1 King Saud University College of Computer & Information Sciences IS 335 Database Management System Lecture 6 Query Processing and Optimization (Practice) Dr. Mourad YKHLEF The slides content is derived from many references

2 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Motivation (1) We would like to find the cheapest way to calculate the join of three tables: Sailors  Reserves  Boats

3 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Motivation (2) We need to decide on the order of operations: (Sailors  Reserves)  Boats or Sailors  (Reserves  Boats) We need to decide which join algorithm to use for each of the operations What information do we need?

4 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Statistics Maintained by DBMS for Relations Cardinality NTuples(R) : Number of tuples in each relation R Size NPages(R) : Number of pages in each relation R

5 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Statistics Maintained by DBMS for Indexes Index Cardinality: Number of distinct key values NKeys(I) for each index I Index Size: Number of pages INPages(I) in each index I Index Height: Number of non-leaf levels IHeight(I) in each B+ Tree index I Index Range: The minimum value ILow(I) and maximum value IHigh(I) for each index I

6 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Note The statistics are updated periodically ( not every time the underlying relations are modified). We cannot use the cardinality for computing select count(*) from R

7 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Estimating Result Sizes Consider The maximum number of tuples is the product of the cardinalities of the relations in the FROM clause The WHERE clause is associating a reduction factor with each term. It reflects the impact of the term in reducing result size. SELECT attribute-list FROM relation-list WHERE term 1 and... and term n

8 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Result Size Estimated result size: maximum size X the product of the reduction factors

9 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Assumptions Containment of value sets: i f NKeys(I1)<NKeys(I2) for attribute Y, then every Y-value of R will be a Y-value of S Empirically-obtained reduction factor is 1/10 if no additional info is available

10 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Estimating Reduction Factors column = value: 1/NKeys(I) –There is an index I on column. –This assumes a uniform distribution. –Otherwise, use 1/10. column1 = column2: 1/Max(NKeys(I1),NKeys(I2)) –There is an index I1 on column1 and an index I2 on column2. –Containment of value sets assumption –If only one column has an index, we use it to estimate the value. –Otherwise, use 1/10.

11 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Estimating Reduction Factors column > value: (High(I)-value)/(High(I)- Low(I)) if there is an index I on column.

12 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Example Cardinality(R) = 100,000 Cardinality(S) = 40,000 NKeys(Index on R.agent) = 100 High(Index on Rating) = 10, Low = 0 Reserves (sid, agent), Sailors(sid, rating) SELECT * FROM Reserves R, Sailors S WHERE R.sid = S.sid and S.rating > 3 and R.agent = ‘Joe’

13 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Example (cont.) Maximum cardinality: 100,000 * 40,000 Reduction factor of R.sid = S.sid: 1/40,000 –sid is a primary key of S Reduction factor of S.rating > 3: (10–3)/(10-0) = 7/10 Reduction factor of R.agent = ‘Joe’: 1/100 Total Estimated size: 700

Creating Indexes Using Oracle

15 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Index Map between –the row key –the row location Oracle has two kinds of indexes –B* tree –Bitmap Sorted

16 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef B* tree Root * 3*5* 7*14*16* 19*20*22*24*27* 29*33*34* 38* 39* 14

17 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Creating an Index Syntax: create [bitmap] [unique] index iname on table(column [, column ]...)

18 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Unique Indexes Create an index that will guarantee the uniqueness of the key. Fail if any duplicate already exists. When you create a table with a –primary key constraint or –unique constraint a "unique" index is created automatically create unique index rating_bit on Sailors(rating);

19 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Bitmap Indexes Appropriate for columns that may have very few possible values For each value c that appears in the column, a vector v of bits is created, with a 1 in v[i] if the i -th row has the value c – Vector length = number of rows Oracle can automatically convert bitmap entries to RowIDs during query processing

20 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Bitmap Indexes: Example create bitmap index rating_bit on Sailors(rating); Corresponding bitmaps: – 3: – 7: –10: SidSnameagerating 12Jim553 13John467 14Jane Sam373

