P_QB TB_TYP B_TM -------- -- --- ----- -- ------ ---- ------ --------------------------
PLAN_TABLE Query
NNNNN SELECT 0 T 2007-08-13 15:47:24.150000NNNNN SELECT 0 T 2007-08-13 15:47:24.150000
Advanced EXPLAIN QueryPROGNAME QQP MTH P_NO MJC TBCREATOR TBNAME CORR_NM ROWS_POST_FILTER ATYP A_NM IXO -------- ----------- --- ---- --- --------- ------------------ -------- ---------------- ---- --------------- ---DSNESM68 00003-01-01 0 2 ---- DSN8710 EMP A 1.0 I XEMP1 NDSNESM68 00003-01-02 1 3 ---- DSN8710 DEPT B 14.0 I XDEPT1 N
The number of rows the optimizer thinks will
qualify after only local predicates are applied (take the lowest value)The query’s predicate
number
Whether or not the predicate is Boolean Term
© YL&A 1997-2009
MIX MCOL STAGE FF BT PRED_TEXT30 NJ_CUJOG PF CFE PGRNG JT QB_TYP P_QB TB_TYP ------ ------ --------- ------------ -- ------------------------------ -------- -- --- ----- -- ------ ---- ------0 1 MATCHING 0.0238095223 Y A.EMPNO='000010' NNNNN SELECT 0 T0 1 MATCHING 0.0714285969 Y A.WORKDEPT=B.DEPTNO NNNNN SELECT 0 T
The predicate stage (where processed within DB2).
Matching, Screening, Stage 1, or Stage 2
The predicate filter factor (percent of rows to be returned based on this predicate alone)
The actual predicate text including rewritten or generated predicates
Whether or not the predicate is Boolean Term
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Simple Query Improvement EXPLAINed!
� The following query has no Boolean Term predicatesEXPLAIN PLAN SET QUERYNO=1 FORSELECT *FROM DSN8710.EMPWHERE EMPNO = ?OR (EMPNO = ? AND
� This improved query has a redundant predicated added
(LASTNAME >= ?)
OR EMPNO > ?ORDER BY EMPNO, LASTNAME;
EXPLAIN PLAN SET QUERYNO=2 FORSELECT *FROM DSN8710.EMP
© YL&A 1997-2009
WHERE (EMPNO = ?OR (EMPNO = ? AND
LASTNAME >= ?)OR EMPNO > ?)AND EMPNO >= ?ORDER BY EMPNO, LASTNAME;
This redundant predicate was added to the query to get
index access on the EMPNO indexed column
Advanced Explain Query Results
Advanced EXPLAIN for Query 1PROGNAME QQP MTH P_NO MJC TBCREATOR TBNAME CORR_NM ROWS_POST_FILTER ATYP A_NM IXO -------- ----------- --- ---- --- --------- ------- -------- -------- -------- ----- ------- ---DSNESM68 00001-01-01 0 2 --- DSN8710 EMP -------- 15.3 I XEMP1 N DSNESM68 00001-01-01 0 4 --- DSN8710 EMP -------- 15.3 I XEMP1 N DSNESM68 00001-01-01 0 5 --- DSN8710 EMP -------- 15.3 I XEMP1 N DSNESM68 00001-01-01 0 6 --- DSN8710 EMP -------- 15.3 I XEMP1 N
MIX MCOL STAGE FF BT PRED_TEXT30 NJ_CUJOG PF CFE PGRNG JT QB_TYP P_QB TB_TYP ------ ------ --------- ------------ -- ------------------------------ -------- -- --- ----- -- ------ ---- ------0 0 STAGE1 .0238095223 N DSN8710.EMP.EMPNO=(EXPR) NNNNN SELECT 0 T0 0 STAGE1 .0238095223 N DSN8710.EMP.EMPNO=(EXPR) NNNNN SELECT 0 T0 0 STAGE1 .3333333134 N DSN8710.EMP.LASTNAME>=(EXPR) NNNNN SELECT 0 T0 0 STAGE1 .3333333134 N DSN8710.EMP.EMPNO>(EXPR) NNNNN SELECT 0 T
Advanced EXPLAIN for Query 2PROGNAME QQP MTH P_NO MJC TBCREATOR TBNAME CORR_NM ROWS_POST_FILTER ATYP A_NM IXO -------- ----------- --- ---- --- --------- ------- -------- -------- -------- ----- ------- ---DSNESM68 00002-01-01 0 3 --- DSN8710 EMP -------- 5.1 I XEMP1 N DSNESM68 00002-01-01 0 5 --- DSN8710 EMP -------- 5.1 I XEMP1 N DSNESM68 00002-01-01 0 6 --- DSN8710 EMP -------- 5.1 I XEMP1 N
© YL&A 1997-2009
DSNESM68 00002-01-01 0 7 --- DSN8710 EMP -------- 5.1 I XEMP1 N DSNESM68 00002-01-01 0 8 --- DSN8710 EMP -------- 5.1 I XEMP1 N
