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Oracle Optimizer

Oracle Optimizer. Types of Optimizers. There are different modes for the optimizer RULE: Rule-based optimizer (RBO) Deprecated; not updated since 1994 CHOOSE: Default option. Chooses cost-based optimizer (CBO) if statistics available, rule-based optimizer otherwise.

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Oracle Optimizer

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  1. Oracle Optimizer

  2. Types of Optimizers • There are different modes for the optimizer • RULE: Rule-based optimizer (RBO) • Deprecated; not updated since 1994 • CHOOSE: Default option. Chooses cost-based optimizer (CBO) if statistics available, rule-based optimizer otherwise. ALTER SESSION SET optimizer_mode = {choose|rule|first_rows(_n)|all_rows}

  3. Types of Optimizers • ALL_ROWS (CBO): Execute the query so that all of the rows are returned as quickly as possible • Merge Join has priority over Block Nested Loop Join • FIRST_ROWS(n) (CBO): execute the query so that all of the first n rows are returned as quickly as possible • Block Nested Loop Join has priority over Merge Join

  4. Analyzing the Data analyze table | index <table_name> | <index_name> compute statistics | estimate statistics [sample <integer> rows | percent] | delete statistics; analyze table Sailors estimate statistics sample 25 percent;

  5. Viewing the Execution Plan • You need a PLAN_TABLE table. So, the first time that you want to see execution plans, run the command: • Set autotrace on to see all plans • Display the execution path for each query, after being executed @$ORACLE_HOME/rdbms/admin/utlxplan.sql

  6. Operations that Access Tables • TABLE ACCESS FULL: sequential table scan • Oracle optimizes by reading multiple blocks • Used whenever there is no where clause on a query select * from Sailors • TABLE ACCESS BY ROWID: access rows by their RowID values. • How do you get the rowid? From an index! select * from Sailors where sid > 10

  7. Types of Indexes • Unique: each row of the indexed table contains a unique value for the indexed column • Nonunique: the row’s indexed values can repeat

  8. Operations that Use Indexes • INDEX UNIQUE SCAN: Access of an index that is defined to be unique • INDEX RANGE SCAN: Access of an index that is not unique or access of a unique index for a range of values

  9. When are Indexes Used/Not Used? • If you set an indexed column equal to a value, e.g., sname = 'Jim' • If you specify a range of values for an indexed column, e.g., sname like 'J%' • sname like '%m': will not use an index • UPPER(sname) like 'J%' : will not use an index • sname is null: will not use an index, since null values are not stored in the index • sname is not null: will not use an index, since every value in the index would have to be accessed

  10. When are Indexes Used? (cont) • 2*age = 20: Index on age will not be used. Index on 2*age will be used. • sname != 'Jim': Index will not be used. • MIN and MAX functions: Index will be used • Equality of a column in a leading column of a multicolumn index. For example, suppose we have a multicolumn index on (sid, bid, day) • sid = 12: Can use the index • bid = 101: Cannot use the index

  11. Optimizer Hints • You can give the optimizer hints about how to perform query evaluation • Hints are written in /*+ */ right after the select • Note: These are only hints. The Oracle optimizer can choose to ignore your hints

  12. Hints • FULL hint: tell the optimizer to perform a TABLE ACCESS FULL operation on the specified table • ROWID hint: tell the optimizer to perform a TABLE ACCESS BY ROWID operation on the specified table • INDEX hint: tells the optimizer to use an index-based scan on the specified table

  13. Examples Select /*+ FULL (sailors) */ sid From sailors Where sname=‘Joe’; Select /*+ INDEX (sailors) */ sid From sailors Where sname=‘Joe’; Select /*+ INDEX (sailors s_ind) */ sid From sailors S, reserves R Where S.sid=R.sid AND sname=‘Joe’;

  14. Combining Output From Multiple Index Scans • Suppose we have 2 indexes: sname, rating • select * from sailorswhere sname = 'Jim' and rating = 10 ----------------------------------0 SELECT STATEMENT Optimizer=CHOOSE1 0 TABLE ACCESS (BY INDEX ROWID) OF 'SAILORS‘2 1 AND-EQUAL3 2 INDEX (RANGE SCAN) OF 'SNAME_IND' (NON-UNIQUE)4 2 INDEX (RANGE SCAN) OF 'RAT' (NON-UNIQUE)

  15. Operations that Manipulate Data Sets • Up until now, all operations returned the rows as they were found • There are operations that must find all rows before returning a single row • Try to avoid these operations for online users! • SORT ORDER BY: query with order by select sname, age from Sailors order by age;

  16. Operations that Manipulate Data Sets • SORT UNIQUE: sorting records while eliminating duplicates (e.g., query with distinct; query with minus, intersect or union) • SORT AGGREGATE, SORT GROUP BY: queries with aggregate or grouping functions (like MIN, MAX)

  17. Is the table always accessed? What if there is no index?

  18. Operations that Manipulate Data Sets • Consider the query: • select sname from sailors union select bname from boats;

  19. Operations that Manipulate Data Sets • Consider the query: • select sname from sailors minus select bname from boats; How do you think that Oracle implements intersect? union all?

  20. Operations that Manipulate Data Sets • Select MIN(age), COUNT(*) from Sailors GROUP BY rating ------------------------------------------0 SELECT STATEMENT Optimizer=CHOOSE1 0 SORT (GROUP BY)2 1 TABLE ACCESS (FULL) OF 'SAILORS'

  21. Distinct • What should Oracle do when processing the query (assuming that sid is the primary key): • select distinct sid from Sailors • Try it!!

