lecture1-intro-handout-6-per

14 introducontoinformaonretrieval sec 14 26 sec 13

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Unformatted text preview: we
process?
 Today’s focus   Consider
processing
the
query:
 Brutus
AND
Caesar
   Locate
Brutus
in
the
Dic)onary;
   Retrieve
its
pos)ngs.
   Locate
Caesar
in
the
Dic)onary;
   Retrieve
its
pos)ngs.
   “Merge”
the
two
pos)ngs:
 2
 4
 8
 16
 1
 2
 3
 5
 32
 8
 64
 13
 21
 21
 Sec. 1.3 Introduc)on to Informa)on Retrieval The
merge
 128
 Brutus
 34
 Caesar
 22
 Introduc)on to Informa)on Retrieval Intersec)ng
two
pos)ngs
lists
 (a
“merge”
algorithm)
   Walk
through
the
two
pos)ngs
simultaneously,
in
 )me
linear
in
the
total
number
of
pos)ngs
entries
 2
 8
 2
 4
 8
 16
 1
 2
 3
 5
 32
 8
 64
 13
 21
 128
 Brutus
 34
 Caesar
 If list lengths are x and y, merge takes O(x+y) operations. Crucial: postings sorted by docID. 23
 24
 4 Introduc)on to Informa)on Retrieval Sec. 1.3 Boolean
queries:
Exact
match
 Example:
WestLaw


http://www.westlaw.com/   The
Boolean
retrieval
model
is
being
able
to
ask
a
 query
that
is
a
Boolean
expression:
   Boolean
Queries
use
AND, OR
and
NOT
to
join
query
terms
  ...
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