Ch 1 Boolean Retrieval Modified by Dongwon Lee from slides by

1. Ch 1 Boolean Retrieval Modified by Dongwon Lee from slides by Christopher Manning and Prabhakar Raghavan

2. In the Last Class • Ch 19 • Index size estimation • Near-duplicate detection using shingling • Ch 21 • PageRank • Hubs and Authorities

3. Next 2 Classes • Ch 1 • IR Basics • Index: Incidence Matrix, Inverted Index • Boolean vs. ranked retrieval models • Ch 2 • Processing terms of documents • Improving inverted index • Supporting keyword vs. phrase queries

4. Information Retrieval • Information Retrieval (IR) is finding material (usually documents) of an unstructured nature (usually text) that satisfies an information need from within large collections (usually stored on computers).

5. IR vs. databases:Structured vs unstructured data • Structured data tends to refer to information in “tables” Employee Manager Salary Smith Jones 50000 Chang Smith 60000 Ivy Smith 50000 Typically allows numerical range and exact match (for text) queries, e.g., Salary < 60000 AND Manager = Smith.

6. Unstructured data • Typically refers to free text • Allows • Keyword queries including operators • More sophisticated “concept” queries e.g., • find all web pages dealing with drug abuse • Classic model for searching text documents

7. Unstructured (text) vs. structured (database) data in 1996

8. Unstructured (text) vs. structured (database) data in 2009

9. Sec. 1.1 Unstructured data in 1680 • Which plays of Shakespeare contain the words BrutusANDCaesar but NOTCalpurnia? • One could grep all of Shakespeare’s plays for Brutus and Caesar, then strip out lines containing Calpurnia? • Why is that not the answer? • Slow (for large corpora) • NOTCalpurnia is non-trivial • Other operations (e.g., find the word Romans nearcountrymen) not feasible • Ranked retrieval (best documents to return)

10. Sec. 1.1 Term-document incidence 1 if play contains word, 0 otherwise BrutusANDCaesarBUTNOTCalpurnia

11. Sec. 1.1 Incidence vectors • So we have a 0/1 vector for each term. • To answer query: take the vectors for Brutus, Caesar and Calpurnia (complemented)  bitwise AND. • 110100 AND 110111 AND 101111 = 100100.

12. Sec. 1.1 Answers to query • Antony and Cleopatra,Act III, Scene ii Agrippa [Aside to DOMITIUS ENOBARBUS]: Why, Enobarbus, When Antony found Julius Caesar dead, He cried almost to roaring; and he wept When at Philippi he found Brutus slain. • Hamlet, Act III, Scene ii Lord Polonius: I did enact Julius Caesar I was killed i' the Capitol; Brutus killed me.

13. Sec. 1.1 Basic assumptions of Information Retrieval • Collection: Fixed set of documents • Goal: Retrieve documents with information that is relevant to the user’s information needand helps the user complete a task

14. Misconception? Mistranslation? Misformulation? The classic search model Get rid of mice in a politically correct way TASK Info Need Info about removing mice without killing them Verbal form How do I trap mice alive? Query mouse trap SEARCHENGINE QueryRefinement Results Corpus

15. Sec. 1.1 Can’t build the incidence matrix • Consider • 1 M documents • 500K unique terms in all documents • 500K x 1M matrix has half-a-trillion 0’s and 1’s. • But the matrix is extremely sparse • What’s a better representation? • We only record the 1 positions.

16. Sec. 1.2 1 2 4 11 31 45 173 1 2 4 5 6 16 57 132 Inverted index • For each term t, we must store a list of all documents that contain t. • Identify each by a docID, a document serial number • Can we used fixed-size arrays for this? Brutus 174 Caesar Calpurnia 2 31 54 101 What happens if the word Caesar is added to document 14?

17. Sec. 1.2 1 2 4 11 31 45 173 1 2 4 5 6 16 57 132 Dictionary Postings Inverted index • We need variable-size postings lists • On disk, a continuous run of postings is normal and best • In memory, can use linked lists or variable length arrays • Some tradeoffs in size/ease of insertion Posting Brutus 174 Caesar Calpurnia 2 31 54 101 Sorted by docID (more later on why).