  22. Join Methods • Select * from Sailors, Reserves where Sailors.sid = Reserves.sid • Oracle can use an index on Sailors.sid or on Reserves.sid (note that both will not be used) • Join Methods: MERGE JOIN, NESTED LOOPS, HASH JOIN

  23. Nested Loops Joins • Block nested loop join NESTED LOOPSTABLE ACCESS FULL OF our_outer_tableTABLE ACCESS FULL OF our_inner_table • Index nested loop join NESTED LOOPS TABLE ACCESS FULL OF our_outer_table TABLE ACCESS BY ROWID OF our_inner_tableINDEX RANGE SCAN OF inner_table_index

  24. Nested Loops Joins • Select *from reserves r, sailors swhere r.sid=s.sid; -------------------------------------------------------0 SELECT STATEMENT Optimizer=CHOOSE1 0 NESTED LOOPS2 1 TABLE ACCESS (FULL) OF 'RESERVES‘3 1 TABLE ACCESS (BY INDEX ROWID) OF 'SAILORS‘4 3 INDEX (UNIQUE SCAN) OF 'SYS_C00628777' (UNIQUE)

  25. When Are Nested Loops Joins Used? • If tables are of unequal size • If results should be returned online

  26. Hash Join Plan HASH JOINTABLE ACCESS FULL OF table_ATABLE ACCESS FULL OF table_B

  27. When Are Hash Joins Used? • If tables are small • If results should be returned online

  28. Sort-Merge Join Plan MERGE JOINSORT JOINTABLE ACCESS FULL OF table_ASORT JOINTABLE ACCESS FULL OF table_B

  29. When Are Sort/Merge Joins Used? • Performs badly when tables are of unequal size. Why?

  30. Hint Examples for Joins inner table Select /*+ USE_NL (sailors) */ sid From sailors S, reserves R Where S.sid=R.sid AND sname=‘Joe’; Select /*+ USE_MERGE (sailors, reserves) */ sid From sailors S, reserves R Where S.sid=R.sid AND sname=‘Joe’; Select /*+ USE_HASH */ sid From sailors S, reserves R Where S.sid=R.sid AND sname=‘Joe’;

  31. XML (Extensible Markup Language) andthe Semi-Structured Data Model

  32. Motivation • We have seen that relational databases are very convenient to query. However: • There is a LOT of data not in relational databases!! • Perhaps the most widely accessed database is the web, and it certainly isn’t a relational database.

  33. Querying the Web • The web can be queried using a search engine, however, we can’t ask questions like: • What is the lowest price for which a Jaguar is sold on the web? • Problems: • There are no facilities for asking complex questions, such as aggregation of data

  34. Understanding the Web • In order to query the web, we must be able to understand it. • 2 Computer Science Approaches: • Artificial Intelligence Approach • Database Approach

  35. Database Approach “The web is unstructured and we will structure it” • Sometimes problems that are very difficult can be solved easily by enforcing a standard • Encourage the use of XML as a standard for data exchange on the web

  36. Example XML Document <addresses> <person friend="yes"> <name> Jeff Cohen</name> <tel> 04-828-1345 </tel> <tel> 054-470-778 </tel> <email> jeffc@cs.technion.ac.il </email> </person> <person friend="no"> <name> Irma Levy</name> <tel> 03-426-1142 </tel> <email>irmal@yourmail.com</email> </person> </addresses> Opening Tag Element Attribute Closing Tag

  37. Very Unstructured XML <?xml version=“1.0”?> <DamageReport> The insured’s <Vehicle Make = “Toyota”> Corolla </Vehicle> broke through the guard rail and plummeted into the ravine. The cause was determined to be <Cause>faulty brakes </Cause>. Amazingly there were no casualties. </DamageReport>

  38. XML Vs. HTML • XML and HTML are brothers. They are both special cases of SGML. • HTML has specific tag and attribute names. These are associated with a specific meaning • XML can have any tag and attribute name. These are not associated with any meaning • HTML is used to specify visual style • XML is used to specify meaning

  39. Characteristics of Semistructured Data (From http://www2.cs.uh.edu/~ceick/6340/intro-xml.ppt) • Missing or additional attributes • Multi-valued attributes • Different types in different objects • Heterogeneous collections Self-describing, irregular data, no a priori structure

  40. Relational vs Semi-Structured

  41. Data Exchange • Problem: Many data sources, each of a different type (different vendor), with a different schema. • How can the data be combined and used together? • How can different companies collaborate on their data? • What format should be used to exchange the data?

  42. Separating Content from Style • Web sites develop over time • Important to separate style from data in order to allow changes to the site structure and appearance • Using XML, we can store data alone • CSS separates style from data only in a limited way • Using XSL, this data can be translated into HTML • The data can be translated differently as the site develops

  43. XSL XSL XSL WML (hand-held devices) HTML (web browser TEXT (Excel) Write Once Use Everywhere XML Data

  44. Using XML • Quering and Searching XML: There are query languages and search engines that query XML and return XML. Examples: Xpath, Xquery /SQL4X, Equix, XSEarch • Displaying XML:An XML document can have an associated style-sheet which specifies how the document should be translated to HTML. Examples: CSS, XSL

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