MIX MCOL STAGE FF BT PRED_TEXT30 NJ_CUJOG PF CFE PGRNG JT QB_TYP P_QB TB_TYP ------ ------ --------- ------------ -- ------------------------------ -------- -- --- ----- -- ------ ---- ------0 1 STAGE1 .0238095223 N DSN8710.EMP.EMPNO=(EXPR) NNNNN SELECT 0 T0 1 STAGE1 .0238095223 N DSN8710.EMP.EMPNO=(EXPR) NNNNN SELECT 0 T0 1 STAGE1 .3333333134 N DSN8710.EMP.LASTNAME>=(EXPR) NNNNN SELECT 0 T0 1 STAGE1 .3333333134 N DSN8710.EMP.EMPNO>(EXPR) NNNNN SELECT 0 T0 1 MATCHING .3333333134 Y DSN8710.EMP.EMPNO>=(EXPR) NNNNN SELECT 0 T
The improvement is detailed in the advanced EXPLAIN
output via the access method, matchcols, boolean
term, and text columns
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Complicated Output
� The examples shown here are not perfect� Output can be influenced by
–Compound predicates–Multi Index access–Multi-Index access
� More rows of data than are needed can appear–Be careful and adjust queries as needed
� Joining to additional EXPLAIN tables can further complicate the output
© YL&A 1997-2009
Reading the Tables Separately
� If the output from the Advanced Query is too complicated–Can reading each table separately–Always read the PLAN_TABLE first to get the ‘Big y _ g g
Picture’ on the access path� Reading each table separately
–Easier to do–Have to put the data together yourself–Can provide more information than the advanced
explain join query or even the Explain Products
© YL&A 1997-2009
explain join query, or even the Explain Products
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© YL&A 1999 - 2009 15
Reading Tables Separately Example
� Here are queries to read the Filter and Predicate Tables– Do this when EXPLAINing a single statement and be sure to
Delete all rows from the Explain Tables first
SELECT SUBSTR(F.PROGNAME, 1, 8) AS PROGNAME,SUBSTR(DIGITS(F.QUERYNO), 6) CONCAT '-' The predicate number and,SUBSTR(DIGITS(F.QUERYNO), 6) CONCAT CONCAT SUBSTR(DIGITS(F.QBLOCKNO), 4)CONCAT '-' CONCATSUBSTR(DIGITS(F.PLANNO), 4) AS QQP,PREDNO, STAGE, EXPLAIN_TIME AS B_TM
FROM DSN_FILTER_TABLE FORDER BY PROGNAME, B_TM, QQP, PREDNO;
SELECT SUBSTR(P.PROGNAME, 1, 8) AS PROGNAME,SUBSTR(DIGITS(P.QUERYNO), 6) CONCAT '-'
The predicate number and stage is pulled from the
DSN_FILTER_TABLE
The predicate number, filter factor and boolean term flag
is pulled from the DSN_PREDICATE_TABLE
© YL&A 1997-2009
CONCAT SUBSTR(DIGITS(P.QBLOCKNO), 4) AS QQ,PREDNO, FILTER_FACTOR, BOOLEAN_TERM AS BT,JOIN, AFTER_JOIN AS AJ, ADDED_PRED AS AP,REDUNDANT_PRED AS RP,KEYFIELD, substr(TEXT,1,40), EXPLAIN_TIME AS B_TM
FROM DSN_PREDICAT_TABLE PORDER BY PROGNAME, B_TM, QQ, PREDNO;
Addition predicate information is pulled: join
predicate, after join predicate, added (generated)
predicate, and redundant predicate flags
Reading Tables Separately Example
SELECT A.FIRSTNME, A.LASTNAME, B.DEPTNAMEFROM DSN8810.EMP aINNER JOIN
DSN8810.DEPT BON A.WORKDEPT = B.DEPTNOWHERE A.EMPNO = '000010'; Our join predicate is
identified as using a key field
PROGNAME QQP PREDNO STAGE B_TM---------+---------+---------+---------+---------+---------+---------+---DSNESM68 00001-01-01 2 MATCHING 2008-08-06-13.25.46.040000DSNESM68 00001-01-02 3 MATCHING 2008-08-06-13.25.46.040000
PROGNAME QQ PREDNO FILTER FACTOR BT JOIN AJ AP RP KEYFIELD
DSN_FILTER_TABLE Query Output
DSN_PREDICAT_TABLE Query Output
© YL&A 1997-2009
PROGNAME QQ PREDNO FILTER_FACTOR BT JOIN AJ AP RP KEYFIELD---------+---------+---------+---------+---------+---------+---------+---------+--------DSNESM68 00001-01 1 +0.1000000000000000E+01 Y N N N N DSNESM68 00001-01 2 +0.2380952239036560E-01 Y N N N Y DSNESM68 00001-01 3 +0.7142859697341919E-01 Y Y N N Y
TEXT B_TM---------+---------+---------+---------+---------+---------+--------(A.EMPNO='000010' AND A.WORKDEPT=B.DEPTN 2008-08-06-13.25.46.040000A.EMPNO='000010' 2008-08-06-13.25.46.040000A.WORKDEPT=B.DEPTNO 2008-08-06-13.25.46.040000
The compound predicate is displayed
