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Database creation

Database creation. Using a script: Connect internal as sysdba; create database NAME….; Using the OEM GUI Parameters stored in a small DB Access to other utility pgms – eg: RMAN Can accommodate several DBAs Key decisions on sizing Eg: assign a block size – once and for all. Block size.

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Database creation

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  1. Database creation • Using a script: Connect internal as sysdba; create database NAME….; • Using the OEM GUI • Parameters stored in a small DB • Access to other utility pgms – eg: RMAN • Can accommodate several DBAs • Key decisions on sizing • Eg: assign a block size – once and for all

  2. Block size • Smallest unit of storage in oracle DB • Critical perf. Factor as inefficient space use leads to increase I/O activity • Block overhead • Integrity section (90 bytes) • Runtime overhead (136 bytes) • Transactional header (24 bytes and growing) • Directory section (Pointers…) • And then………..data • Big is beautiful? Percentage of overhead decreases when block size increases, but no right answer here } Approx. 256 b

  3. 2 K, 4K, 8K or 16K? • OLTP prefer smaller – small granularity increase likelihood that data is in buffer (indexed access) • DW or DSS prefer larger – physically close data used together + less movement • In practice constraint on memory will also dictate • Buffer size + now split in 3 areas: • KEEP pool • RECYCLE pool • DEFAULT pool

  4. Block size and row size • Table with average row length = 2236 b (10m records) • Block size 4k = 4006 b => one row per block • 8 k => 3 rows per block

  5. Opening and closing the DB Startup open NAME; Or Startup mount NAME; Alter database open; Shutdown; Else – use OEM menus

  6. page846 When DB is started • System Global Area (SGA) created: • Data block buffer cache created • Size is key for performance of DB • fraction of whole DB • DB_BLOCK_BUFFERS / DB_BLOCK_SIZE • Least Recently Used (LRU) method • Too small means fight for space between objects => increase in misses => increase in I/O • Too big = crippling for other operations

  7. When DB created (2) • Shared SQL Pool created: • Stores DD cache: info about data structure • Stores library cache: info about SQL statements run • LRU algorithm • Too small => no re-use and low performance • SHARED_POOL_SIZE in init.ora

  8. Parameter files: INIT.ORA • Initialisation file for each DB • MYDB = > initmydb.ora • Contains sizing of SGA and number of parameters, DB name and block size • Read at startup means changes only implemented at next startup

  9. The control file • Each DB also has a control file • Contains Dbname, name and location of files and redo logs, timestamp of DB creation • Can be mirrored for added security • Identifies all files that must be opened when Db is loaded • Also used in DB recovery

  10. Data hierarchy in Oracle • Data hierarchy in Oracle is specific (and demented!) • Normally, unit of storage is file… • Managed by OS • Can grow / shrink • Physical sections logically connected by OS • In Oracle a DB = one or more tablespaces • One or more objects, indexes, clusters… • Fixed size – gets full • Extended by DBA only • One file can support only ONE tablespace See figure 3.1

  11. About tablespaces • Kind of partitioning • Except fixed size • Extended by dba • Oracle very strong at TS level • Optimal seems to be around 2GB • Strategy = isolate aged data into one tablespace => can be made read-only • Should store similar objects together

  12. Reasoning behind TS creation • Business relevance • Size of objects • Type of activity • Volume of activity • Backup • Transportability • Time-based changes in activity

  13. Reasoning behind TS creation P 148 • Business relevance • Size of objects • Type of activity • Volume of activity • Backup • Transportability • Time-based changes in activity e.g. modular design / keep data togegther Object of similar sizes – reuse of space e.g. index VS table / full scan VS rowID Low I/O VS high I/O objects TS is smallest unit for Bcup and restore Cloning copy is done by TS as well DW env. Aged archived system better – access time related Bias towards more rather than less = > compromise Also perf reading

  14. Create a tablespace • Create tablespace FREDDY datafile ‘path\freddy.dbf’ size 1000K Default storage (initial 10K next 10K Minextent 1 maxextents 100 Pctincrease 0) Permanent; • 10 10 10 10 10 10 10 10 10 10 • 20 30 40 50 60 70 80 90 100 • 10 10 12 14.4 17.2 20.7 25 30 36 42.3 • 10 20 32 46.4 63.6 84.3 109.3 139.3 175.3 218.5

