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Chap 7 . Indexing

File Structures by Folk, Zoellick, and Ricarrdi. Chap 7 . Indexing. 서울대학교 컴퓨터공학과 객체지향시스템연구실 SNU-OOPSLA-LAB 김 형 주 교수. Chapter Objectives(1). Introduce concepts of indexing that have broad applications in the design of file systems

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Chap 7 . Indexing

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  1. File Structures by Folk, Zoellick, and Ricarrdi Chap 7. Indexing 서울대학교 컴퓨터공학과 객체지향시스템연구실 SNU-OOPSLA-LAB 김 형 주 교수 SNU-OOPSLA Lab.

  2. Chapter Objectives(1) • Introduce concepts of indexing that have broad applications in the design of file systems • Introduce the use of a simple linear index to provide rapid access to records in an entry-sequenced, variable-length record file • Investigate the implementation of the use of indexes for file maintenance • Introduce the template features of C++ for object I/O • Describe the object-oriented approach to indexed sequential files SNU-OOPSLA Lab.

  3. Chapter Objectives(2) • Describe the use of indexes to provide access to records by more than one key • Introduce the idea of an inverted list, illustrating Boolean operations on lists • Discuss of when to bind an index key to an address in the data file • Introduce and investigate the implications of self-indexing files SNU-OOPSLA Lab.

  4. Contents(1) 7.1 What is an Index? 7.2 A Simple Index for Entry-Sequenced Files 7.3 Using Template Classes in C++ for Object I/O 7.4 Object-Oriented Support for Indexed, Entry- Sequenced Files of Data Objects 7.5 Indexes That Are Too Large to Hold in Memory SNU-OOPSLA Lab.

  5. Contents(2) 7.6 Indexing to Provide Access by Multiple Keys 7.7 Retrieval Using Combinations of Secondary Keys 7.8 Improving the Secondary Index Structure: Inverted Lists 7.9 Selective Indexes 7.10 Binding SNU-OOPSLA Lab.

  6. 7.1 What Is an Index? Overview: Index(1) • Index: a data structure which associates given key values with corresponding record numbers • It is usually physically separate from the file (unlike for indexed sequential files tight binding). • Linear indexes (like indexes found at the back of books) • Index records are ordered by key value as in an ordered relative file • Best algorithm for finding a record with a specific key value is binary search • Addition requires reorganization SNU-OOPSLA Lab.

  7. 7.1 What Is an Index? Index File k1 k2 k4 k5 k7 k9 k1 k2 k4 k5 k7 k9 AAA ZZZ CCC XXX EEE FFF Data File Overview: Index(2) SNU-OOPSLA Lab.

  8. 7.1 What Is an Index? Overview: Index(3) • Tree Indexes (like those of indexed sequential files) • Hierarchical in that each level • Beginning with the root level, points to the next record • Leaves POINTs only the data file • Indexed Sequential File • Binary Tree Index • AVL Tree Index • B+ tree Index SNU-OOPSLA Lab.

  9. 7.1 What Is an Index? Roles of Index? • Index: keys and reference fields • Fast Random Accesses • Uniform Access Speed • Allow users to impose order on a file without actually rearranging the file • Provide multiple access paths to a file • Give user keyed access to variable-length record files SNU-OOPSLA Lab.

  10. 7.2 A Simple Index for E-S Files A Simple Index(1) • Datafile • entry-sequenced, variable-length record • primary key : unique for each entry in a file • Search a file with key (popular need) • cannot use binary search in a variable-length record file(can’t know where the middle record) • construct an index object for the file • index object : key field + byte-offset field SNU-OOPSLA Lab.

