Dale Roberts 8/20/ Department of Computer and Information Science, School of Science, IUPUI Dale Roberts, Lecturer Computer Science, IUPUI SQL Tuning Introduction

Dale Roberts 8-2 Oracle Data Dictionary Data Dictionary: stores all information that is used to manage the objects in the database. Nomenclature: USER_%objects you own ALL_%objects you own, have been granted access to, or public DBA_%all objects in database. Road Map Tables: DICTIONARY (DICT) - describes the Oracle data dictionary tables. select table_name, comments from DICT where table_name like ‘USER_%’;

Dale Roberts 8-3 Tables about Database Objects: –USER_CATALOG (CAT) - tables, views, synonyms, sequences –USER_OBJECTS (OBJ) - clusters, links, functions, indexes, packages, procedures, sequences, synonyms, tables, triggers, views –USER_TABLES (TABS) –USER_TAB_COLUMNS (COLS) –USER_VIEWS –USER_SYNONYMS (SYN) –USER_SEQUENCES (SEQ) Constraints: –USER_CONSTRAINTS –USER_CONS_COLUMNS –EXCEPTIONS Comments: –USER_TAB_COMMENTS –USER_COL_COMMENTS Indexes: –USER_INDEXES (IND) –USER_IND_COLUMNS Clusters: –USER_CLUSTERS (CLU) –USER_CLU_COLUMNS Oracle Data Dictionary

Dale Roberts 8-4 Links, Snapshots: –USER_DB_LINKS –USER_SNAPSHOTS –USER_SNAPSHOT_LOGS Procedures: –USER_TRIGGERS –USER_SOURCE –USER_ERRORS –USER_OBJECT_SIZE –USER_ARGUMENTS Space Allocation: –USER_TABLESPACES –USER_SEGMENTS –USER_EXTENTS –USER_FREE_SPACE User Tables: –USER_USERS –USER_RESOURCE_LIMITS –USER_TAB_PRIVS –USER_SYS_PRIVS Roles: –USER_ROLE_PRIVS –ROLE_TAB_PRIVS - DBA privilege required –ROLE_SYS_PRIVS - DBA privilege required –ROLE_ROLE_PRIVS - DBA privilege required Oracle Data Dictionary

Dale Roberts 8-5 Auditing: (only if auditing turned on by DBA) –USER_AUDIT_OBJECT –USER_AUDIT_SESSION –USER_AUDIT_STATEMENT –USER_OBJ_AUDIT_OPTS Dynamic Tables: (DBA ONLY) –V$ACCESS - locks currently held on objects –V$FILESTAT - file read/write statistics –V$LOCK - non-DDL locks and resources –V$PROCESS - currently active processes –V$SESSION - currently active sessions –V$SGA - size of each component in the SGA (System Global Area) –V$SYSSTAT - current statistics on entire database –V$TRANSACTION- info about transactions. Chained Rows: –Create table CHAINED_ROWS from $ORA_HOME/rdbms/admin/utlchain.sql –Analyze a table like this: analyze LEDGER list chained rows into CHAINED_ROWS; Plan Table: –Create table PLAN_TABLE from $ORA_HOME/rdbms/admin/utlxplan.sql –Analyze a query like this: explain plan set Statement_id = ‘A’ for select...; Oracle Data Dictionary

Dale Roberts 8-6 Understand the Applications Tasks –What data is to be captured? –How should the data be processed? –How should the data be reported? Good Design Has a Human Touch

Dale Roberts 8-7 Ways to Increase DB Performance: –Relax normalization rules by using meaningful keys, pp –Put redundant data into 3NF tables, pp –More memory, p. 614 –Combining smaller tables, “Kitchen Junk Drawer” approach, p. 615 –Computation Table, p. 616 –Snapshots, Chapter 23 Performance and Design

Dale Roberts 8-8 Why tune SQL statements: –Improve response time of interactive programs. –Improve batch throughput. –To ensure scalability of applications (load v. perf.) –Reduce system load for other uses besides DB. –Avoid hardware upgrades. Scalability: –Well-tuned applications deliver good performance as number of users or data volume increases. –Applications which have a linear degradation pattern degrade predictably, usually they are problems responsive to hardware upgrades. –Exponential degradation patterns are more serious. They tend to be problems that creep up over time, starting benign and unrecognized, later becoming huge problems. –Bottlenecks are performance problems which are abrupt, like “hitting a brick wall”. Usually no warning, and no hardware solution. SQL Tuning

