What is K-gram indexes for wildcard queries?
characters. Thus cas, ast and stl are all 3-grams occurring in the term castle. We use a special character $ to denote the beginning or end of a term, so the full set of 3-grams generated for castle is: $ca, cas, ast, stl, tle, le$.
Which indices can be used for solving wildcard queries?
Finally, we use the standard inverted index to retrieve all documents containing any terms in this intersection. We can thus handle wildcard queries that contain a single * symbol using two B-trees, the normal B-tree and a reverse B-tree.
What is a K-gram index?
A k-gram index maps a k-gram to a postings list of all possible vocabulary terms that contain it. The figure below shows the k-gram postings list corresponding to the bigram “ur”. It is noteworthy that the postings list is sorted alphabetically.
What is a Permuterm index useful for?
Now that the permuterm index enables us to identify the original vocabulary terms matching a wildcard query, we look up these terms in the standard inverted index to retrieve matching documents. We can thus handle any wildcard query with a single * symbol.
What is wildcard queries in information retrieval?
We have seen that a wildcard query can result in multiple terms being enumerated, each of which becomes a single-term query on the standard inverted index. Search engines do allow the combination of wildcard queries using Boolean operators, for example, re*d AND fe*ri.
What is a wild card query?
To locate a specific item when you can’t remember exactly how it is spelled, try using a wildcard character in a query. Wildcards are special characters that can stand in for unknown characters in a text value and are handy for locating multiple items with similar, but not identical data.
What is Permuterm index in information retrieval?
The Permuterm index [Garfield 1976] is a time-efficient and elegant solution to the string dictionary problem in which pattern queries may possibly include one wild-card symbol (called Tolerant Retrieval problem). Unfortunately the Permuterm index is space inefficient because it quadruples the dictionary size.
Does Permuterm index require Postfiltering?
k-gram index is more space-efficient permuterm index does not require postfiltering.
What is blocked sort based indexing?
One solution is the blocked sort-based indexing algorithm or BSBI in Figure 4.2 . BSBI (i) segments the collection into parts of equal size, (ii) sorts the termID-docID pairs of each part in memory, (iii) stores intermediate sorted results on disk, and (iv) merges all intermediate results into the final index.