1. 7202ICT Database Administration Lecture 7 Managing Database Storage Part 2 Orale Concept Manuel Chapter 3 & 4

2. INDEX SEGMENTS B-TREE If indexes are created on a table, then an index segment is created for each index. Index Segment Features  Balanced tree indexes can quickly locate key values  ROWID addresses (hexadecimal values) can provide direct access to the data block  Queries that reference indexes columns can be resolved in the index without even touching the table data segment  Index searches are an alternative to full table scans  Index segments are physically separate and independent from their associated table segments  Each index can have its own storage parameters  Indexes are generally much smaller than their associated table  Indexes can be stored in a different tablespace for optimization

3. Example: Create an index on the EMP table SQL> CREATE INDEX emp_ename /* Index name */ ON emp (ename)/* table and column name */ STORAGE (INITIAL 500K NEXT 500K PCTINCREASE 0) TABLESPACE IDX_SPACE;

4. TEMPORARY SEGMENTS Temporary segments provide workspace for the RDBMS to sort and join tables while performing complex searches. Temporary Segments  Are created automatically by ORACLE to perform complex operations such as joins, group by, creating an index, and anything that requires sorting  Are created in memory if enough memory space is available, or in data files if not  Created when needed on a per transaction basis  Space is reclaimed at the end of the transaction by the SMON background process  Each tablespace may contain temporary segments

5. TEMPORARY SEGMENTS (cont)  Space allocation for temporary segments is determined by the DEFAULT STORAGE clause of the tablespace that the temporary segments are being created in  Dynamic extension of temporary tables during sort operations may cause tablespace fragmentation  The DBA may define which tablespace each user uses for temporary segments, thus it is possible to manage the temporary segments and even have a dedicated tablespace for them  Temporary segments are not protected by use of the REDO LOG since they pose no threat to data integrity

6. ROLLBACK SEGMENTS Rollback segment entries are written in a circular fashion for rollback, read consistency, and recovery. –Each Rollback Segment  Consists of several rollback entries from multiple transactions  Stores block information such as file and block ID, as well as data as it existed before being modified  Must be created under special circumstances and brought online before being used by any transaction  May grow in size due to large or long running transactions  Will automatically shrink to OPTIMAL if extended beyond it  Can be assigned to a transaction automatically or explicitly

7. Rollback Segment Storage Parameters  INITIAL  NEXT  MINEXTENTS  MAXEXTENTS  OPTIMAL

8. Parameter Notes  OPTIMAL specifies the optimal size of a rollback segment in bytes  ORACLE de-allocates extents when a rollback segment is larger than OPTIMAL  PCTINCREASE cannot be specified for rollback segments and is always zero  MINEXTENTS must be at least 2 to accommodate the segment being written to in a circular fashion

9. Example Create a rollback segment using the space utilization parameters, then bring the rollback segment online. SQL> CREATE ROLLBACK SEGMENT rbs01 TABLESPACE RBS STORAGE (INITIAL 10K NEXT 10K OPTIMAL 20KMAXEXTENTS 121); SQL> ALTER ROLLBACK SEGMENT rbs01 ONLINE;

10. SYSTEM SEGMENT The system segment contains data dictionary tables to be loaded when the database is opened. System Segment  Also referred to as the cache segment  It cannot be read modified or dropped  Exists in the STSTEM tablespace and is owned by the user SYS  Requires no action on the part of the DBA  Usually needs less than 50 ORACLE blocks

11. TABLESPACES AND DATA FILES Data in an ORACLE database is logically stored in tablespaces and physically stored in database files. An ORACLE can be subdivided into smaller logical areas of space known as tablespaces.

12. Tablespaces  Each tablespace contains one or more operating system files  Tablespace can be brought online while the database is running  Tablespace can be taken offline while the tablespace is running, except for the SYSTEM tablespace which must always remain online  Objects created in a tablespace can never be allocated space outside of their original tablespace

13. Useful Features of the Tablespace  Control of space allocation and assigning space quotas to users  Control availability of data by taking individual tablespaces online or offline  Distribute data storage across devices to improve I/O performance and reduce I/O against a single disk  Perform partial backup and partial recovery operations

14. Note : A database always consists of at least one tablespace, SYSTEM. Generally, additional tablespaces are added to the database for increased control and long term ease of maintenance. Essentially, ORACLE sees the entire database made up of two types of tablespaces: SYSTEM and non- SYSTEM.

