Monitoring SQL Performance Using the DB2 Statement Cache

Transcription

1 Session: I02 Monitoring SQL Performance Using the DB2 Statement Cache Mike Perry BMC Software May 19, :00 a.m. 12:00 p.m. Platform: DB2 for z/os This session discusses a low cost method of detecting and diagnosing SQL performance problems. The V8 EXPLAIN STMTCACHE and the DB2 9 EXPLAIN MONITORED STMTS statements form the basis of the technique. You will learn how to use the DSN_STATEMENT_CACHE_TABLE to find unexpected statistics values, to obtain the statement text and to execute an EXPLAIN PLAN statement. The session reveals the cost of collecting the statistics. 1

2 Outline Activating statement cache monitoring Detecting exceptions Obtaining access paths Activating profile monitoring (DB2 9) Keeping a performance database 2 Objectives 1. Describe the DB2 statement cache, including structure and contents. Includes discussion on DB2 9 changes 2. Show how cached performance metrics are activated and accessed for analysis 3. Identify techniques for detecting exception conditions for statements in the cache 4. Show how SQL statement text from the cache can be analyzed using EXPLAIN processes 5. Describe keeping a performance database to be used for long term analysis 2

3 CACHEDYN=YES Statement cache DBM1 EDM pool 3 ** Dynamic statement cache An EDM pool in which DB2 saves prepared SQL statements for sharing among different threads, plans, and packages. Saves re-preparation cost. Cast-out strategy = Least Recently Used (LRU) ** Eligible statements SELECT UPDATE INSERT DELETE MERGE Not statements in plans or packages bound with REOPT(ALWAYS). ** Conditions for sharing Identical statements Same authorization ID or role Same bind options Same values of special registers ** Static SQL (DB2 9 only) 3

4 EXPLAIN STMTCACHE ALL Statement cache owner.dsn_statement_cache_table 4 SYSADM authority inserts all statements in the dynamic statement cache. Otherwise DB2 inserts only statements with the current authorization ID. 4

5 DSN_STATEMENT_CACHE_TABLE Statement ID section Bind section Stats section SQL text 5 Description in DB2 for z/os SQL Reference Explain Chapter Also DB2 for z/os Performance Monitoring and Tuning Guide JCL in SDSNSAMP(DSNTESC) ** Statement ID collection ID, program name, current sqlid ** Bind info qualifier, isolation, currentdata, dynamicrules, degree ** Stats executions, get pages, synchronous reads, writes, elapsed time, CPU time, wait time ** SQL text goes to auxiliary table (CLOB) Stats not collected unless start trace(mon) ifcid(318) 5

6 -START TRACE(MON) IFCID(318) DEST(GTF) for DB2 9 PLAN(planname) AUTHID(authorization_id) 6 DEST(OPX) for DB2 9 gets DSNW182I *DEDK AN INTERNAL STOP HAS BEEN ISSUED FOR TRACE NUMBER 03 BECAUSE THE OP BUFFER OWNER HAS TERMINATED No actual Instrumentation Facility Component ID (IFCID) 318 record exists. IFCID enables collection of IFCID 316 and 317. You can only use IFI READS command to get them. See DB2 for z/os and OS/390 : Squeezing the Most Out of Dynamic SQL, SG and DB2 UDB for z/os Version 8: Everything You Ever Wanted to Know,... and More You can Limit scope of tracing with PLAN or AUTHID. 6

7 CPU time overhead of trace Worst case microseconds/fetch off on 7 Worst case 18% increase in CPU time with IFCID(318) trace on Benchmark strategy, measure job step CPU time for 500,000 fetches Select current timestamp from sysibm.sysdummy1 1.4 microsecond overhead per fetch 7

8 Query cache_table to get stmt_id SELECT STMT_ID, STAT_EXEC AS EXEC, DEC(STAT_ELAP,5,2) AS ELAPSED, DEC(STAT_CPU,5,2) AS CPU, SUBSTR(STMT_TEXT,1,80) FROM DSN_STATEMENT_CACHE_TABLE WHERE STAT_CPU > 1.0 ORDER BY STAT_CPU DESC 8 Look for tuning candidates with high CPU time. 8

