ROLLBACK SEGMENTS. In this chapter, you will learn about: Rollback Segment Management Page 272

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1 C H A P T E R 1 2 ROLLBACK SEGMENTS CHAPTER OBJECTIVES In this chapter, you will learn about: Rollback Segment Management Page 272 The information in a rollback segment is used for query read consistency, canceling (rolling back) a transaction, and recovering a transaction. Rollback segments store before-image information of data whenever a transaction modifies data. This before-image information is referred to as a snapshot. It is the image of the data as it existed before the start of a transaction, and it is the image that remains consistent and available to the user throughout the life of a transaction. (The use of the term, snapshot, in this chapter, is not to be confused with the Oracle snapshot object that is used in distributed transaction processing. Distributed transaction processing is not discussed in this book). The System Change Number (SCN) is the unique number assigned to each new transaction by the Oracle Server. It serves as a relative timestamp that defines the state of the database at a precise moment in time. When data is modified through a transaction, the previous values of the data (along with their SCNs) get stored in rollback segments. This way, the Server can obtain values that correspond to the SCN noted when a query began. If a row has been modified since the query began, the Server obtains the data for the query from the rollback segment. 271

2 272 Chapter 12: Rollback Segments ROLLBACK SEGMENT MANAGEMENT OBJECTIVES After this lab, you will be able to: Configure and Maintain Rollback Segments EXERCISES CONFIGURE AND MAINTAIN ROLLBACK SEGMENTS UNDERSTANDING TRANSACTIONS Rollback segments are very important in terms of transactions. A transaction is often referred to as a logical unit of work. This logical unit of work is an SQL statement or related set of statements that Oracle treats as a single unit. Modified data is written permanently to the redo logs when a user issues the COMMIT statement. And part or all of a transaction can be canceled, or undone, when a user issues the ROLLBACK statement. Transactions are managed by ORACLE with locking (discussed in Chapter 13, Locking ) and by use of the multi-version consistency model. In short, this model allows the Oracle Server to act as though each transaction has its own copy of the data. In other words, at any given time, there exist multiple, overlapping

3 Lab : Rollback Segment Management 273 versions of the database. A transaction begins with the first SQL statement in the transaction and completes with a COMMIT or ROLLBACK statement. For querying, ORACLE guarantees a transaction statement-level read consistency. Statement-level read consistency means that when a user executes a query, the current view of the data remains the same while ORACLE is fetching and returning the result set. However, if a transaction consists of multiple queries, each individual query is internally consistent, but not with any other query. For a multiple-query transaction to have data consistency, it must be started with the command SET TRANSACTION READ ONLY. This command guarantees the transaction transaction-level read consistency, which means that the transaction does not see the results of the changes brought about by other transactions. WHAT GETS STORED IN A ROLLBACK SEGMENT When a transaction modifies data, it modifies the data in the data blocks associated with the changed rows. To understand the significance of the rollback segment, one must understand what is stored in the rollback segment when a transaction modifies data. The rollback segment only stores the before-image snapshot of the row(s) within a particular data block if it was modified. For example, if a transaction modifies a data block by changing a value in one of the columns of one of its rows from 200 to 100, then the old value of 200 is stored in the rollback segment and the original data block contains the new value of 100. If the transaction is rolled back, then the value of 200 is copied from the rollback segment back to the data block. Part of the information in rollback segments consists of undo entries. If a row is inserted into a table, the undo created by that transaction would include, among other information, the rowid of that row so that an undo operation only needs the rowid to restore the row back to its original state. For update transactions, rollback segments store the old values of the updated columns. Every rollback segment header has a transaction table. Every data block has a transaction directory in its header. When a transaction modifies data, it updates the transaction directory in the data block header of the data block that contains the data s row or set of rows. This header, in turn, points to the rollback segment containing the undo information for the transaction. Concurrently, the transaction places an entry in the transaction table of the header of the rollback segment. Along with other information, the transaction table entry contains the address of the modified data block, the status of the transaction (commit the transaction is complete and the space used for its undo entries is now available to be overwritten by new transaction undo entries, or active the transaction has not yet completed with the issuance by the user of either a COMMIT or ROLL- BACK command), and the location within the rollback segment where the undo entries for the particular transaction are stored.