21 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef When to Create an Index Large tables, on columns that are likely to appear in where clauses as a simple equality where s.sname = ‘John’ and s.age = 50 where s.age = r.age

22 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Function-Based Indexes You can't use an index on sname for the following query: select * from Sailors where UPPER(sname) = 'SAM'; You can create a function-based index to speed up the query: create index upp_sname on Sailors(UPPER(sname));

23 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Index-Organized Tables An index organized table keeps its data sorted by the primary key Rows do not have RowIDs They store their data as if they were an index create table Sailors( sid number primary key, sname varchar2(30), age number, rating number) organization index;

24 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Index-Organized Tables (2) What advantages does this have? –Primary key is not duplicated in the index –Improve performance of queries based on the primary key What disadvantages? –expensive to add columns, dynamic data When to use? –where clause on the primary key –static data

25 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Clustering Tables Together You can ask Oracle to store several tables with common columns together on the disk This is useful if you often join these tables Cluster: area on the disk where the rows of the tables are stored Cluster key: the columns by which the tables are usually joined in a query

26 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Clustering Tables Together: Syntax create cluster sailor_reserves (X number); –Create a cluster with nothing in it create table Sailors( sid number primary key, sname varchar2(30), age number, rating number) cluster sailor_reserves(sid); –create the table in the cluster

27 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Clustering Tables Together: Syntax (cont.) create index sailor_reserves_index on cluster sailor_reserves –Create an index on the cluster create table Reserves( sid number, bid number, day date, primary key(sid, bid, day) ) cluster sailor_reserves(sid); –A second table is added to the cluster

28 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Reserves sidbidday /7/ /10/ /12/96 Sailors sidsnameratingage 22Dustin Lubber Rusty Stored sidsnameratingagebidday 22Dustin /7/ /10/96 31Lubber Rusty /12/96

The Oracle Optimizer

30 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Types of Optimizers There are different modes for the optimizer RULE: Rule-based optimizer (RBO) –deprecated CHOOSE: Cost-based optimizer (CBO); picks a plan based on statistics (e.g. number of rows in a table, number of distinct keys in an index) –Need to analyze the data in the database using analyze command ALTER SESSION SET optimizer_mode = {choose|rule|first_rows(_n)|all_rows}

31 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Types of Optimizers ALL_ROWS: 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): 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

32 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef analyze table | index | compute statistics | estimate statistics [sample rows | percent] | delete statistics; analyze table Sailors estimate statistics sample 25 percent; Analyzing the Data

33 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Viewing the Execution Plan (Option 1) 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 Or C:\oracle\ora92\rdbms\admin\utlxplan.sql

34 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef PLAN_TABLE create table PLAN_TABLE ( statement_id varchar2(30), plan_id number, timestamp date, remarks varchar2(4000), operation varchar2(30), options varchar2(255), object_node varchar2(128), object_owner varchar2(30),

35 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef PLAN_TABLE object_name varchar2(30), object_alias varchar2(65), object_instance numeric, object_type varchar2(30), optimizer varchar2(255), search_columns number, id numeric, parent_id numeric, depth numeric,

36 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef PLAN_TABLE position numeric, cost numeric, cardinality numeric, bytes numeric, other_tag varchar2(255), partition_start varchar2(255), partition_stop varchar2(255), partition_id numeric, other long,

37 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef PLAN_TABLE distribution varchar2(30), cpu_cost numeric, io_cost numeric, temp_space numeric, access_predicates varchar2(4000), filter_predicates varchar2(4000), projection varchar2(4000), time numeric, qblock_name varchar2(30) );

38 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef Viewing the Execution Plan (Option 2) Another option: explain plan set statement_id='test' for SELECT * FROM Sailors S WHERE sname='Joe'; explain plan set statement_id=‘ ’ for Select … from Plan_Table where statement_id= ‘ test ’ ;

39 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef 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

40 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef 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

41 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef 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

42 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef

43 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef

44 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef

45 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef 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

46 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef 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

47 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef 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

48 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef 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

49 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef 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’;

50 IS 335 – Query Processing and Optimization - Dr. Mourad Ykhlef