15. SYSTEM Tablespace  Required in all databases for database operation  Contains data dictionary information, definitions of stored procedures, packages, and database triggers  Contains the SYSTEM rollback segment  Can contain user data if so desired

16. Non-System Tablespace  Allows more flexibility in database administration  Consists of:  rollback segments,  temporary segments,  application data,  application indexes,  user space,  and so forth

17. PHYSICAL DATABASE STRUCTURE The total amount of physical space allotted to ORACLE database objects depends upon the size of the physical operating system files created on behalf of each individual tablespace.  Each logical tablespace is physically made up of one or more operating system files  A segment, such as a table segment, can span multiple files as long as those files belong to the same tablespace

18. Resolving Disk Contention Disk contention can be minimized, by separating groups of segments that will contend for disk resources among different tablespaces. Naturally, those tablespace should not all map to data files on the same disk. To Minimize Disk Contention  Separate dictionary segments from other segments  Separate rollback segments from other segments  Separate data segments from their corresponding index segments

19. Data Dictionary Views DISPLAYING EXTENT AND SEGMENT INFORMATION The data dictionary contains a set of views, which can be queried to see how many extents and segments exist in the database.  DBA_EXTENTS  DBA_FREE_SPACE  DBA_SEGMENTS  DBA_TABLESPACES  DBA_DATA_FILES

20. Example : The DBA_SEGMENTS view can be used to gain summary information about extent allocation in the database. It is also possible to run reports on this view to determine the level of fragmentation of a segment and the danger of it reaching MAX_EXTENTS. SQL> SELECT tablespace_name, count(*) SEGMENTS, SUM(bytes) BYTES FROM dba_segments GROUP BY tablespace_name; TABLESPACE_NAMESEGMENTSBYTES ----------------------------------------- APPL_DATA1010240 RBS240960 SYSTEM1082240512 TEMP020480 As can be seen above, each row displays a tablespace name, the number of segments contained in that tablespace and the total number of bytes consumed by those segments from that tablespace.

21. List the names of the columns in DBA_TABLESPACES. SQL> DESCRIBE dba_tablespaces NAME NULLTYPE ---------------------- ----------------------- TABLESPACE_NAMEVARCHAR2(30) INITIAL_EXTENTNUMBER NEXT_EXTENTNUMBER MIN_EXTENTSNUMBER MAX_EXTENTSNUMBER PCT_INCREASENUMBER STATUSVARCHAR2(9) More Examples :

22. List the name and status of each tablespace. SQL> SELECT tablespace_name, status FROM dba_tablespaces; TABLESPACE_NAMESTATUS ------------------------------ SYSTEMONLINE APPL_DATAONLINE RBSONLINE TEMPONLINE More Examples :

23. List the names of the columns in the DBA_DATA_FILES view. SQL> DESCRIBE dba_data_files NAMENULLTYPE --------------------------------------------- FILE_NAMEVARCHAR2(257) FILE_IDNUMBER TABLESPACE_NAMEVARCHAR2(30) BYTESNUMBER BLOCKSNUMBER STATUSVARCHAR2(9) More Examples :

24. List general information about the data files belonging to each tablespace. SQL> SELECT file_name, file_id, tablespace_name, bytes FROM dba_data_files; FILE_NAME FILE_ID TABLESPACE_NAME BYTES ----------------- --------- ------------------ --------- /exp1/ora1/data /user_01.dbf 1 SYSTEM10485760 … More Examples :

25. List the extents of free space in each tablespace. SQL> SELECT * FROM dba_free_space ORDER BY file_id, block_id; TABLESPACE_NAMEFILE_ID BLOCK_ID BYTES BLOCKS ------------------------- --------- -------- ---------- -- SYSTEM1900 184329 SYSTEM1909 40962 SYSTEM1 1912 6572032 3209 … More Examples :

26. List the same free space information, as in the above example, using the names of each data file instead of its file id. SQL> SELECT free.tablespace_name, free.block_id, free.bytes, free.blocks, df.file_name FROM dba_free_space free, dba_data_files df WHERE free.file_id = df.file_id ORDER BY 1,2; TABLESPACE_NAME BLOCK_ID BYTES BLOCKS FILE_NAME ---------------- --------- ------ ------ ----------- SYSTEM 900 18432 9 user01.dbf SYSTEM 909 4096 2 user01.dbf SYSTEM 1912 6572032 32 user01.dbf … More Examples :

27. List general information about all segments in the database, using the DBA_SEGMENTS view. SQL> SELECT owner, segment_name, extents, max_extents FROM dba_segments ORDER BY 1,2; OWNERSEGMENT_NAME EXTENTS MAX_EXTENTS ------------------------------------------- ----------- SCOTTEMP 5 25 SYSTEMRBS1 20 121 SYSTEMRBS2 2 121 SYSUNDO$ 50 99 … More Examples :