4 274 Chapter 12: Rollback Segments HOW ROLLBACK SEGMENTS EXTEND Rollback segments consist of multiple extents. Unlike other segments, a rollback segment is ring-shaped and writes to its extents in an ordered, circular fashion, moving from one extent to the next, after the current extent is full. The current extent being written to is called the head of the rollback segment. The extent that contains the oldest active undo information (the first undo entry of the oldest transaction) is called the tail. The undo information created by a transaction is guaranteed to remain in the rollback segment until the transaction commits or rolls back. Figure illustrates how rollback segments extend. A few important rules to keep in mind when allocating space for rollback segments are: One rollback segment extent may have multiple transactions written to it. The head of the rollback segment never moves into a rollback extent currently occupied by the tail. (The exception to this rule is the ORA-1555: snapshot too old (rollback segment too small) error Figure How rollback segments extend.

5 Lab : Rollback Segment Management 275 message. Please see the sidebar entitled ORA-1555: snapshot too old.) If the head cannot use the next extent, it allocates another extent and inserts it into the ring-shaped rollback segment. A rollback segment block contains only one active transaction at a time. This way, contention on the rollback segment block is reduced. A transaction cannot span multiple rollback segments. All of its undo entries can be stored in only one rollback segment. As the head tries to advance, extents cannot be skipped or used out of order. ORA-1555: snapshot too old This error message occurs when data manipulation transactions are performed simultaneously with long-running queries. Both online transaction processing and batch processing are being executed in the database at the same time. Long-running queries, therefore, may receive the snapshot too old error message. The occurrence of this error has to do with the definitions of active and inactive data. While a transaction is executing, the data being modified by the transaction is considered active. Once a transaction is complete, its data continues to remain in the rollback segment to support the queries and transactions that began executing before it was committed. So, if a query accesses the same table as a transaction, it will need to use the data stored by the transaction s rollback segment entry. However, once the transaction is complete, its data is then inactive. This means that the transaction s data blocks are now available to be overwritten by new transactions, regardless of the fact that the longrunning query is still using the data from those blocks. Therefore, when the query attempts to read the blocks that have been overwritten, it will fail. The solution is to schedule batch processing to take place during a time period when you expect the least amount of online transaction processing.

6 276 Chapter 12: Rollback Segments HOW ROLLBACK SEGMENTS USE SPACE When a transaction begins, ORACLE begins writing an undo entry to a rollback segment. The entry cannot expand into another rollback segment and it cannot switch to another rollback segment. ORACLE begins writing its undo entry to the first available extent. When an entry requires space in more than one extent, the rollback segment tries to locate the next available extent for the entry. It tries to extend into the next sequential extent within the rollback segment. If the current extent is the last extent in the rollback segment, ORACLE attempts to extend the entry for the current transaction into the first extent. In essence, it attempts to overwrite old undo entries in the first extent with the continuing undo entry for the current transaction. However, that extent may currently be in use (active). If so, the rollback segment is forced to extend and acquire a new extent. The entry is then continued in this new extent. CREATING AND TESTING ROLLBACK SEGMENTS A few tests should be conducted by the DBA to find out how large and how numerous the rollback segments must be to handle normal database processing. Type the following at the SQL*Plus command line while logged on as user SYSTEM: SELECT segment_name, tablespace_name, status FROM dba_rollback_segs; a) Write down the names of your rollback segments as well as their associated tablespaces. What resulting statuses do you see, and what do you think those statuses indicate? Choose the first rollback segment that does not begin with SYS (it is most likely RB01 ), and execute the following query: SELECT initial_extent, next_extent, min_extents, max_extents, pct_increase FROM dba_rollback_segs WHERE segment_name = 'RB01';