Dale Roberts 8-9 Objections to SQL tuning: –“The Oracle optimizer will tune my statements for me automatically.” –“I’m not a SQL programmer, I’m a... VB/PowerBuilder/C++/Java programmer.” –“I’ll write the SQL, someone else (DBA) can tune it for me later.” –“I’ll tune it later” –“We can’t afford to tune it” When to tune: –Early is least costly & better. –Changing SQL/table designs in the design phase means that no applications need to be re-written. –Tuning SQL performance when SQL is first written usually means lower testing costs. –In production systems, testing SQL can sometimes be difficult... change control, production system availability, extra time to deal with larger data volumes. –Tune SQL early as possible to be most effective and economical. SQL Tuning

Dale Roberts 8-10 SQL Tuning

Dale Roberts 8-11 SQL Tuning Retrieving Data: –Full Table Scan - gets data from row 1 to high water mark –Row ID - gets data by physical location. Quickest way to get a row. –Index Lookup - matches up key value with Row ID –Hash Key Lookup - computes a Row ID with a mathematical formula applied to key value. Joining Data: –Sort Merge Join sorts each table’s key columns merges data together does not use indexes –Nested Loops Join full table scan used on smaller table key values of 1st table joined with the larger table’s index –Hash Join hash key is built for larger table, constructs index on the fly smaller table is then fully scanned data from smaller table is joined with the hash key index.

Dale Roberts 8-12 SQL Tuning Oracle Optimizers: –Rule Based Optimizer older optimizer, used since first versions of Oracle. set of rules which rank access paths. always picks an index over doing full table scan. –Cost Based Optimizer new in Oracle 7. takes advantage of database statistics to pick optimal access path. To collect table statistics: ANALYZE TABLE tablename CALCULATE STATISTICS; Optimizer Goals: –RULE, picks only rule based optimizer. –CHOOSE, picks cost based optimizer if any table in query has been analyzed. –ALL_ROWS, picks the cost based optimizer and finds an execution plan which is best for the entire query. Good for batch reporting. –FIRST_ROWS, pick the cost based optimizer and finds an execution plan which is best for the first row. Good for interactive applications.

Dale Roberts 8-13 SQL Tuning How to set the optimizer goal: –Change the database configuration file (init.ora). OPTIMIZER_MODE=FIRST_ROWS –Change settings for your session in SQL*Plus. ALTER SESSION SET OPTIMIZER_GOAL=RULE; –“Influence” the optimizer with “hints”. Example, use rule based optimizer: SELECT /*+ RULE */ * FROM EMPLOYEE WHERE SALARY > ; Example, use full table scan: select /*+ FULL(E) FULL(D) */ e.employee_id, e.surname, e.firstname from employee e, department d where d.location=‘Indianapolis’ and d.dept_id = e.dept_id; Example, recommend an index: select /*+ index(E, emp_dept_idx) */ e.employee_id, e.surname, e.firstname from employee e, department d where d.location=‘Indianapolis’ and d.dept_id = e.dept_id;

Dale Roberts 8-14 SQL Tuning Tracing Oracle SQL: –EXPLAIN PLAN, shows the execution plan. –SQL_TRACE, generates a trace file containing SQL executed by your session and resources used. –tkprof, formats SQL_TRACE output. –AUTOTRACE, SQL*Plus command to show exuction plans and statistics in your SQL*Plus session. –3rd Party Tools. Numerous GUI tools to quickly show this information, usually expensive.

Dale Roberts 8-15 EXPLAIN PLAN Executing: explain plan for select /*+RULE */ e.surname, e.firstname, e.date_of_birth from employee e, customers c where e.surname=c.contact_surname and e.firstname=c.contact_firstname and e.date_of_birth=c.date_of_birth order by e.surname, e.firstname; Formatting Plan Table in a Query: select rtrim(lpad(‘ ‘,2*level)|| rtrim(operation)||‘ ‘|| rtrim(options)||‘ ‘|| object_name) as query_plan from plan_table connect by prior id=parent_id start with id=0; Execution Plan Query: query_plan SELECT STATEMENT SORT ORDER BY NETED LOOPS TABLE ACCESS FULL CUSTOMERS TABLE ACCESS BY ROWID EMPLOYEES INDEX RANGE SCAN EMP_BOTH_IDX SQL Tuning

Dale Roberts 8/20/ Acknowledgements Loney, Oracle Database 10g The Complete Reference