9 Most expensive statements STMT_ID EXEC ELAPSED CPU SELECT SELECT SELECT select 9 Make note of STMT_ID to get access path. 9

10 EXPLAIN STMTCACHE STMTID nnn Statement cache PLAN_TABLE DSN_STATEMNT_TABLE DSN_FUNCTION_TABLE 10 Statement does not go through access path selection again. Does not populate extended plan tables. 10

11 Query plan_table Select qblockno, planno, method, accesstype, tname, accessname, matchcols From plan_table Where queryno = Order by qblockno, planno, mixopseq 11 Use stmt_id to query the plan_table. Get PROCSU, service units, from the dsn_statemnt_table. 11

12 Access path steps QB PL ME AC TB IX MC I SYSINDEXES DSNDXX I SYSCOLUMNS DSNDCX R SYSKEYS 0 12 Three way join First two steps use indexes with 2 matching columns Method 1 = nested loop join Planno 3 uses tablespace scan 12

13 Use DSNREXX to retrieve the SQL sqlstmt = "select stmt_text from dsn_statement_cache_table where stmt_id = " stmt_id "execsql prepare s1 from :sqlstmt" "execsql open c1" "execsql fetch c1 into :stmt_text" "execsql close c1" call fmtsql(stmt_text) 13 /* rexx to get stmt_text from dsn_statement_cache_table INPUT: 1. db2 subsystem id 2. stmt_id */ parse upper arg ssid stmt_id 'subcom dsnrexx' if rc then S_RC = RXSUBCOM('ADD','DSNREXX','DSNREXX') address dsnrexx 'CONNECT' ssid "execsql set current packageset='dsnrexcs'" sqlstmt = "select stmt_text from dsn_statement_cache_table ", "where stmt_id = " stmt_id "execsql prepare s1 from :sqlstmt" "execsql open c1" "execsql fetch c1 into :stmt_text" "execsql close c1" call fmtsql(stmt_text) 'DISCONNECT' exit rc; 13

14 Formatted SQL text SELECT A.* FROM "SYSIBM"."SYSCOLUMNS" A, "SYSIBM"."SYSINDEXES" B, "SYSIBM"."SYSKEYS" C WHERE C.IXCREATOR = 'NSU911C1' AND C.IXNAME = 'DXRSSX0' AND C.COLSEQ = 1 AND B.CREATOR = C.IXCREATOR AND B.NAME = C.IXNAME AND A.TBCREATOR = B.TBCREATOR AND A.TBNAME = B.TBNAME AND A.COLNO = C.COLNO FOR FETCH ONLY 14 Formatted text much easier to understand /* subroutine to format SQL text */ fmtsql: parse arg sql data = space(sql) do while data <> "" rstart = pos(" SELECT ",data); if rstart = 0 then rstart = pos(" FROM ",data); if rstart = 0 then rstart = pos(" WHERE ",data); if rstart = 0 then rstart = pos(" AND ",data); if rstart = 0 then rstart = pos(" ORDER ",data); if (rstart > 0) then do; j = rstart; k = 1 do while j > 72; say substr(data,k,72); j = j - 72; k = k + 72; end say substr(data,k,j) data = substr(data,rstart+1); end else do; say data; data = ; end end return 14

15 What if? SYSIBM.SYSKEYS has index on IXCREATOR, IXNAME, COLNAME AND A.COLNO = C.COLNO AND A.NAME = C.COLNAME 15 If we use column name instead of column number, we might induce the DB2 optimizer to use an index on SYSKEYS. 15

16 EXPLAIN PLAN SET QUERYNO=nnn FOR sql-statement PLAN_TABLE DSN_STATEMNT_TABLE DSN_FUNCTION_TABLE DSN_PREDICAT_TABLE DSN_DETCOST_TABLE 9 OTHER TABLES 16 Inserts into regular plan tables and extended ones too. JCL in SDSNSAMP(DSNTIJOS) for DB2 9. Description in DB2 for z/os Performance Monitoring and Tuning Guide Extended plan tables: DSN_DETCOST_TABLE DSN_FILTER_TABLE DSN_PGRANGE_TABLE DSN_PGROUP_TABLE DSN_PREDICAT_TABLE DSN_PTASK_TABLE DSN_QUERY_TABLE DSN_SORTKEY_TABLE DSN_SORT_TABLE DSN_STRUCT_TABLE DSN_VIEWREF_TABLE 16