7 Lab : Rollback Segment Management 277 Oracle strongly recommends that DBAs create rollback segments with initial and next extents that are all the same size. b) Write down the results of this query. Are the initial and next extent parameter values the same or different? What is the value specified for the PCTINCREASE parameter? Why do you think Oracle recommends that DBAs create rollback segments with initial and next extents of the same size? Each rollback segment must have a minimum of two extents.therefore, the MINEXTENTS parameter for a rollback segment should never be set to a value less than two. During this lab, you are selecting from the dynamic performance views (i.e., the ones that begin with V$). Do not shut down the database unless you are instructed to do so, since the statistics in the V$ views are reset when you restart the database. Type the following while logged on as user SYSTEM: DESC v$rollstat c) Write down at least three types of information that can be obtained from this view. To find out how the values in this table change,type the following two statements: ALTER ROLLBACK SEGMENT RB04 OFFLINE; SELECT DISTINCT status FROM v$rollstat;

8 278 Chapter 12: Rollback Segments d) Compare the results you receive here with the action you performed in Question c). What can you deduce about the information that is represented in V$ROLLSTAT? Now take a look at another related dynamic performance view by typing the following: DESC v$rollname e) What kind of information does V$ROLLNAME contain? f) Using V$ROLLNAME and V$ROLLSTAT, write a query to display the names and statuses of your rollback segments and execute it. What are the names and statuses of your rollback segments? Of the rollback segments listed, how many are offline? Why? You need an adequate number of rollback segments to prevent contention between transactions. Determine whether your system is running into contention by issuing the following query: SELECT n.name, ROUND(100 * s.waits/s.gets) "%Contention" FROM v$rollname n, v$rollstat s WHERE n.usn = s.usn; If %Contention is greater than 1%, you will need to add more rollback segments.

9 Lab : Rollback Segment Management 279 g) Do you see any value for %Contention greater than 1? Why do you think it is necessary to perform this test? Determine the amount of undo information that has been created in each of the rollback segments since the database was started by issuing the following query: SELECT n.name, s.writes "Bytes written to date" FROM v$rollname n, v$rollstat s WHERE n.usn = s.usn; h) Write down the results. Do you see all of your rollback segments listed with corresponding results? Why or why not? Note that the information you gather in this question will be used in Question i) below. Determine the amount of undo information created for a particular transaction (i.e., a rollback segment entry). Open another SQL*Plus session and connect to your database as user SCOTT. The password for user SCOTT is TIGER. If you are unable to connect as user SCOTT, create this user while logged into SQL*Plus as user SYSTEM by executing the following set of statements: CREATE USER scott IDENTIFIED BY tiger DEFAULT TABLESPACE tools TEMPORARY TABLESPACE temp QUOTA UNLIMITED ON tools; GRANT CONNECT TO scott; Run the demobld.sql script to create user SCOTT s schema. If you are using UNIX, this script can be found in a directory structure similar to the following: $ORACLE_HOME/sqlplus/demo/demobld.sql

10 280 Chapter 12: Rollback Segments If you are using NT, this script can be found in a directory structure similar to the following: e:\oracle\ora81\sqlplus\demo\demobld.sql Force a rollback segment selection by issuing the following statement: SET TRANSACTION USE ROLLBACK SEGMENT RB03; Now create some undo entries by typing the following DML statements. INSERT INTO DEPT (deptno, dname, loc) VALUES (50, 'PAYROLL', 'SAN FRANCISCO'); COMMIT; In your SQL*Plus session where you are already logged on as user SYSTEM, reissue the query listed just before Question h). i) Write down the results. For rollback segment RB03, subtract the Bytes written to date value received in your result set for Question h) from the Bytes written to date value received in your result set for this question and write down the result. The value you receive as a result of this subtraction is roughly the number of bytes of undo information that was created in the rollback segment for this transaction. j) What do you think this information is useful for? Determine the number of rollback segments needed in your system with the aid of the following chart from the Oracle Database Administrator s Guide of the Oracle documentation set.

