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Records and Files

Records and Files. Storage Technology: Topic 3. Record Formats: Fixed Length. Information about field types stored in system catalogs. Direct access to i’th field. F1. F2. F3. F4. L1. L2. L3. L4. Base address (B). Address = B+L1+L2. 4. $. $. $. $.

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Records and Files

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  1. Records and Files Storage Technology: Topic 3

  2. Record Formats: Fixed Length • Information about field types stored in systemcatalogs. • Direct access to i’th field. F1 F2 F3 F4 L1 L2 L3 L4 Base address (B) Address = B+L1+L2

  3. 4 $ $ $ $ Record Formats: Variable Length • Two alternative formats (# fields is fixed): F1 F2 F3 F4 Fields Delimited by Special Symbols Field Count F1 F2 F3 F4 Array of Field Offsets • Second offers direct access to i’th field, efficient storage • of nulls(special don’t know value); small directory overhead.

  4. Files of Records • Page or block is OK when doing I/O, but higher levels of DBMS operate on records, and files of records. • FILE: a collection of pages, each containing a collection of records. Must support: • insert/delete/modify record • read a particular record (specified using record id) • scan all records (possibly with some conditions on the records to be retrieved)

  5. Page Formats: Fixed Length Records Slot 1 Slot 1 Slot 2 Slot 2 • Record id = <page id, slot #>. In first alternative, moving records for free space management changes rid; may not be acceptable. Free Space . . . . . . Slot N Slot N Slot M N . . . 1 1 1 M 0 M ... 3 2 1 number of records number of slots PACKED UNPACKED, BITMAP

  6. Rid = (i,N) Page i Rid = (i,2) Rid = (i,1) Page Formats: Variable Length Records • Can move records on page without changing rid; so, attractive for fixed-length records too. 20 16 24 Pointer to start of free space N . . . 2 1 SLOT DIRECTORY

  7. Unordered (Heap) Files • Simplest file structure contains records in no particular order. • As file grows and shrinks, disk pages are allocated and de-allocated. • To support record level operations, we must: • keep track of the pages in a file • keep track of free space on pages • keep track of the records on a page • There are many alternatives for keeping track of this.

  8. Heap File Implemented as a List • Each page contains 2 `pointers’ plus data. Data Page Data Page Data Page Pages with Free Space Header Page Data Page Data Page Data Page Full Pages

  9. Heap File Using a Page Directory Data Page 1 Header Page • The entry for a page can include the number of free bytes on the page. • The directory is a collection of pages; linked list implementation is just one alternative. Data Page 2 Data Page N DIRECTORY

  10. Indexes • A Heap file allows us to retrieve records: • by specifying the rid, or • by scanning all records sequentially • Sometimes, we want to retrieve records by specifying the values in one or more fields, e.g., • Find all students in the “CS” department • Find all students with a gpa > 3 • Indexes enable us to answer value-based (associative) queries efficiently.

  11. Alternative File Organizations Many alternatives exist, each ideal for some situation , and not so good in others: • Heap files: Suitable when typical access is a file scan retrieving all records. • Sorted Files: Best if records must be retrieved in some order, or only a `range’ of records is needed. • Hashed Files: Good for equality selections. • File is a collection of buckets. Bucket = primary page plus zero or moreoverflow pages. • Hashing functionh: h(r) = bucket in which record r belongs. h looks at only some of the fields of r, called the search fields.

  12. Cost Model for Analysis Ignore CPU costs, for simplicity: • B: The number of data pages • R: Number of records per page • D: (Average) time to read or write disk page • Measuring number of page I/O’s ignores gains of sequential I/O; thus, even I/O cost is only approximated. • Average-case analysis; based on several simplistic assumptions. • Good enough to show the overall trends!

  13. Cost of Operations • Several assumptions underlie these (rough) estimates!

  14. Assumptions • Single record insert and delete. • Heap Files: • Equality selection on key; exactly one match. • Insert always at end of file. • Sorted Files: • Files compacted after deletions. • Selections on sort field(s). • Hashed Files: • No overflow buckets, 80% page occupancy.

  15. Summary • Variable length record format with field offset directory offers support for direct access to i’th field and null values. • Slotted page format supports variable length records and allows records to move on page. • File layer keeps track of pages in a file, and supports abstraction of a collection of records. • Linked list or directory data structure • Sorted and hashed files for query processing

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