lecture2-dictionary-handout-6-per

Clearlyposionalindexescanbeusedforsuch

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Unformatted text preview: merge takes O(m+n) operations. Can we do better? Yes (if index isn’t changing too fast). 29
 1 64 128 31 11 2 48 11 17 21 31   Why?
   To
skip
pos)ngs
that
will
not
figure
in
the
search
 results.
   How?
   Where
do
we
place
skip
pointers?
 30
 5 Sec. 2.3 Introduc)on to Informa)on Retrieval Query
processing
with
skip
pointers
 4 8 41 2 3 8   Tradeoff:
 48 64 128   More
skips
→
shorter
skip
spans
⇒
more
likely
to
skip.

 But
lots
of
comparisons
to
skip
pointers.
   Fewer
skips
→
few
pointer
comparison,
but
then
long
skip
 spans
⇒
few
successful
skips.
 31 11 1 Sec. 2.3 Where
do
we
place
skips?
 128 41 2 Introduc)on to Informa)on Retrieval 11 17 21 31 Suppose we’ve stepped through the lists until we process 8 on each list. We match it and advance. We then have 41 and 11 on the lower. 11 is smaller. But the skip successor of 11 on the lower list is 31, so we can skip ahead past the intervening postings. 31
 Introduc)on to Informa)on Retrieval Sec. 2.3 32
 Introduc)on to Informa)on Retrieval Placing
skips
   Simple
heuris)c:
for
pos)ngs
of
length
L,
use
√L
 evenly‐spaced
skip
pointers.
   This
ignores
the
distribu)on
of
query
terms.
   Easy
if
the
index
is
rela)vely
sta)c;
harder
if
L
keeps
 changing
because
of
updates.
   This
definitely
used
to
help;
with
modern
hardware
it
 may
not
(Bahle
et
al.
2002)
unless
you’re
memory‐ based
 PHRASE
QUERIES
AND
POSITIONAL
 INDEXES
   The
I/O
cost
of
loading
a
bigger
pos)ngs
list
can
outweigh
 the
gains
from
qu...
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This document was uploaded on 02/26/2014.

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