DB2 9 for z/OS 6/14/2009
© YL&A 1999 - 2009 16
Reading Tables Separately Example
EXPLAIN PLAN SET QUERYNO=1 FORSELECT A.FIRSTNME, A.LASTNAME, B.DEPTNAMEFROM DSN8810.EMP aINNER JOIN
DSN8810.DEPT BON A.WORKDEPT = B.DEPTNOWHERE A WORKDEPT = 'B01'; Our join predicate is
id ifi d 2WHERE A.WORKDEPT = B01 ;
PROGNAME QQP PREDNO STAGE B_TM---------+---------+---------+---------+---------+---------+---------+---DSNESM68 00001-01-01 4 MATCHING 2008-08-06-13.08.53.170000DSNESM68 00001-01-02 2 MATCHING 2008-08-06-13.08.53.170000DSNESM68 00001-01-02 3 STAGE2 2008-08-06-13.08.53.170000
PROGNAME QQ PREDNO FILTER_FACTOR BT JOIN AJ AP RP KEYFIELD---------+---------+---------+---------+---------+---------+---------+---------+--------
DSN_FILTER_TABLE Query Output
DSN_PREDICAT_TABLE Query Output
identified as stage 2
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DSNESM68 00001-01 1 +0.1000000000000000E+01 Y N N N N DSNESM68 00001-01 2 +0.2380952239036560E-01 Y N N N Y DSNESM68 00001-01 3 +0.7142859697341919E-01 Y Y N N N DSNESM68 00001-01 4 +0.7142859697341919E-01 Y N Y N Y
TEXT B_TM---------+---------+---------+---------+---------+---------+--------(B.DEPTNO='B01' AND (A.WORKDEPT='B01' AN 2008-08-06-13.08.53.170000A.WORKDEPT='B01' 2008-08-06-13.08.53.170000A.WORKDEPT=B.DEPTNO 2008-08-06-13.08.53.170000B.DEPTNO='B01' 2008-08-06-13.08.53.170000
This fourth predicate was generated by DB2 via
predicate transitive closure
Addition Information Not Covered
� This is only an introduction to the use of these tables� May require more information
– Compliments the PLAN_TABLE information� Filter, Detailed Cost, and Predicate tables provide enhanced
informationinformation
DSN_VIEWREF_TABLEDSN_QUERY_TABLE
DSN_PGRANGE_TABLEDSN_SORTKEY_TABLE
Views and MQTs used in a query
Query text before and after query transformation
Qualified partitions for a page range scan
The keys for all sorts
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DSN_SORT_TABLEDSN_PTASK_TABLE
DSN_PGROUP_TABLEDSN_STRUCT_TABLE
yin a query
Sort operations required
Information about query parallelism
Information about each query block in a query
DB2 9 for z/OS 6/14/2009
© YL&A 1999 - 2009 17
� Explain has been enhanced– Allowing for EXPLAINs to be done for statements in the global
prepare cache� The access path used by the statement is written to the PLAN_TABLE
– COLLID - DSNDYNAMICSQLCACHE
Global Prepare Cache – V8
� The statement does NOT go through access path selection (as opposed to normal explain processing of dynamic SQL)
– Statement Id (STMTID) available from trace records with IFCID 316, 124
– Statement Token (STMTTOKEN) assigned by application that prepares statement
� RRSAF SET_ID function
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� SQLESETI function for remote applicationsEXPLAIN STMTCACHE
STMTID = int
EXPLAIN STMTCACHESTMTTOKEN = string
DSN_STATEMENT_CACHE_TABLE – V8
� To populate use keyword ALL is on EXPLAIN STMTCACHE� DSN_STATEMENT_CACHE_TABLE is created to hold the output of
IFCID 316 and IFCID 318� Two different sets of information that can be collected from the
SQL statements in the dynamic statement cache y� STMTCACHE with the STMTID or STMTTOKEN
– Traditional access path information to be written to the PLAN_TABLE for the associated SQL statement
– Single row written to DSN_STATEMENT_CACHE_TABLE if it exists� STMTCACHE with the ALL keyword
– Information is written only DSN_STATEMENT_CACHE_TABLE– Consists of one row per SQL statement in the dynamic statement cache
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Co s sts o o e o pe SQ state e t t e dy a c state e t cac efor which the current authorization ID is authorized to execute.
