Seminar: Presenter: Oracle Database Objects Internals. Oren Nakdimon.

Transcription

1 Seminar: Oracle Database Objects Internals Presenter: Oren Nakdimon 1 Oren Nakdimon

2 Who Am I? Chronology by Oracle years When What Where Oracle 6/ Developer IAF Oracle Server group manager Golden Screens Oracle 8i/9i DBA group manager TELEknowledge Oracle 10g/11g VP R&D and Israel Site Manager Today: Freelance database expert Database Administration Development Courses 2 Olista An expert is a person who has made all the mistakes that can be made in a very narrow field Niels Bohr ( )

3 Agenda Database objects and segments The storage hierarchy Most common segment types Other types of tables Other types of indexes Partitioning Advanced features 3

4 The Building Blocks Segments 4

5 Segments Segments are objects that consume storage Segment types Table Table partition Index Undo Temporary And more A segment may be created as soon as the corresponding object is created (IMMEDIATE) or when it gets data for the first time (DEFERRED, new in version 5

6 The Building Blocks Oracle Storage Hierarchy 6

7 Storage Hierarchy A database contains tablespaces Tablespace A logical container of segments Composed of one or more data files 7

8 Storage Hierarchy A segment is composed of one or more extents Extent A logically contiguous space in a data file Composed of blocks Block The smallest unit of space allocation in Oracle Block size is usually 2KB, 4KB, 8KB, 16KB or 32KB All the blocks within a specific tablespace have the same size 8

9 Storage Hierarchy 9

10 Block Structure Block overhead Header Block type Transaction information Block address Table directory Row directory Data Free space 10

11 Most Common Segment Types Heap Organized Tables 11

12 Tables A table may contain up to 1000 columns and virtually infinite number of rows Data (rows) are stored in blocks Controlling block usage PCTFREE PCTUSED 12

13 Tables There are 9 major types of tables The most common type is Heap Organized Table 13

14 Heap Organized Tables The default table type The location of rows is random Data is returned in random order Good for Insert/Update/Delete operations What about data 14

15 Most Common Segment Types B*Tree Indexes 15

16 Indexes Supportive objects aimed to quickly locate the data There are different types of indexes The most common type is B*Tree 16

17 B*Tree Indexes 17

18 B*Tree Indexes 2 kinds of blocks: Branch and Leaf Always balanced The leaf blocks are doubly linked list Controlling block usage PCTFREE (relevant only to index creation) FREELISTS 18

19 Other Types of Tables Let s See Some Use Cases 19

20 Use Case 1a PROJECT_EMPLOYEES Classic many-to-many relationship Usually retrieving all the employees of a specific 20

21 Index Organized Tables IOT is a table stored in an index structure Data is sorted by the Primary 21

22 Use Case 1b PROJECT_EMPLOYEES With additional columns 22

23 Index Organized Tables Some (or all) of the non-primary-key columns may be located in a different segment OVERFLOW PCTTHRESHOLD 23

24 Use Case 1c PROJECT_EMPLOYEES Querying from both sides 24

25 Index Organized Tables Secondary indexes may be defined Using logical 25

26 Use Case 2 PROJ_PROGRESS Rows for the same project are added over time All rows for the same project are retrieved 26

27 Index Organized Tables IOT is advantageous when All the columns are in the primary key The table is always accessed by (the leading part of) the primary key Co-location of data that arrive randomly is needed Disadvantages Maintaining a B*Tree is more expensive than maintaining a Heap Secondary indexes are somewhat less efficient than on Heap Organized Tables 27

28 Use Case 3 DEPT and EMP Department and its employees are usually retrieved together 28

29 Cluster Segments A cluster stores related data, from one or more tables, in the same block 29

30 Cluster Segments The storage parameters are defined in the cluster level, not the tables level SIZE defines the expected size of each cluster key value Blocks are chained if the data of a single cluster key value requires more than one block Note: in a cluster, rows of different tables may have the same ROWID 30

31 Cluster Segments Limitations No TRUNCATE TABLE No partitioning There are 3 types of clustered tables Index Clustered Tables Hash Clustered Tables Sorted Hash Clustered Tables 31

32 Index Clustered Tables The cluster key is indexed by a B*Tree (it s like the cluster s primary key) The creation order is: CREATE CLUSTER with key column(s) CREATE INDEX ON CLUSTER CREATE TABLE in the cluster Optionally, CREATE additional TABLEs in the 32

33 Index Clustered Tables Advantageous when Data is mostly read, via indexes, joined frequently Disadvantages Full table scans read more data than needed Wrong SIZE estimation will lead to inefficient storage and/or bad performance 33

34 Use Case 4 PRODUCTS Maximum number of products is known A product is usually accessed by its ID 34

35 Hash Clustered Tables The cluster key is hashed to determine the data location You can write your own hash function, or use the default one Advantages Less I/O to get to the data (no 35

36 Hash Clustered Tables Restrictions: Access by hash works only for equality, not range Number of hash keys must be set when creating the cluster (HASHKEYS) Space is pre-allocated [HASHKEYS/trunc(blocksize/SIZE) blocks] Advantageous when Maximum number of final rows is known, not many INSERTs, no UPDATEs of the hash key, access mainly by the hash key value For example: lookup tables 36

