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Token graphs

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Token graphs

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When a tokenizer converts a text into a stream of tokens, it also records the following:

The position of each token in the stream
The positionLength, the number of positions that a token spans

Using these, you can create a directed acyclic graph, called a token graph, for a stream. In a token graph, each position represents a node. Each token represents an edge or arc, pointing to the next position.

Synonyms

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Some token filters can add new tokens, like synonyms, to an existing token stream. These synonyms often span the same positions as existing tokens.

In the following graph, quick and its synonym fast both have a position of 0. They span the same positions.

Multi-position tokens

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Some token filters can add tokens that span multiple positions. These can include tokens for multi-word synonyms, such as using "atm" as a synonym for "automatic teller machine."

However, only some token filters, known as graph token filters, accurately record the positionLength for multi-position tokens. These filters include:

Some tokenizers, such as the nori_tokenizer, also accurately decompose compound tokens into multi-position tokens.

In the following graph, domain name system and its synonym, dns, both have a position of 0. However, dns has a positionLength of 3. Other tokens in the graph have a default positionLength of 1.

Using token graphs for search

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Indexing ignores the positionLength attribute and does not support token graphs containing multi-position tokens.

However, queries, such as the match or match_phrase query, can use these graphs to generate multiple sub-queries from a single query string.

Example

A user runs a search for the following phrase using the match_phrase query:

domain name system is fragile

During search analysis, dns, a synonym for domain name system, is added to the query string’s token stream. The dns token has a positionLength of 3.

The match_phrase query uses this graph to generate sub-queries for the following phrases:

dns is fragile
domain name system is fragile

This means the query matches documents containing either dns is fragile or domain name system is fragile.

Invalid token graphs

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The following token filters can add tokens that span multiple positions but only record a default positionLength of 1:

This means these filters will produce invalid token graphs for streams containing such tokens.

In the following graph, dns is a multi-position synonym for domain name system. However, dns has the default positionLength value of 1, resulting in an invalid graph.

Avoid using invalid token graphs for search. Invalid graphs can cause unexpected search results.

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