18. Sec. 1.2 Tokenizer Friends Romans Countrymen Token stream. Linguistic modules More on these later. friend friend roman countryman Modified tokens. roman Indexer 2 4 countryman 1 2 Inverted index. 16 13 Inverted index construction Documents to be indexed. Friends, Romans, countrymen.

19. Sec. 1.2 Indexer steps: Token sequence • Sequence of (Modified token, Document ID) pairs. Doc 1 Doc 2 I did enact Julius Caesar I was killed i' the Capitol; Brutus killed me. So let it be with Caesar. The noble Brutus hath told you Caesar was ambitious

20. Sec. 1.2 Indexer steps: Sort • Sort by terms • And then docID Core indexing step

21. Sec. 1.2 Indexer steps: Dictionary & Postings • Multiple term entries in a single document are merged. • Split into Dictionary and Postings • Doc. frequency information is added.

22. Sec. 1.2 Where do we pay in storage? Lists of docIDs Terms and counts Pointers

23. Sec. 1.3 2 4 8 16 32 64 1 2 3 5 8 13 21 Query processing: AND • Consider processing the query: BrutusANDCaesar • Locate Brutus in the Dictionary; • Retrieve its postings. • Locate Caesar in the Dictionary; • Retrieve its postings. • “Merge” the two postings: 128 Brutus Caesar 34

24. Sec. 1.3 Brutus Caesar 13 128 2 2 4 4 8 8 16 16 32 32 64 64 8 1 1 2 2 3 3 5 5 8 8 21 21 13 34 The merge • Walk through the two postings simultaneously, in time linear in the total number of postings entries 128 2 34 If the list lengths are x and y, the merge takes O(x+y) operations. Crucial: postings sorted by docID. What is this?

25. Intersecting two postings lists(a “merge” algorithm) The time complexity of the “merge” algorithm: O(x+y) = O(N)

26. Sidetrack: What is Algorithm? • Algorithm: a sequence of finite instructions for doing some task • unambiguous and simple to follow • Eg, Euclid’s algorithm to determine the greatest common divisor (gcd) of two integers greater than 1 • Divide the larger number (A) by the smaller number (B) to get the remainder A’ • Divide the larger number (B) by the smaller number (A’) to get the remainder B’ … • Divide the larger number (A’) by the smaller number (B’) to get the remainder A’’ … • Repeat until you get 0 or 1: Then, gcd is the last divider

27. Sidetrack: Eg, Optimal Wedding • When a person has a chance to date K persons, the optimal wedding algorithm is: • Date upto (K/3)-th persons • Let the best person among K/3 as B using a criteria C • Start dating again from (K/3 + 1)-th person, p • If p is better than B using C • Stop and Marry p • Otherwise, keep dating till K-th person

28. Sidetrack: How good is an Algorithm? • Measure the rate of growth of an algorithm or problem based upon the size of the input • Compare algorithms to determine which is better for the situation • Input N • Describing the relationship as a function f(N) • f(N): the number of steps required by an algorithm

29. Sidetrack: Why Input Size N? • What are other factors that affect computation time? • Speed of the CPU • Size of memory • Choice of programming languages • Size of the input • … • Since an algorithm can be implemented in different machines, by different programming languages, and run on different platforms, we are interested in comparing algorithms using factors that are not affected by implementations

30. Sidetrack: Growth of different f(N) N2 N Work required to finish log N 1 Size of input N

31. Sidetrack: Informal Definition of Big-O • Q: How long does it take from State College to Washington DC if you drive? • About 3 hours • More than 60 minutes • Less than 10 days Lower Bound Upper Bound

32. Sidetrack: Formal Definition of Big-O • For a given function g(n), O(g(n)) is defined to be the set of functions O(g(n)) = {f(n) : there exist positive constants c and n0 such that 0  f(n)  cg(n) for all n  n0} “f(n) = O(g(n))” means that f(n) is no worse than the function g(n) when n is large. That is, asymptotic upper bound