11 Lab : Rollback Segment Management 281 Number of Concurrent Transactions Recommended Number of Rollback (n) Segments n < <= n <= <= n n/4, but no more than 50 Given: There is an average of five concurrent transactions for each table in user SCOTT s schema at any one moment. Determine the total number of expected concurrent transactions. HINT: You need to look at SCOTT s USER_TABLES data dictionary view or the DBA_TABLES data dictionary view, using the appropriate WHERE clause, to find out how many tables are in user SCOTT s schema. k) Write down the query you used to find out the number of tables contained in user SCOTT s schema. Issue the query. How many tables are listed? l) Multiply the result you receive in Question k) by the average number of concurrent transactions for each table in user SCOTT s schema. How many concurrent transactions can you expect in user SCOTT s schema at any one moment? Compare this product with the values in the Number of Concurrent Transactions column in the chart above. How many rollback segments are needed to support the expected transactions? If you do not have enough rollback segments, add additional ones as needed. Size them the same as the other rollback segments already in the database. Use the DBA_ROLLBACK_SEGS data dictionary view to obtain the name of the tablespace and segment/extent information supporting your non-system rollback segments. Use this information to fill in the blanks in Question m).

12 282 Chapter 12: Rollback Segments m) Fill in the blanks in the following CREATE ROLLBACK SEGMENT statement: CREATE ROLLBACK SEGMENT RB05 TABLESPACE STORAGE (INITIAL NEXT MINEXTENTS MAXEXTENTS OPTIMAL [HINT: This OPTIMAL value is typically obtained with the following formula: INITIAL + (NEXT * MINEXTENTS) ] ); Notice the last storage clause, OPTIMAL. Remember that rollback segments dynamically extend when necessary. Without specifying the OPTIMAL storage clause for a rollback segment, once the transaction requiring the rollback segment to extend completes, the rollback segment simply keeps the extra space it has acquired. This phenomenon can cause rollback segment space management problems. One single large transaction can potentially use all of the space available in the rollback segment tablespace. The OPTIMAL storage clause allows DBAs to specify an optimal size for a rollback segment. When a rollback segment extends beyond the length specified in the OPTIMAL clause, it later dynamically shrinks itself back to this length specification. It performs this shrinking action by eliminating its oldest inactive extent. Create rollback segment RB05 using the above CREATE ROLLBACK SEG- MENT statement with your sizing values. Create any necessary additional rollback segments with the same statement, ensuring that the rollback segment name contains the appropriate next sequential number (for example, RB06, RB07, etc.). You now know how many rollback segments there are and how many concurrent transactions there will be. Assume that all of the transactions are the same size (the same number of bytes) as the transaction investigated in Question i).

13 Lab : Rollback Segment Management 283 n) If the space usage is evenly distributed across the rollback segments, how much space is needed in each rollback segment? Ensure that all of your rollback segments are online by issuing the following five commands: ALTER ROLLBACK SEGMENT RB04 ONLINE; ALTER ROLLBACK SEGMENT RB05 ONLINE; ALTER ROLLBACK SEGMENT RB06 ONLINE; ALTER ROLLBACK SEGMENT RB07 ONLINE; ALTER ROLLBACK SEGMENT RB08 ONLINE; Ensure that your new rollback segments are listed in your init.ora file by completing the following instructions. Make a backup copy of your init.ora file in your pfile directory. The name of your backup file should be init.ora.06, or whatever the next highest version number is. Open your init.ora file for editing and change the value of the ROLLBACK_SEGMENTS parameter to include all newly created rollback segments. o) Why do you think this step is necessary? IMPORTANT: You must restart your database before continuing. EXERCISE ANSWERS ANSWERS a) Write down the names of your rollback segments as well as their associated tablespaces. What resulting statuses do you see, and what do you think those statuses indicate?

