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L01: Corpuses, Terms and Search
Basic terminology
The need for unstructured text search
Boolean Retrieval Model
Algorithms for compressing data
Algorithms for answering Boolean queries
Read Chapter 1 of MRS
Unstructured (text) vs. structured (database) data
31996
Unstructured (text) vs. structured (database) data
42006
Text Information Retrieval
You have all the Shakespeare plays stored in files
Which plays contain the word Caesar? Your algorithm here:
Which plays of Shakespeare contain the words Brutus AND Caesar ?
Processing unstructured data
Which plays of Shakespeare contain the words Brutus AND Caesar but NOT Calpurnia?
One could grep all of Shakespeare’s plays for Brutus and Caesar, then strip out lines containing Calpurnia, but…
It Is Slow! (for large corpora)Computing NOT Calpurnia is non-trivial
Other operations not feasible (e.g., find Romans near countrymen)
Ranked retrieval (find “best” documents to return)also not possible
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Term-document incidence
Antony and Cleopatra Julius Caesar The Tempest Hamlet Othello Macbeth
Antony 1 1 0 0 0 1
Brutus 1 1 0 1 0 0
Caesar 1 1 0 1 1 1
Calpurnia 0 1 0 0 0 0
Cleopatra 1 0 0 0 0 0
mercy 1 0 1 1 1 1
worser 1 0 1 1 1 0
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1 if file contains word, 0 otherwise
Brutus AND Caesar but NOT Calpurnia
N documents
m t
erm
s
Incidence vectors
So we have a 0/1 binary vector for each term.
To answer query: take the vectors for Brutus, Caesar and (complement) Calpurnia bitwise AND.
110100 AND 110111 AND 101111 100100.
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N documents
m t
erm
s
Brutus AND Caesar but NOT Calpurnia
Answers to query
Antony and Cleopatra, Act III, Scene iiAgrippa [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 iiLord Polonius: I did enact Julius Caesar I was killed i' the
Capitol; Brutus killed me.
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Space requirements
Say, N = 1M documents, each with about 1K terms. How many terms all together?
Average 6 bytes/term including spaces/punctuation.How many GB of data?
Say there are m = 500K distinct terms among these.How many 0’s and 1’s in the Term-doc incidence matrix?
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N=1mil documents
M=
0.5
mil t
erm
s
Does the matrix fits in memory?
500K x 1M matrix has 500 Billion 0’s and 1’s.
But matrix is extremely sparse it has no more than 1 Billion 1’s.
What’s a better representation than a matrix?
We only record the 1 positions.11
Why?
N documents
m t
erm
s 1 1 1
1 1
1
1 1
1 1
1 1
1 1
1 1
1 1
1 1
1 1
1 1
1 1
1
1 1
1 1
1 1
1 11 1
1
1 1
1
1 1
1 1
“Inverted” Index
For each term T, we must store a listing of all document id’s that contain T.
Do we use an array or a linked list for this?
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Brutus
Calpurnia
Caesar
1 2 3 5 8 13 21 34
2 4 8 16 32 64128
13 16
What happens if the word Caesar is added to document 14?
Inverted index
Linked lists generally preferred to arrays + Dynamic space allocation + Insertion of terms into documents easy – Space overhead of pointers
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Brutus
Calpurnia
Caesar
2 4 8 16 32 64 128
2 3 5 8 13 21 34
13 16
1
Dictionary Postings lists
Sorted by docID
Posting
Inverted index construction
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Tokenizer
Token stream. Friends Romans countrymen
Linguistic modules
Modified tokens. friend roman countryman
Indexer
Inverted index.
friend
roman
countryman
2 4
2
13 16
1
More onthese later.
Documents tobe indexed.
Friends, Romans, countrymen.
Indexer step 1: Token sequence
INPUT: Sequence of pairs:(Modified token, Document ID)
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I did enact JuliusCaesar I was killed
i' the Capitol; Brutus killed me.
Doc 1
So let it be withCaesar. The noble
Brutus hath told youCaesar was ambitious
Doc 2
Indexer step 2: Sort
Sort by terms.
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Core indexing step.
Indexer step 3: Dictionary & Postings
Merge multiple term entries in a single document
Note two entries for caesar! We merge PER DOCUMENT.
Add frequency information.
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Why frequency?Will discuss later.
Dictionary
Postings
Query processing: AND
Consider processing the query:Brutus AND Caesar Locate Brutus in the Dictionary;
Retrieve its postings. Locate Caesar in the Dictionary;
Retrieve its postings. “Intersect” the two postings:
18
128
34
2 4 8 16 32 64
1 2 3 5 8 13
21
Brutus
Caesar
The Intersection
Walk through the two postings simultaneously.
What did we call this process in CS230?
Time? _______ in the total number of postings entries
19
34
1282 4 8 16 32 64
1 2 3 5 8 13 21
128
34
2 4 8 16 32 64
1 2 3 5 8 13 21
Brutus
Caesar2 8
If the list lengths are x and y, the merge takes O(x+y)
operations.
Crucial: postings sorted by docID.
Intersecting two postings lists
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Query Optimization
Best order for processing pairs of postings? (Brutus AND Calpurnia) AND Caesar (Calpurnia AND Caesar) AND Brutus (Brutus AND Caesar) AND Calpurnia
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Query: Brutus AND Calpurnia AND Caesar
Brutus
Calpurnia
Caesar
1 2 3 5 8 13 21 34
2 4 8 16 32 64128
13 16
Query optimization example
Process in order of increasing freq: start with smallest set, then keep cutting further.
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Brutus
Calpurnia
Caesar
1 2 3 5 8 13 21 34
2 4 8 16 32 64128
13 16
This is why we keptfreq in dictionary
Execute the query as (Caesar AND Brutus) AND Calpurnia.
Query: Brutus AND Calpurnia AND Caesar
More General Optimization
Query: (madding OR crowd) AND (ignoble OR strife)
Get freq’s for all terms.
Estimate the size of each OR by the sum of its freq’s (conservative).
Process in increasing order of OR sizes.
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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.
Primary commercial retrieval tool for 3 decades.
Professional searchers (e.g., lawyers) still like Boolean queries: (e.g. Westlaw)
You know exactly what you’re getting.
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