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ISAM: Indexed-Sequential-Access-Method

ISAM: Indexed-Sequential-Access-Method. Adapted from Prof Joe Hellerstein’s notes http://db.cs.berkeley.edu/dbcourse/notes.html. A Note of Caution. ISAM is an old-fashioned idea B+-trees are usually better, as we’ll see Though not always But, it’s a good place to start

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ISAM: Indexed-Sequential-Access-Method

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  1. ISAM: Indexed-Sequential-Access-Method Adapted from Prof Joe Hellerstein’s notes http://db.cs.berkeley.edu/dbcourse/notes.html

  2. A Note of Caution • ISAM is an old-fashioned idea • B+-trees are usually better, as we’ll see • Though not always • But, it’s a good place to start • Simpler than B+-tree, but many of the same ideas • Upshot • Don’t brag about being an ISAM expert on your resume • Do understand how they work, and tradeoffs with B+-trees

  3. Index File kN k2 k1 Range Searches • ``Find all students with gpa > 3.0’’ • If data is in sorted file, do binary search to find first such student, then scan to find others. • Cost of binary search can be quite high. • Simple idea: Create an `index’ file. • Level of indirection again! Data File Page N Page 3 Page 1 Page 2 • Can do binary search on (smaller) index file!

  4. Overflow page index entry ISAM P K P P K P K m 2 0 1 m 1 2 • Index file may still be quite large. But we can apply the idea repeatedly! Non-leaf Pages Leaf Pages Primary pages • Leaf pages contain data entries.

  5. Root 40 51 63 20 33 46* 55* 40* 51* 97* 10* 15* 20* 27* 33* 37* 63* Example ISAM Tree • Each node can hold 2 entries; no need for `next-leaf-page’ pointers. (Why?)

  6. Data Pages Index Pages Overflow pages Comments on ISAM • File creation: Leaf (data) pages allocated sequentially, sorted by search key.Then index pages allocated.Then space for overflow pages. • Index entries: <search key value, page id>; they `direct’ search for data entries, which are in leaf pages. • Search: Start at root; use key comparisons to go to leaf. Cost log F N ; F = # entries/index pg, N = # leaf pgs • Insert: Find leaf where data entry belongs, put it there.(Could be on an overflow page). • Delete: Find and remove from leaf; if empty overflow page, de-allocate. • Static tree structure: inserts/deletes affect only leaf pages.

  7. Example ISAM Tree • Each node can hold 2 entries; no need for `next-leaf-page’ pointers. (Why?) Root 40 51 63 20 33 46* 55* 40* 51* 97* 10* 15* 20* 27* 33* 37* 63*

  8. After Inserting 23*, 48*, 41*, 42* ... Root 40 Index Pages 51 63 20 33 Primary Leaf 10* 15* 20* 27* 46* 55* 40* 51* 97* 33* 37* 63* Pages 41* 48* 23* Overflow Pages 42*

  9. ... then Deleting 42*, 51*, 97* Root 40 51 63 20 33 46* 55* 40* 10* 15* 20* 27* 33* 37* 63* 41* 48* 23* • Note that 51 appears in index levels, but 51* not in leaf!

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