  15. Object creation • Tables = one physical space on disk [segment] – same for any object • Each segment = space set out in tablespace [initial extent] • When table grows allocate next extent – set of adjacent blocks • Until tablespace is full • see figure 20.3

  16. What a segment looks like • Each segment contains a header block • Space directory for other blocks • Properly sized tables = few extents (low fragmentation) • Additional extents are never lost until drop or delete commands are used • Can also adjust pctfree value (to fill a Null value column)

  17. Sizing extents • Try to match the max capacity of your storage device to limit I/O operations • For single objects, It is beneficial to keep extents of same size (deletion) • Try to limit the number of extents below 50 (measure of the growth of the object) • If too many, rebuild object with larger size extents

  18. Oracle “space allocation” • DBA allocates space for table after 2 days of hard thinking • Oracle ignores it blatantly as follows: Table name initial size next extent oracle sizing next extent Smalltab 7K 7K 20K 20K Mediumtab 103K 103K 120K 120K Total size after growth smalltab = 40K instead of 14K mediumtab = 206K instead of 240K

  19. Creating and using tables • Table creation with CREATE TABLE statement • Syntax: Create table NAME ( field1 datatype special constraint field2 datatype etc… ); • Special constraint: e.g. NOT NULL or PRIMARY KEY or CHECK

  20. Constraints • Candidate key: Create table fred ( name varchar2(10) NOT NULL firstname varchar2(10) NOT NULL age number Constraint FRED_UQ unique (name, firstname) );

  21. Primary Key Create table fred ( name varchar2(10) firstname varchar2(10) age number Constraint FRED_PK Primary Key (name, firstname) ); • Same apart: always NOT NULL + can only have one of them in a table • If only one field just list primary key as special constraint

  22. Foreign key – aka referential integrity constraint Create table fred ( ID number NOT NULL name varchar2(10) firstname varchar2(10) age number Constraint FRED_PK Primary Key (ID), foreign key (age) references OTHERTAB(age) ); • Use ON DELETE CASCADE to maintain constraint

  23. Check constraint Create table fred ( ID number NOT NULL name varchar2(10) firstname varchar2(10) age number CHECK (age between 18 and 65) );

  24. Deleting data • Drop table statement – remove table • Or truncate table statement – empty it • Warning: neither can be rolled back • Otherwise: Alter table NAME Add (…) Alter table NAME modify (…) drop column fieldname • See rules on handout

  25. Now what? Clusters • To store different tables in physically close areas on disk • Create a cluster to store both objects • Cluster key is any useful field that links the tables • Can keep name of field or pick new one • e.g. Create cluster fred (name datatype) • then assign tables: create table…cluster fred (name)

  26. What this cluster looks like • 12 Cork fred adam French • Midleton Bob Fitz Irish • Bally… Brian O’reilly English Table 1 Table 2 Cluster Key

  27. Finally… • Users can have different logical views on a database • These views are called…..? • They allow for a re-organisation of physical DB objects for each users • Each ? can contain elements from different tablespaces • Has limited size allocated to each user

  28. Summary: storage in Oracle • Keywords: tablespace / datafiles / database objects / segments / extents • When you create an object: • Specify size / tablespace info = storage clause • Otherwise: default values • Pctincrease should be used with care (affects re-usability of reclaimed space – i.e. extents) • Wise use of tablespace eases admin (like partitioning) • sizing follows sizing of device – e.g. 2GB • Many utilities to work with tablespaces • Isolate processes – e.g. a report running inside a TS • Also for archiving

  29. Performance issues • Extent sizing a key • Oracle has 2 ways to read data: • By rowID – typically thru an index • Full table scan • In second case, oracle reads several blocks at a time • Set with DB_FILE_MULTIBLOCK_READ_COUNT • Limited only by I/O device buffer capacity • E.g. block = 4K, I/O buffer = 64K => ????? • Extent size should be multiple of 64K

  30. example • Table with 10 extents 64 K each • I/O device buffer = 64K => 10 reads • If extent = 640K => ???? Reads • If 8 extents 80K each => ??? Reads • Reads cannot span extents => 16 reads or an increase of 60% in I/O operations • Cl: either use extents much larger than buffer or use multiple of buffer size

  31. And for roll back segments • Should be kept small enough • Current wisdom is to ensure TS for rollback is normally 50% free • Not always possible to use only one RB size • For different types of processes – e.g. TPS • Use two different TB to isolate activites • Easier trouble shooting • Discarded rollback segments go into the log file

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