14 284 Chapter 12: Rollback Segments Answer: Upon executing this query, you should have received a result similar to the following: SQL> SELECT segment_name, tablespace_name, status 2 FROM dba_rollback_segs; SEGMENT_NAME TABLESPACE_NAME STATUS SYSTEM SYSTEM ONLINE SYSROL SYSTEM OFFLINE RB01 RBS ONLINE RB02 RBS ONLINE RB03 RBS ONLINE RB04 RBS ONLINE 6 rows selected. The status ONLINE means that a rollback segment has been activated, or made available to the database users. If a rollback segment s status shows that it is OFFLINE, then it has been deactivated. Deactivated does not have the same meaning as dropped. If a rollback segment has been deactivated as a result of being taken offline, it still exists as a database object. It is just not currently available. b) Write down the results of this query. Are the INITIAL and NEXT extent parameter values the same or different? What is the value specified for the PCTINCREASE parameter? Why do you think Oracle recommends that DBAs create rollback segments with initial and next extents of the same size? Answer: Upon executing this query, you should have received a result similar to the following: SQL> SELECT initial_extent, next_extent, min_extents, 2 max_extents, pct_increase 3 FROM dba_rollback_segs 4 WHERE segment_name = 'RB01';

15 Lab : Rollback Segment Management 285 INITIAL_EXTENT NEXT_EXTENT MIN_EXTENTS MAX_EXTENTS PCT_INCREASE E+09 0 The INITIAL and NEXT extent parameters are the same size (as they should be), 100 kilobytes. The PCTINCREASE parameter value is zero. Oracle recommends that DBAs create rollback segments with initial and next extents of the same size so that, without the possibility of extents growing exponentially, expansion and shrinkage can be manageable and predictable. c) Write down at least three types of information that can be obtained from this view. Answer: Upon issuing this command, you should have received a result similar to the following: SQL> DESC v$rollstat Name Null? Type USN EXTENTS RSSIZE WRITES XACTS GETS WAITS OPTSIZE HWMSIZE SHRINKS WRAPS EXTENDS AVESHRINK AVEACTIVE STATUS VARCHAR2(15) CUREXT CURBLK Some of the types of information that can be obtained from this view include: USN The rollback segment number. EXTENTS The number of current extents in the rollback segment. RSSIZE The current size of the rollback segment, in bytes.

16 286 Chapter 12: Rollback Segments WRITES The number of bytes of undo entries written to the rollback segment. XACTS The number of active transactions. GETS The number of requests for the rollback segment header. WAITS The number of requests for the rollback segment header that were not immediately serviced. SHRINKS The number of shrinks that the rollback segment has been made to perform to stay at the size specified with the OPTIMAL parameter. WRAPS The number of times an undo entry has wrapped from one extent into another. EXTENDS The number of times the rollback segment had to acquire a new extent. STATUS The status of the rollback segment. d) Compare the results you receive here with the action you perform in Question c). What can you deduce about the information that is represented in V$ROLLSTAT? Answer: Upon executing this series of statements you should have received a result similar to the following: SQL> ALTER ROLLBACK SEGMENT RB04 OFFLINE; Rollback segment altered. SQL> SELECT DISTINCT status FROM v$rollstat; STATUS ONLINE The dynamic performance table,v$rollstat, contains information about only those rollback segments whose status is not currently OFFLINE. e) What kind of information does V$ROLLNAME contain? Answer: Upon issuing this command, you should have received a result similar to the following:

17 Lab : Rollback Segment Management 287 SQL> DESC v$rollname Name Null? Type USN NAME NOT NULL VARCHAR2(30) The information contained in V$ROLLNAME includes: USN The rollback segment number. NAME The name of the rollback segments that are currently online. f) Using V$ROLLNAME and V$ROLLSTAT, write a query to display the names and statuses of your rollback segments and execute it. What are the names and statuses of your rollback segments? Of the rollback segments listed, how many are offline? Why? Answer: One possible solution for this query is: SELECT name, status FROM v$rollstat s, v$rollname n WHERE s.usn = n.usn; Upon executing this query, you should receive a result similar to the following: SQL> SELECT name, status 2 FROM v$rollstat s, v$rollname n 3 WHERE s.usn = n.usn; NAME STATUS SYSTEM ONLINE RB01 ONLINE RB02 ONLINE RB03 ONLINE RB04 ONLINE None of the rollback segments listed are offline. Remember,V$ROLLSTAT contains information about only those rollback segments that are online. If you want to see status information about both online and offline rollback segments, you should query the DBA_ROLLBACK_SEGS data dictionary view.

