Biword & Positional Index Quiz and Flashcards

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Questions and Answers

What is the main difference between postings lists in a nonpositional index and a positional index?

The type of query being performed

The size of the index

The presence or absence of a list of positions (correct)

The number of postings in the list

What type of search does a positional index support?

Range search

Wildcard search

Phrase search and proximity search (correct)

Boolean search

What is an example of a query that requires a proximity search?

employment /4 place (correct)

employment AND place

employment OR place

employment -place

What is the purpose of a positional index?

To support phrase and proximity searches Signup and view all the answers

What is an example of a document that would match the query 'employment /4 place'?

Employment agencies that place healthcare workers are seeing growth. Signup and view all the answers

What is the simplest algorithm for proximity search using positional index?

Looking at the cross-product of positions of words in a document Signup and view all the answers

What is a limitation of using positional indexes for proximity search?

They can be very inefficient for frequent words, especially stop words Signup and view all the answers

What is the advantage of combining biword indexes with positional indexes?

It results in faster queries for frequent biwords Signup and view all the answers

What is the benefit of using Next Word Index over positional indexes?

It is faster than positional indexes, but at a cost of more space Signup and view all the answers

What is the purpose of including frequent biwords as vocabulary terms in the index?

To increase the speed of queries for frequent biwords Signup and view all the answers

What is the result of using a combination scheme of frequent biwords and positional intersection?

Improved efficiency for frequent biwords Signup and view all the answers

What operation takes O(m+n) time complexity in merging two postings lists?

Merging sorted lists Signup and view all the answers

What is the purpose of skip pointers in postings lists?

To skip postings that will not figure in the search results Signup and view all the answers

What is the tradeoff in placing skip pointers?

Between the length of skip spans and the number of comparisons Signup and view all the answers

What is the advantage of using skip pointers in query processing?

Improving the efficiency of query processing Signup and view all the answers

When is it possible to do better than O(m+n) time complexity in merging postings lists?

If the index isn’t changing too fast Signup and view all the answers

What happens when we match a posting on both lists?

We match it and advance to the next posting on both lists Signup and view all the answers

Study Notes

Index Types and Structures

Nonpositional index: each posting consists of just a document ID (docID).
Positional index: each posting includes a docID and a list of specific positions where the term appears.

Positional Index Example

Queries can be structured with terms sorted by their occurrence and relevance (e.g. "to1 be2 or3 not4 to5 be6").
For example, the term ‘TO’ in document 993427 has multiple positions: 7, 18, 33, etc.
Document 4 matches the query based on the positional index.

Proximity Search

Positional indexes support proximity searches, allowing searches for documents where two terms occur within a specified distance (e.g., EMPLOYMENT /4 PLACE).
Example hit: "Employment agencies that place healthcare workers are seeing growth."
Example miss: "Employment agencies that have learned to adapt now place healthcare workers."

Proximity Search Implementation

Simplest algorithm involves the cross-product of positions for the queried terms within individual documents.
The approach is inefficient for commonly used or stop words, which require returning actual matching positions.

Combination Indexing Scheme

Biword and positional indexes can be effectively combined for efficiency.
Frequent biwords (e.g. "Michael Jackson") enable faster searches than typical positional postings intersection.
A mixed indexing scheme, such as the Next Word Index, can improve speed while requiring more storage (26% more space).

Merge Operations in Indexing

Basic merge involves traversing two postings simultaneously, with a time complexity of O(m+n), where m and n are the lengths of the posting lists.
The merge process can be optimized through techniques like skip pointers.

Skip Pointers

Skip pointers enhance indexing by allowing for the skipping of irrelevant postings during query processing.
Skip pointers must be carefully placed to effectively reduce the number of comparisons during searches.

Query Processing with Skip Pointers

In a situation where lists are traversed and a match occurs, skip pointers allow the algorithm to jump ahead to relevant entries.
For example, if the matched number is 8, the next number on one list is 11, and the skip pointer allows skipping ahead past irrelevant postings to 31.

Placement and Trade-offs of Skip Pointers

The effectiveness of skip pointers is based on the balance between the number of skip pointers and the length of gaps they create.
More skips can lead to shorter spans and more possible skips, though they can also require additional comparisons with more pointers.

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Description

This quiz covers the basics of indexing in information retrieval, including positional and nonpositional indexes, and querying techniques.

Information Retrieval: Indexing and Querying

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