EXPLAIN STMTCACHE ALL
DB2 9 for z/OS 6/14/2009
© YL&A 1999 - 2009 18
DSN_STATEMENT_CACHE_TABLE – V8STMT_ID Statement ID, EDM unique tokenSTMT_TOKEN Statement token. User-provided identification stringCOLLID Collection id value is DSNDYNAMICSQLCACHEPROGRAM_NAME Program name, Name of package or DBRM that performed the
initial PREPAREINV DROPALT Invalidated by DROP/ALTERINV_DROPALT Invalidated by DROP/ALTERINV_REVOKE Invalidated by REVOKEINV_LRU Removed from cache by LRUINV_RUNSTATS Invalidated by RUNSTATSCACHED_TS TS Timestamp when statement was cachedUSERS Number of current users of statement. These are the users that
have prepared or executed the statement during their current unit of work.
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COPIES Number of copies of the statement owned by all threads in the system
LINES Precompiler line number from the initial PREPAREPRIMAUTH User ID - Primary authorization ID of the user that did the initial
PREPARECURSQLID CURRENT SQLID of the user that did the initial prepare
BIND_QUALIFIER Bind Qualifier, object qualifier for unqualified table names
BIND_ISO ISOLATION BIND option: , 'UR' - Uncommitted Read , 'CS' - Cursor Stability , 'RS' - Read Stability , 'RR' - Repeatable Read
BIND_C DATA CURRENTDATA BIND option: - 'Y' - CURRENTDATA(YES) - 'N' - CURRENTDATA(NO)
BIND_DYNRL DYNAMICRULES BIND option: - 'B' - DYNAMICRULES(BIND), 'R' - DYNAMICRULES(RUN)
BIND DEGRE CURRENT DEGREE value: - 'A' - CURRENT DEGREE = ANY , '1' - CURRENT DEGREE = 1
DSN_STATEMENT_CACHE_TABLE (cont..) – V8
BIND_DEGRE CURRENT DEGREE value: A CURRENT DEGREE ANY , 1 CURRENT DEGREE 1
BIND_SQLRL CURRENT RULES value: ‘D' - CURRENT RULES = DB2, 'S' - CURRENT RULES = SQL
BIND_CHOLD Cursor WITH HOLD bind option 'Y' - Initial PREPARE was done for a cursor WITH HOLD, 'N' - Initial PREPARE was not done for a cursor WITH HOLD
STAT_TS Timestamp of stats when IFCID 318 is started
STAT_EXEC Number of executions of statement. For a cursor statement, this is the number of OPENs
STAT_GPAG Number of getpage operations performed for statement
STAT_SYNR Number of synchronous buffer reads performed for statement
STAT WRIT N b f b ff it ti f d f t t t
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STAT_WRIT Number of buffer write operations performed for statement
STAT_EROW Number of rows examined for statement
STAT_PROW Number of rows processed for statement
STAT_SORT Number of sorts performed for statement
STAT_INDX Number of index scans performed for statement
STAT_RSCN Number of table space scans performed for statement
DB2 9 for z/OS 6/14/2009
© YL&A 1999 - 2009 19
STAT_PGRP Number of parallel groups created for statementSTAT_ELAP Accumulated elapsed time used for statementSTAT_CPU Accumulated CPU time used for statementSTAT_SUS_SYNIO Accumulated wait time for synchronous I/OSTAT_SUS_LOCK Accumulated wait time for lock and latch request
DSN_STATEMENT_CACHE_TABLE (cont..) – V8
STAT_SUS_SWIT Accumulated wait time for synchronous execution unit switchSTAT_SUS_GLCK Accumulated wait time for global locksSTAT_SUS_OTHR Accumulated wait time for read activity done by another thread STAT_SUS_OTHW Accumulated wait time for write activity done by another threadSTAT_RIDLIMT Number of times a RID list was not used because the number of RIDs would
have exceeded one or more DB2 limitsSTAT_RIDSTOR Number of times a RID list was not used because not enough storage was
available to hold the list of RIDs
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EXPLAIN_TS When statement cache table is populatedSCHEMA CURRENT SCHEMA valueSTMT_TEXT Statement textSTMT_ROWID Statement ROWID
� New Columns in V9
BIND RA TOT
DSN_STATEMENT_CACHE_TABLE – V9
BIND_RA_TOTTotal number of rebinds that have been issued for the dynamic
statement due to REOPT(AUTO)BIND_TO_TYPE
N – REOPT(NONE) or its equivalent1 – REOPT(ONCE) or its equivalentA – REOPT(AUTO)