18 288 Chapter 12: Rollback Segments g) Do you see any value for %Contention greater than 1? Why do you think it is necessary to perform this test? Answer: Upon executing this query, you should have received a result similar to the following: SQL> SELECT n.name, ROUND(100 * s.waits/s.gets) 2 "%Contention" 3 FROM v$rollname n, v$rollstat s 4 WHERE n.usn = s.usn; NAME %Contention SYSTEM 0 RB01 0 RB02 0 RB03 0 In this example, the value of %Contention for all the online rollback segments is 0. A %Contention value <=1 means that there is no contention. A %Contention value >1, indicates that contention exists, and therefore corrective action is required. The existence of contention means that at some point in the rollback segment s history, there was not enough room for transaction undo entries in the header of the rollback segment. The percentage value indicates approximately how often (i.e., what percentage of time during which the rollback segment was being used) this happened. New transactions will be forced to wait for space to become free in the rollback segment header. Users will experience this situation in the form of system performance degradation. If contention is indicated, a corrective action such as the creation of additional rollback segments should be performed. h) Write down the results. Do you see all of your rollback segments listed with corresponding results? Why or why not? Note that the information you gather in this question will be used in Question i) below. Answer: Upon executing this query, you should have received a result similar to the following: SQL> SELECT n.name, s.writes "Bytes written to date" 2 FROM v$rollname n, v$rollstat s 3 WHERE n.usn = s.usn;

19 Lab : Rollback Segment Management 289 NAME Bytes written to date SYSTEM 9660 RB RB02 54 RB03 54 You will not see ALL of your rollback segments listed with current results. Notice that rollback segment RB04 is not listed.this is because, like V$ROLLSTAT,V$ROLLNAME provides information for only those rollback segments whose status is not currently OFFLINE. i) Write down the results. For rollback segment RB03, subtract the Bytes written to date value received in your result set for Question h) from the Bytes written to date value received in your result set for this question and write down the result. Answer: Upon executing this query again, your new result set should look similar to the following: SQL> SELECT n.name, s.writes "Bytes written to date" 2 FROM v$rollname n, v$rollstat s 3 WHERE n.usn = s.usn; NAME Bytes written to date SYSTEM 9660 RB RB02 54 RB For rollback segment RB03, = bytes of undo information were created in rollback segment RB03 after user SCOTT inserted a row in the DEPT table. Do not worry if the values you receive do not exactly match the values you see listed here.the point of this exercise is that after you force the rollback segment selection of RB03 and issue a transaction in user SCOTT s schema, you should notice an increase in the value for the Bytes written to date column upon reissuing the query above. The values for all other listed rollback segments should remain the same for both query result sets.

20 290 Chapter 12: Rollback Segments j) What do you think this information is useful for? Answer: This information is useful for predicting how large your transactions will be so that you may size your rollback segments appropriately. k) Write down the query you used to find out the number of tables contained in user SCOTT S schema. Issue the query. How many tables are listed? Answer: One possible query to find out the number of tables contained in user SCOTT s schema is (using your SQL*Plus session where you are already logged on as user SCOTT): SELECT COUNT(*) FROM user_tables; Upon issuing this query, you should receive a result similar to the following: SQL> SELECT COUNT(*) 2 FROM user_tables; COUNT(*) There are six tables in user SCOTT s schema. l) Multiply the result you receive in Question k) by the average number of concurrent transactions for each table in user SCOTT s schema. How many concurrent transactions can you expect in user SCOTT s schema at any one moment? Compare this product with the values in the Number of Concurrent Transactions column in the chart above. How many rollback segments are needed to support the expected transactions? Answer: Since user SCOTT has an average number of five concurrent transactions for each of the six tables in his schema, he can expect 30 concurrent transactions at any one moment. According to the chart above, eight non-system rollback segments are needed to support the expected transactions. m) Fill in the blanks in the following CREATE ROLLBACK SEGMENT statement. Answer: After filling in the blanks, your CREATE ROLLBACK SEGMENT statement should appear similar to the following:

21 Lab : Rollback Segment Management 291 CREATE ROLLBACK SEGMENT RB05 TABLESPACE RBS STORAGE (INITIAL 100K NEXT 100K MINEXTENTS 2 MAXEXTENTS UNLIMITED OPTIMAL 300K ); Except for the value for the OPTIMAL storage parameter, the rest of these values can be obtained by querying the DBA_ROLLBACK_SEGS data dictionary view as follows: SQL> SELECT tablespace_name, initial_extent, 2 next_extent, min_extents, 3 max_extents 4 FROM dba_rollback_segs 5 WHERE segment_name = 'RB01'; TABLESPACE_NAME INITIAL_EXTENT NEXT_EXTENT MIN_EXTENTS MAX_EXTENTS RBS E+09 n) If the space usage is evenly distributed across the rollback segments, how much space is needed in each rollback segment? Answer: Assuming that all of the transactions are the same size (the same number of bytes) as the transaction investigated in Question i), then 196 (the expected number of bytes of undo information per transaction) * 30 (the number of concurrent transactions at any one moment) = / 8 (the number of non-system rollback segments) = 735. Therefore, if the space usage is evenly distributed across the rollback segments, 735 bytes of space are needed in each rollback segment. o) Why do you think this step is necessary? Answer: This step is necessary because, although you bring your new rollback segments online manually in this exercise, once the database is shut down, the rollback segments are automatically taken offline. Upon database startup, the only way to ensure that your rollback segments are brought back online (without your having to explicitly set each of them online with an ALTER ROLLBACK SEGMENT statement) is to include their names in the list of rollback segment names following the ROLLBACK_ SEGMENTS parameter in your init.ora configuration file.

22 292 Chapter 12: Rollback Segments SELF-REVIEW QUESTIONS To test your progress, you should be able to answer the following questions. 1) A transaction is often referred to as: a) an SQL statement or set of SQL statements b) a logical unit of work c) any DML statement that modifies data d) all of the above 2) Statement-level read consistency means that: a) this type of consistency refers to multiple-query transactions b) the data remains the same while ORACLE is fetching and returning it c) if you execute a query once, then re-execute it, the data returned in both result sets is exactly the same d) you receive your result set in a textual ASCII format 3) Rollback segments, like other types of segments, should be created with a minimum of one extent. a) True b) False 4) The transaction table in a rollback segment header contains which of the following: (Check all that apply.) a) rowids for inserted or deleted rows b) the location in the rollback segment where a transaction s undo entries may be found c) the address of the modified data block d) the status of the transaction e) old values for updated columns

23 Lab : Rollback Segment Management 293 5) Which of the following descriptions for the way rollback segments operate is NOT true? a) a rollback segment block contains only one active transaction at a time b) as the head tries to advance, the extents cannot be skipped or used out of order c) one rollback segment extent may have multiple transactions written to it d) a transaction can span multiple rollback segments for purposes of load balancing e) the head of the rollback segment never moves into an extent currently occupied by the tail 6) Once a transaction is complete, its data is deleted from the rollback segment. a) True b) False 7) A transaction in a rollback segment always has one of the two following statuses: a) ACTIVE or COMPLETE b) COMMIT or ROLLBACK c) COMMIT or ONLINE d) ACTIVE or INACTIVE e) ACTIVE or COMMIT Quiz answers appear in Appendix A, Lab.

24 294 Chapter 12: Rollback Segments CHAPTER 12 TEST YOUR THINKING 1) If you create a rollback segment with the OPTIMAL clause, in which data dictionary view(s) can that value be found? Write a query to show the names and storage parameters of your non-system rollback segments, including the OPTIMAL clause values. 2) Open your create script createdb00_02.sql for viewing. Notice that when creating the first four non-system rollback segments, RB01, RB02, RB03 and RB04, there are no specifications for either the MINEXTENTS or MAXEXTENTS storage parameter. From where do these rollback segments obtain the values for these parameters? 3) Starting with Oracle 7.x, the value for PCTINCREASE may not be specified when creating or altering rollback segments. The value is fixed at PCTINCREASE 0. Why do you think this restriction has been introduced?