  11. 7.2 A Simple Index for E-S Files Datafile Indexfile Reference Address of Key Actual data record field record ANG3795 167 LON|2312|Romeo and Juliet|Prokofiev . . . 32 COL31809 353 RCA|2626|Quarter in C Sharp Minor . . . 77 DG139201 396 WAR|23699|Touchstone|Corea . . . 132 COL38358 211 ANG|3795|Sympony No. 9|Beethoven . . . 167 DG18807 256 COL|38358|Nebeaska|Springsteen . . . 211 FF245 442 DG|18807|Symphony No. 9|Beethoven . . . 256 LON2312 32 MER|75016|Coq d'or Suite|Rimsky . . . 300 MER75016 300 COL|31809|Symphony No. 9|Dvorak . . . 353 RCA2626 77 DG|139201|Violin Concerto|Beethoven . . . 396 WAR23699 132 FF|245|Good News|Sweet Honey In The . . . 442 A Simple Index (2) SNU-OOPSLA Lab.

  12. 7.2 A Simple Index for E-S Files Key Reference field A Simple Index (3) • Index file: fixed-size record, sorted • Datafile: not sorted because it is entry sequenced • Record addition is quick (faster than a sorted file) • Can keep the index in memory • find record quickly with index file than with a sorted one • Class TextIndex encapsulates the index data and index operations SNU-OOPSLA Lab.

  13. 7.2 A Simple Index for E-S Files Let’s See Figure 7.4 Class TextIndex{ public: TextIndex(int maxKeys = 100, int unique = 1); int Insert(const char*ckey, int recAddr); //add to index int Remove(const char* key); //remove key from index int Search(const char* key) const; //search for key, return recAddr void Print (ostream &) const; protected: int MaxKeys; // maximum num of entries int NumKeys;// actual num of entries char **Keys; // array of key values int* RecAddrs; // array of record references int Find (const chat* key) const; int Init (int maxKeys, int unique); int Unique;// if true --> each key must be unique } SNU-OOPSLA Lab.

  14. Index Implementation • Page 638, 639, 640 • G.1 Recording.h • G.2 Recording.cpp • G.3 Makere.cpp • Page 641, 642 • G.4 Textind.h • G.5 Textind.cpp SNU-OOPSLA Lab.

  15. RetrieveRecording with the Index • RetrieveRecording(KEY...)procedure : retrieve a single record by key from datafile. And puts together the index search, file read, and buffer unpack operations into single function int RetriveRecording (Recording & recording, char * key, TextIndex & RecordingIndex, BufferFile & RecordingFile) // read and unpack the recording, return TRUE if succeeds { int result; result = RecordingFile . Read (RecordingIndex.Search(key)); if (result == -1) return FALSE; result = recording.Unpack (RecordingFile.GetBuffer()); return result; } SNU-OOPSLA Lab.

  16. 7.3 Using Template Classes in C++ for Object I/O Template Class for I/O Object(1) • Template Class RecordFile • we want to make the following code possible • Person p; RecordFile pFile; pFile.Read(p); • Recording r; RecordFile rFile; rFile.Read(r); • difficult to support files for different record types without having to modify the class • Template class which is derived from BufferFile • the actual declarations and calls • RecordFile <Person> pFile; pFile.Read(p); • RecordFile <Recording> rFile; rFile.Read(p); SNU-OOPSLA Lab.

  17. 7.3 Using Template Classes in C++ for Object I/O Template Class for I/O Object(2) template <class RecType> class RecordFile : public BufferFile{ public: int Read(RecType& record, int recaddr = -1); int Write(const RecType& record, int recaddr = -1); int Append(const RecType& record); RecordFile(IOBuffer& buffer) : BufferFile(buffer) {} }; //The template parameter RecType must have the following methods //int Pack(IOBuffer &); pack record into buffer //int Unpack(IOBuffer &); unpack record from buffer • Template Class RecordFile SNU-OOPSLA Lab.

  18. 7.3 Using Template Classes in C++ for Object I/O Template Class for I/O Object(3) • Adding I/O to an existing class RecordFile • add methods Pack and Unpack to class Recording • create a buffer object to use in the I/O • DelimFieldBuffer Buffer; • declare an object of type RecordFile<Recording> • RecordFile<Recording> rFile (Buffer); • Declaration and Calls • Recording r1, r2; • rFile.Open(“myfile”); • rFile.Read(r1); • rFile.Write(r2); Directly open a file and read and write objects of class Recording SNU-OOPSLA Lab.

