170120107066 dbms
- 1. Gandhinagar Institute of Technology(012) Subject : Active Learning Assignment Branch : CE DIV. : B1 Prepared by : - By:Harsh kothari Enrolment no.:170120107066 Guided By: Prof. Kiran Shah
- 2. Selection Operation 13.2 File scan – search algorithms that locate and retrieve records that fulfill a selection condition. Algorithm A1 (linear search). Scan each file block and test all records to see whether they satisfy the selection condition. Cost estimate (number of disk blocks scanned) = br br denotes number of blocks containing records from relation r If selection is on a key attribute, cost = (br /2) stop on finding record Linear search can be applied regardless of selection condition or ordering of records in the file, or availability of indices
- 3. Selection Operation (Cont.) 13.3 A2 (binary search). Applicable if selection is an equality comparison on the attribute on which file is ordered. Assume that the blocks of a relation are stored contiguously Cost estimate (number of disk blocks to be scanned): log2(br) — cost of locating the first tuple by a binary search on the blocks Plus number of blocks containing records that satisfy selection condition – Will see how to estimate this cost in Chapter 14
- 4. Selections Using Indices 13.4 Index scan – search algorithms that use an index selection condition must be on search-key of index. A3 (primary index on candidate key, equality). Retrieve a single record that satisfies the corresponding equality condition Cost = HTi + 1 A4 (primary index on nonkey, equality) Retrieve multiple records. Records will be on consecutive blocks Cost = HTi + number of blocks containing retrieved records A5 (equality on search-key of secondary index). Retrieve a single record if the search-key is a candidate key Cost = HTi + 1 Retrieve multiple records if search-key is not a candidate key Cost = HTi + number of records retrieved – Can be very expensive! each record may be on a different block – one block access for each retrieved record
- 5. Selections Involving Comparisons 13.5 Can implement selections of the form AV (r) or A V(r) by using a linear file scan or binary search, or by using indices in the following ways: A6 (primary index, comparison). (Relation is sorted on A) For A V(r) use index to find first tuple v and scan relation sequentially from there For AV (r) just scan relation sequentially till first tuple > v; do not use index A7 (secondary index, comparison). For A V(r) use index to find first index entry v and scan index sequentially from there, to find pointers to records. For AV (r) just scan leaf pages of index finding pointers to records, till first entry > v In either case, retrieve records that are pointed to – requires an I/O for each record – Linear file scan may be cheaper if many records are to be fetched!
- 6. Implementation of Complex Selections Conjunction: 1 2. . . n(r) A8 (conjunctive selection using one index). Select a combination of i and algorithms A1 through A7 that results in the least cost fori (r). Test other conditions on tuple after fetching it into memory buffer. A9 (conjunctive selection using multiple-key index). Use appropriate composite (multiple-key) index if available. A10 (conjunctive selection by intersection of identifiers). Requires indices with record pointers. Use corresponding index for each condition, and take intersection of all the obtained sets of record pointers. Then fetch records from file If some conditions do not have appropriate indices, apply test in memory.
- 7. Algorithms for Complex Selections Disjunction:1 2 . . . n (r). A11 (disjunctive selection by union of identifiers). Applicable if all conditions have available indices. Otherwise use linear scan. Use corresponding index for each condition, and take union of all the obtained sets of record pointers. Then fetch records from file Negation: (r) Use linear scan on file If very few records satisfy , and an index is applicable to Find satisfying records using index and fetch from file