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( )O – Current plan is deemed as optimal and no need for further
REOPT(AUTO)
DB2 9 for z/OS 6/14/2009
© YL&A 1999 - 2009 20
DSN_STATMENT_CACHE_TABLE Usage
SELECT cached_ts, STAT_EXEC, dec(stat_elap,12,2) as stat_elap, dec(STAT_CPU,12,2) as stat_cpu, left(STMT_TEXT,100) as short_text
from UID1.DSN_STATEMENT_CACHE_TABLEwhere primauth = ‘ABC'and explain_ts > '2008-08-27-00.00.00.000000'order by stat_cpu desc
An example statement extracting elapsed and CPU time f i i th d i SQL h i l di th fi t
© YL&A 1997-2009
for queries in the dynamic SQL cache, including the first 100 characters of the SQL text.
Could also get STMTID and then obtain the access path information from the cache.
Example Output
CACHED_TS STAT_EXEC STAT_ELAP STAT_CPU SHORT_TEXT--------------------------------------------------------------------------------------------------------------------------2008-08-26 01:55:29.340787 341295 7050.42 1924.97 SELECT
2008-08-26 01:55:24 243727 252134 5508 16 1741 26 SELECT2008-08-26 01:55:24.243727 252134 5508.16 1741.26 SELECT
2008-08-26 01:55:30.130466 193977 2943.68 930.89 SELECT
An example statement extracting elapsed and CPU time
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An example statement extracting elapsed and CPU time for queries in the dynamic SQL cache, including the SQL text (just the first few characters in this example).
DB2 9 for z/OS 6/14/2009
© YL&A 1999 - 2009 21
Conclusions
� IBM's advanced SQL analysis tools– Nice if you have them available
� The EXPLAIN tables explained– Many new tables with additional information
� Defining the EXPLAIN tables you need – Can be defined and populated outside the tools
� Running EXPLAINs under SPUFI – Or anywhere an EXPLAIN can be executed
� Queries for the EXPLAIN tables – Basic and advanced
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Basic and advanced� Selecting only at the fields needed for query tuning
– Many very useful fields for additional information � EXPLAIN facilities for dynamic SQL tuning
– Ability to look at statement cache
DB2 9 for z/OS DBA Certification Guide
“DB2 9 for z/OS DBA Certification Guide” McPressOctober 2007 Authored By Susan Lawson and Dan Luksetich
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DB2 9 for z/OS 6/14/2009
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DB2 Information on the Web� IBM
� Ibm.com� IBM Software
� Ibm.com/software� DB2 Family
� Ibm.com/software/db2DB2 S l ti Di t A li ti� DB2 Solutions Directory Applications
� Ibm.com/developerworks/db2� "Red Books"
� ibm. com/ redbooks� DB2 for z/OS
� Ibm. Com/software/db2zos� DB2 Support
� Ibm.com/software/db2zos/support.html� DB2 for z/OS Papers
f //f f / f / /
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� ftp://ftp.software.ibm.com/software/data/db2zos� DB2 Magazine
� http:// www. db2mag. Com� DB2 Certification
� http://www.ibm.com/certify� DB2 Experts
� www.db2expert.com
Courses by YL&A - Taught by skilled instructors world-wide!
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CPU Reduction Through Performance Audits� DB2 Performance Audits
– Existing or new database designs and applications– Certification of design and implementation acceptance– Evaluation of all the performance ‘points’ in a DB2 environment
� Physical Design� Subsystem� Application Code and SQL
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� What was acceptable performance in the past is NOT acceptable in an e-business environment
– Experienced in ‘fighting fires’ – many performance problems do not become reality until production
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become reality until production– Results: problems identified, solutions proposed (many implemented
immediately), continual knowledge transfer during the process
Cost Avoidance Through Performance Tuning!!!!