33. Sidetrack: Visual Illustration of Big-O cg(n) Upper Bound f(n) f(n) = O(g(n)) Work required to finish Our Algorithm n0 Size of input

34. Sidetrack: Tight Upper-Bound We say Big O complexity of 3n2 + 2 = O(n2) drop constants! because we can show that there is a n0 and a c such that: 0  3n2 + 2  cn2 for n  n0 e.g., c = 4 and n0 = 2 yields: 0  3n2 + 2  4n2 for n  2 You could say 3n2 + 2 = O(n6) or 3n2 + 2 = O(n700) But this is like answering: • How long does it take to drive to Washington DC? • Less than 11 years vs. Less than 5 hours

35. Sidetrack: Correct Interpretation of O() • Does not mean that it takes only one operation • Does mean that the work doesn’t change as N changes • Is notation for “constant work” • Does not mean that it takes N operations • Does mean that the work changes in a way that is proportional to N • Is a notation for “work grows at a linear rate” O(1) or “Order One” O(N) or “Order N”

36. Sidetrack: Ex: Sequential Steps • If steps appear sequentially (one after another), then add their respective O(). loop . . . endloop loop . . . endloop N O(N + M) M

37. Sidetrack: Ex: Embedded Steps • If steps appear embedded (one inside another), then multiply their respective O(). loop loop . . . endloop endloop O(N*M) M N

38. Sidetrack: Classes of Big O Functions O(1): O(log log n): O(log n): O((log n)c), c>1: O(n): O(n logn) = O(log n!): O(n2): O(n3): O(nc), c>1: O(cn): O(n!) = O(nn): cO(c^n) : • constant • double logarithmic • logarithmic • polylogarithmic • linear • quasilinear • quadratic • cubic • polynomial • exponential (or geometric) • combinatorial • double exponential

39. Sec. 1.3 Boolean queries: Exact match • The Boolean retrieval model is being able to ask a query that is a Boolean expression: • Boolean Queries are queries using AND, OR and NOT to join query terms • Views each document as a set of words • Is precise: document matches condition or not. • Perhaps the simplest model to build an IR system on • Primary commercial retrieval tool for 3 decades. • Many search systems still use are Boolean: • Email, library catalog, Mac OS X Spotlight

40. Sec. 1.3 2 4 8 16 32 64 128 1 2 4 5 8 16 21 32 Query optimization • What is the best order for query processing? • Consider a query that is an AND of n terms. • For each of the n terms, get its postings, then AND them together. Brutus Caesar Calpurnia 13 16 Query: BrutusANDCaesar ANDCalpurnia 40

41. Sec. 1.3 2 4 8 16 32 64 128 1 2 4 5 8 16 21 32 Query optimization example • Process in order of increasing freq: • start with smallest set, then keepcutting further. This is why we kept document freq. in dictionary Brutus Caesar Calpurnia 13 16 Execute the query as (CalpurniaANDBrutus)AND Caesar.

42. Sec. 1.3 2 4 8 16 32 64 128 1 2 4 5 8 16 21 32 Query optimization example Brutus Caesar Calpurnia 13 16 Execution order: (BrutusANDCaesar)AND Calpurnia Execution order: (CalpurniaANDBrutus)AND Caesar

43. Sec. 1.3 More general optimization • e.g., (madding OR crowd) AND (ignoble OR strife) • Get doc. freq.’s for all terms. • Estimate the size of each OR by the sum of its doc. freq.’s (conservative). • Process in increasing order of OR sizes.

44. More general optimization • If the query is: • friendsAND romans AND (NOT countrymen) • how could we use the freq of countrymen?

45. Example: WestLaw http://www.westlaw.com/ Extended Boolean model w. ranking

46. Sec. 1.4 Example: WestLaw http://www.westlaw.com/ • Largest commercial (paying subscribers) legal search service (started 1975; ranking added 1992) • Tens of terabytes of data; 700,000 users • Majority of users still use boolean queries • Example query: • What is the statute of limitations in cases involving the federal tort claims act? • LIMIT! /3 STATUTE ACTION /S FEDERAL /2 TORT /3 CLAIM • /3 = within 3 words, /S = in same sentence