  19. 7.4 OO Support for Indexed, E-S Files of Data Objects Object-Oriented Approach to I/O • Class IndexedFile • add indexed access to the sequential access provided by class RecordFile • extends RecordFile with Update, Append and Read method • Update & Append : maintain a primary key index of data file • Read : supports access to object by key • TextIndex, RecordFile ==> IndexedFile • Issues of IndexedFile • how to make a persistent index of a file • how to guarantee that the index is an accurate reflection of the contents of the data file SNU-OOPSLA Lab.

  20. 7.4 OO Support for Indexed, E-S Files of Data Objects Basic Operations of IndexedFile(1) • Create the original empty index and data files • Load the index file into memory • Rewrite the index file from memory • Add records to the data file and index • Delete records from the data file • Update records in the data file • Update the index to reflect changes in the data file • Retrieve records SNU-OOPSLA Lab.

  21. 7.4 OO Support for Indexed, E-S Files of Data Objects Basic Operations of TextIndexedFile (1) • Creating the files • initially empty files (index file and data file) created as empty files with header records • implementation ( makeind.cpp in Appendix G ) Create method in class BufferFile • Loading the index into memory • loading/storing objects are supported in the IOBuffer classes • need to choose a particular buffer class to use for an index file ( tindbuff.cpp in Appendix G ) • define class TextIndexBuffer as a derived class of FixedFieldBuffer to support reading and writing of index objects SNU-OOPSLA Lab.

  22. 7.4 OO Support for Indexed, E-S Files of Data Objects Basic Operations of TextIndexedFile(2) • Rewriting the index file from memory • part of the Close operation on an IndexedFile • write back index object to the index file • should protect the index when failure • write changes when out-of-date(use status flag) • Implementation • Rewind and Write operations of class BufferFile • Record Addition Add a new record to data file using RecordFile<Recording>::Write Add an entry to the index Requires rearrangement if in memory, no file access using TextIndex.Insert + SNU-OOPSLA Lab.

  23. 7.4 OO Support for Indexed, E-S Files of Data Objects Basic Operations of TextIndexedFile(3) • Record Deletion • data file: the records need not be moved • index: delete entry really or just mark it • using TextIndex::Delete • Record Updating (2 categories) • the update changes the value of the key field • delete/add approach • reorder both the index and the data file • the update does not affect the key field • no rearrangement of the index file • may need to reconstruct the data file SNU-OOPSLA Lab.

  24. 7.4 OO Support for Indexed, E-S Files of Data Objects Class TextIndexedFile(1) • Members • methods • Create, Open, Close, Read (sequential & indexed), Append, and Update operations • protected members • ensure the correlation between the index in memory (Index),the index file (IndexFile), and the data file (DataFile) • char* key() • the template parameter RecType must have the key method • used to extract the key value from the record SNU-OOPSLA Lab.

  25. 7.4 OO Support for Indexed, E-S Files of Data Objects Class TextIndexedFile(2) Template <class RecType> class TextIndexedFile { public: int Read(RecType& record); // read next record int Read(char* key, RecType& record) // read by key int Append(const RecType& record); int Update(char* oldKey, const RecType& record); int Create(char* name, int mode=ios::in|los::out); int Open(char* name, int mode=ios::in|los::out); int Close(); TextIndexedFile(IOBuffer & buffer, int keySize, int maxKeys=100); ~TextIndexedFile(); // close and delete protected: TextIndex Index; BufferFile IndexFile; TextIndexBuffer IndexBuffer; RecordFile<RecType> DataFile; char * FileName; // base file name for file int SetFileName(char* fName, char*& dFileName, char*&IdxFName); }; SNU-OOPSLA Lab.

  26. 7.4 OO Support for Indexed, E-S Files of Data Objects Enhancements to TextIndexedFile(1) • Support other types of keys • Restriction: the key type is restricted to string (char *) • Relaxation: support a template class SimpleIndex with parameter for key type • Support data object class hierarchies • Restriction: every object must be of the same type in RecordFile • Relaxation: the type hierarchy supports virtual pack methods SNU-OOPSLA Lab.