37 Use Case 2b PROJ_PROGRESS Rows for the same project are added over time All rows for the same project are retrieved together ordered by day 37

38 Sorted Hash Clustered Tables Data is clustered by one key Within each cluster key, the data is stored ordered by some other key Advantageous when Data is usually retrieved by one column value and ordered by another column Data arrives sorted over time (by the SORTed 38

39 Other Types of Tables Global Temporary Tables 39

40 Temporary Tables Hold intermediate resultsets Defined once, not in runtime The lifetime of the data is either a transaction or a session ON COMMIT PRESERVE ROWS ON COMMIT DELETE ROWS The data is visible only to the current session Each session has its own temporary 40

41 Temporary Tables Temporary tables can have: Indexes Check constraints Triggers Temporary tables cannot: Have foreign key constraints Have NESTED TABLE columns Be Index Organized Tables Be in a cluster Be partitioned 41

42 Temporary Undo (12c) In 12c, undo for temporary tables can be stored in the temporary tablespace As a result: No redo for undo of DML on temporary tables Increased real undo retention Ability to use temporary tables for read and write in read-only Active Data Guard TEMP_UNDO_ENABLED = true 42

43 Session Level Sequences (12c) A new type of sequence Returns a range of numbers which is unique within a session Non 43

44 Other Types of Tables External Tables 44

45 External Tables External tables allow querying from flat files (outside the database) The CREATE TABLE statement: Defines how to read and parse the file Maps the file structure to table columns Can be SELECTed just like a regular table (including joins, aggregation, sorting, etc.) As of 11g, a preprocessor can be 45

46 SQL*Loader Express Mode (12c) No need to create a control file A log file is generated for future use, including: Control file CREATE EXTERNAL TABLE statement INSERT statement Data types of all table columns should be numeric, string or datetime 46

47 Other Types of Indexes Bitmap Indexes 47

48 Bitmap Indexes Each index key stores pointers to multiple rows Each row is represented as a bit in every indexed value Efficient bitwise operations Advantageous when Low DML activity Low/medium cardinality High ad-hoc query 48

49 Other Types of Indexes Bitmap-Join Indexes 49

50 Bitmap-Join Indexes Allows to index one table using columns from another 50

51 Partitioning Partitioning Overview 51

52 Partitioning Partitioning allows to physically break a table or index into many smaller, more manageable pieces There is no segment for the table/index itself each partition has a segment Partition Key used to segregate data based on its values 52

53 Partitioning Advantages Increased data availability Easier administration of large objects Performance improvement of certain queries Improved scalability in certain cases 53

54 Partitioning Partitioning Schemes 54

55 Table Partitioning Schemes Range (8) Hash (8i) List (9i) Interval (11g) 55

56 Table Partitioning Schemes Reference (11g) the problem 56

57 Table Partitioning Schemes Reference (11g) the solution 57

58 Table Partitioning Schemes Reference + Interval (12c) 58

59 Table Partitioning Schemes Composite 2-level partitioning: Range-Hash (8i) Range-List (9i) The other combinations (11g) There is no segment for the partition itself each subpartition has a segment 59

60 Partitioning Composite 60

61 Partitioning Partitioning Indexing 61

62 Index Partitioning Local index Each table partition has a corresponding index partition Unique local indexes must include the partitioning key 62

63 Index Partitioning Globally partitioned index The index is partitioned by range or hash Each index partition may point to several table partitions 63

64 Index Partitioning Global (non-partitioned) indexes May be defined on partitioned tables as well 64

65 Partitioning New Partitioning Features in 12c 65

66 New Partitioning Features in 12c New scheme: reference + Interval Asynchronous global index maintenance Truncate/exchange partition cascade Multiple (sub)partitions in single DDL operations Partial indexes 66

67 Advanced Features Virtual Columns 67

68 Virtual Columns Virtual columns are not stored on disk Their values are derived on demand by computing expressions or functions Can be used in queries and DML statements, but cannot be set in INSERT and UPDATE Can be indexed (function based index) Can be used as partitioning 68

69 Advanced Features Invisible Columns 69

70 Invisible Columns (12c) Invisible columns are invisible, unless explicitly specified For example, they will not appear in: SELECT * FROM INSERT with no column list %ROWTYPE 70

71 Invisible Columns May be created as invisible May be modified from visible to invisible and vice versa When an invisible column becomes visible, it appears as the last column of the 71

72 Invisible Columns In SQL*Plus SET COLINVISIBLE controls inclusion or exclusion of invisible columns in DESCRIBE Invisible columns can be also: Virtual Partitioning key Cannot contain invisible columns: External tables Temporary tables Clustered 72

73 Advanced Features Virtual Indexes 73

74 Virtual Indexes A virtual index: Has no segment Is not maintained by DML Is invisible to the optimizer Unless _use_nosegment_indexes is true 74

75 Advanced Features Invisible Indexes 75

76 Invisible Indexes An invisible index is: Maintained by DML Invisible to the optimizer Unless optimizer_use_invisible_indexes is 76

77 80