17 New access path steps QB PL ME AC TB IX MC I SYSINDEXES DSNDXX I SYSCOLUMNS DSNDCX I SYSKEYS DSNDKX01 3 Service Units Plan no 3 access type goes from table space scan to index access with 3 matching columns. Estimated service units dropped from 502 to 2. 17

18 Profile monitoring DB2 9 introduced Input tables control monitoring Lossless pushout Static SQL too But no wildcards 18 As statements leave the cache, DB2 pushes their information out to tables. No easy way exists to monitor every statement. 18

19 SYSIBM.DSN_PROFILE_TABLE AUTHID Authorization ID PLANNAME COLLID PKGNAME IPADDR PROFILE_ENABLED Plan name Collection ID Package name IP address Y or N 19 Primary input table for profile monitoring Set either yellow columns or blue columns to specify what to monitor. JCL in SDSNSAMP(DSNTIJOS) for DB2 9 19

20 SYSIBM.DSN_PROFILE_ATTRIBUTES PROFILEID KEYWORDS Foreign key for PROFILE_TABLE Function keywords ATTRIBUTE1 String attribute ATTRIBUTE2 Integer attribute ATTRIBUTE3 Float attribute 20 Secondary input table for profile monitoring. Function keywords specify how to monitor. 20

21 Function keywords Keyword Attribute1 Attribute2 Attribute3 MONITOR What to record MONITOR RLF What to record MONITOR CPUTIME What to record CPUTIME threshold MONITOR SPIKE PERCENT What to record Percent threshold 21 Normal monitoring Exception monitoring RLF exception CPU time exception CPU time spike exception. Current execution exceeds the average CPU time by the given percent. MAX PUSHOUT limits the number of statement reports for a monitor profile. 21

22 Tables that receive data Statement cache DSN_STATEMENT_RUNTIME_INFO DSN_OBJECT_RUNTIME_INFO PLAN TABLES 22 Qualifier = SYSIBM Rows generated after A stop profile command A statement purged from the statement cache An exception An EXPLAIN statement An start IFCID 318 trace 22

23 ATTRIBUTE2 e=explain o=object info s=statement info e eeooosss 00000eos C The 3 low bits designate the target tables S=DSN_STATEMENT_RUNTIME_INFO 0=DSN_OBJECT_RUNTIME_INFO E=EXPLAIN tables, including PLAN_TABLE, DSN_STATEMNT_TABLE, and DSN_FUNCTION_TABLE. Or Other explain tables if eee=111 The other bits (all on or all off) control either maximum or minimum amount of data. 23

24 -START PROFILE Monitors profiles where PROFILE_ENABLED = Y! -STOP TRACE(MON) DEST(GTF) 24 First stop any IFCID(318) trace. Otherwise, it will interfere with profile monitoring. 24

25 CPU time overhead of monitoring Worst case microseconds/fetch off stmts objects 25 Worst case 20% increase in CPU time with monitoring. 25% increase if monitoring objects too. 25

26 EXPLAIN MONITORED STMTS SCOPE AUTHID authid IPADDR nn.nn.nn.nn PLAN planname COLLECTION collid PACKAGE package Statement cache DSN_STATEMENT_RUNTIME_INFO sqlid.plan_table sqlid.dsn_statemnt_table sqlid.dsn_function_table 26 Use explain monitored stmts in case you don t want to wait for a natural pushout. 26

27 DSN_STATEMENT_RUNTIME_INFO Statement ID section Bind info section Stats section Exception section SQL text 27 Note addition of exception section compared to dsn_statement_cache_table. 27