  27. 7.4 OO Support for Indexed, E-S Files of Data Objects Enhancements to TextIndexedFile(2) • Support multirecord index files • Restriction: the entire index fit in a single record • Relaxation: add protected method Insert, Delete, and Search to manipulate the arrays of index objects • Active optimization of operations • Obvious: the most obvious optimization is to use binary search in the Find method • Active: add a flag to the index object to avoid writing the index record back to the index file when it has not been changed SNU-OOPSLA Lab.

  28. Where are we going? • Plain Stream File • Persistency ==> Buffer support ==> BufferFile <incremental approach> Deriving BufferFile using various other classes • Random Access ==> Index support => IndexedFile <incremental approach> : Deriving TextIndexedFile using RecordFile and TextIndex SNU-OOPSLA Lab.

  29. 7.5 Indexes That Are Too Large to Hold in Memory Too Large Index(1) • On secondary storage (large linear index) • Disadvantages • binary searching of the index requires several seeks(slower than a sorted file) • index rearrangement requires shifting or sorting records on second storage • Alternatives (to be considered later) • hashed organization • tree-structured index (e.g. B-tree) SNU-OOPSLA Lab.

  30. 7.5 Indexes That Are Too Large to Hold in Memory Too Large Index (2) • Advantages over the use of a data file sorted by key even if the index is on the secondary storage • can use a binary search • sorting and maintaining the index is less expensive than doing the data file • can rearrange the keys without moving the data records if there are pinned records SNU-OOPSLA Lab.

  31. 7.6 Indexing to Provide Access by Multiple Keys Index by Multiple Keys(1) • DB-Schema = ( ID-No, Title, Composer, Artist, Label) • Find the record with ID-NO “COL38358” (primary key - ID-No) • Find all the recordings of “Beethoven” (2ndary key - composer) • Find all the recordings titled “Violin Concerto” (2ndary key - title) SNU-OOPSLA Lab.

  32. BEETHOVEN DG18807 7.6 Indexing to Provide Access by Multiple Keys Index by Multiple Keys(2) • Most people don’t want to search only by primary key • Secondary Key • can be duplicated • Figure --> • Secondary Key Index • secondary key --> consult one additional index (primary key index) SNU-OOPSLA Lab.

  33. 7.6 Indexing to Provide Access by Multiple Keys Secondary Index:Basic Operations(1) • Record Addition • similar to the case of adding to primary index • secondary index is stored in canonical form • fixed length (so it can be truncated) • original name can be obtained from the data file • can contain duplicate keys • local ordering in the same key group SNU-OOPSLA Lab.

  34. 7.6 Indexing to Provide Access by Multiple Keys Secondary Index:Basic Operations (2) • Record Deletion (2 cases) • Secondary index references directly record • delete both primary index and secondary index • rearrange both indexes • Secondary index references primary key • delete only primary index • leave intact the reference to the deleted record • advantage : fast • disadvantage : deleted records take up space SNU-OOPSLA Lab.

  35. 7.6 Indexing to Provide Access by Multiple Keys Secondary Index: Basic Operations (3) • Record Updating • primary key index serves as a kind of protective buffer • Secondary index references directly record • update all files containing record’s location • Secondary index references primary key (1) • affect secondary index only when either primary or secondary key is changed Continued. SNU-OOPSLA Lab.

  36. 7.6 Indexing to Provide Access by Multiple Keys Secondary Index: Basic Operations (4) • Secondary index references primary key(2) • when changes the secondary key • rearrange the secondary key index • when changes the primary key • update all reference field • may require reordering the secondary index • when confined to other fields • do not affect the secondary key index SNU-OOPSLA Lab.

  37. 7.7 Retrieval Using Combinations of Secondary Keys Retrieval of Records • Types • primary key access • secondary key access • combination of above • Combination of keys • using secondary key index, it is easy • boolean operation (AND, OR) SNU-OOPSLA Lab.

  38. 7.8 Improving the Secondary Index Structure Inverted Lists(1) • Inverted List • a secondary key leads to a set of one or more primary keys • Disadvantages of 2nd-ary index structure • rearrange when adding • repeated entry when duplicating • Solution A: by an array of references • Solution B: by linking the list of references SNU-OOPSLA Lab.

  39. Revised composer index Secondary key Set of primary key references BEETHOVEN ANG3795 DG139201 DG18807 RCA2626 COREA WAR23699 DVORAK COL31809 PROKOFIEV LON2312 RIMSKY-KORSAKOV MER75016 SPRINGSTEEN COL38358 SWEET HONEY IN THE R FF245 7.8 Improving the Secondary Index Structure Array of References • * no need to rearrange • * limited reference array • * internal fragmentation SNU-OOPSLA Lab.

  40. PROKOFIEV ANG36193 LON2312 7.8 Improving the Secondary Index Structure Inverted Lists (2) • Guidelines for better solution • no reorganization when adding • no limitation for duplicate key • no internal fragmentation • Solution B: by Linking the list of references • A list of primary key references • secondary key field, relative record number of the first corresponding primary key reference SNU-OOPSLA Lab.

  41. Improved revision of the composer index Secondary Index file Label ID List file BEETHOVEN 3 LON2312 -1 0 0 1 2 -1 COREA RCA2626 1 7 2 DVORAK WAR23699 -1 2 PROKOFIEV 3 ANG23699 10 8 3 4 4 RIMSKY-KORSAKOV COL38358 6 -1 5 SPINGSTEEN DG18807 4 1 5 6 SWEET HONEY IN THE R MER75016 9 -1 6 COL31809 -1 7 DG139201 5 8 FF245 -1 9 10 ANG36193 0 7.8 Improving the Secondary Index Structure Linking List of References (1) SNU-OOPSLA Lab.

  42. 7.8 Improving the Secondary Index Structure Linking List of References (2) • The primary key references in a separate, entry-sequenced file • Advantages • rearranges only when secondary key changes • rearrangement is quick • less penalty associated with keeping the secondary index file on secondary storage (less need for sorting) • Label ID List file not need to be sorted • reusing the space of deleted record is easy SNU-OOPSLA Lab.

  43. 7.8 Improving the Secondary Index Structure Linking List of References (3) • Disadvantage • same secondary key references may not be physically grouped • lack of locality • could involve a large amount of seeking • solution: reside in memory • same Label ID list can hold the lists of a number of secondary index files • if too large in memory, can load only a part of it SNU-OOPSLA Lab.

  44. 7.9 Selective Indexes Selective Indexes • Selective Index: Index on a subset of records • Selective index contains only some part of entire index • provide a selective view • useful when contents of a file fall into several categories • e.g. 20 < Age < 30 and $1000 < Salary SNU-OOPSLA Lab.

  45. 7.10 Binding Index Binding(1) • When to bind the key indexes to the physical address of its associated record? • File construction time binding (Tight, in-the-data binding) • tight binding & faster access • the case of primary key • when secondary key is bound to that time • simpler and faster retrieval • reorganization of the data file results in modifications of all bound index files SNU-OOPSLA Lab.

  46. 7.10 Binding Index Binding (2) • Postpone binding until a record is actually retrieved (Retrieval-time binding) • minimal reorganization & safe approach • mostly for secondary key • Tight, in-the-data binding is good when • static, little or no changes • rapid performance during retrieval • mass-produced, read-only optical disk SNU-OOPSLA Lab.

  47. Let’s Review (1) 7.1 What is an Index? 7.2 A Simple Index for Entry-Sequenced Files 7.3 Using Template Classes in C++ for Object I/O 7.4 Object-Oriented Support for Indexed, Entry- Sequenced Files of Data Objects 7.5 Indexes That Are Too Large to Hold in Memory SNU-OOPSLA Lab.

  48. Let’s Review(2) 7.6 Indexing to Provide Access by Multiple Keys 7.7 Retrieval Using Combinations of Secondary Keys 7.8 Improving the Secondary Index Structure: Inverted Lists 7.9 Selective Indexes 7.10 Binding SNU-OOPSLA Lab.

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