28 DSN_OBJECT_RUNTIME_INFO STMT_RUNTIME_INFO_ID QBLOCKNO, PLANNO, MIXOPSEQ PROCESSED_ROWS IX_PROCESSED_ROWS COMPCD_EST Foreign key for DSN_STATEMENT_RUN TIME_INFO Key of matching PLAN_TABLE row Num rows that DB2 processes Num rows that DB2 processes for an index Estimated cardinality 28 Rows only pushed out with exceptions. Exposes object data at the access path step level. 28

29 Keeping a performance database Hourly Daily Monthly Once an hour EXPLAIN STMTCACHE ALL into DSN_STATEMENT_CACHE_TABLE. Delete rows older than 24 hours. Once a day sum the stats for each statement and insert into the daily table. Delete rows older than 30 days. Once a month sum the stats for each statement and insert into the monthly table. Delete rows older than 24 months. 29

30 Scrubbing the SQL text SELECT * FROM SYSTABLES WHERE CARDF < 0.0 AND NAME LIKE DSN% SELECT * FROM SYSTABLES WHERE CARDF <. AND NAME LIKE 30 To compare two statements for equality, first remove extra blanks, numbers and string literals. alpha = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ_@#$abcdefghijklmnopqrstuvwxyz' digits = " " alnum = alpha digits alnumq = alnum "'" pos = verify(sql,alnumq,'m') /* keep spaces and punctuation */ do while pos > 0 c = substr(sql,pos,1) select when datatype(c,'n') then do /* blank out numbers */ endpos = verify(sql,digits,'n',pos) if endpos = 0 then endpos = length(sql)+1 sql = overlay("",sql,pos,endpos-pos) end when c = "'" then do /* blank out string literals */ endpos = verify(sql,"'",'m',pos+1)+1 if endpos = 1 then endpos = length(sql)+1 sql = overlay("",sql,pos,endpos-pos) end when c = '"' then do /* keep delimited names */ endpos = verify(sql,'"','m',pos+1)+1 if endpos = 1 then endpos = length(sql)+1 end otherwise /* keep identifiers */ endpos = verify(sql,alnum,'n',pos) end pos = verify(sql,alnumq,'m',endpos) end Sql = space(sql) /* remove extra blanks */ 30

31 Computing a text hash Scrubbed text + Qualifier SELECT * FROM SYSTABLES WHERE CARDF <. AND NAME LIKE SYSIBM Texthash: hash = 0 pos = 1 len = length(sql) do while pos <= len hash =(hash * c2d(substr(sql,pos,3))) // pos = pos + 3 end return hash 31

32 Updating the performance database MERGE INTO DAILY USING ( VALUES(?,?,?,?,?,?,?) ) AS T ( HASH, PROG, EXEC, CPU, TS, SCHEMA, TEXT) ON (DAILY. STMT_HASH = T.HASH) WHEN MATCHED THEN UPDATE SET DAILY.STAT_EXEC = DAILY.STAT_EXEC + T.EXEC, DAILY.STAT_CPU = DAILY.STAT_CPU + T.CPU WHEN NOT MATCHED THEN INSERT VALUES ( T.HASH, T.PROG, T.EXEC, T.CPU, T.TS, T.SCHEMA, T.TEXT) 32 DB2 9 introduced the merge statement that performs either an update or an insert operation. 32

33 Spikes CPU DAY 33 When looking for CPU spikes, compare the current average CPU to the baseline (average for last 30 days). Select sum(stat_cpu) / sum(stat_exec) Into :avg_cpu From daily Where stmt_hash = :hash If stat_cpu / stat_exec > 3 * avg_cpu then Say CPU spike for statement = hash 33

34 Trends CPU DAY 34 When looking for CPU trends, compute the slope of the line over time. REGR_SLOPE = COVARIANCE(DAYS(EXPLAIN_TS), STAT_CPU) / VARIANCE(DAYS(EXPLAIN_TS)) 34

35 Summary Explain stmtcache all Explain stmtcache stmid= Explain plan for Explain monitored stmts Merge Covariance 35 The main ideas in this presentation: How to get data out of the statement cache How to get access path info for a statement How to change and re-explain a statement How to use profile monitoring How to maintain a performance database How to detect unusual conditions 35

36 Session I02 Monitoring SQL Performance Using the DB2 Statement Cache Mike Perry BMC Software 36 36