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Direct Index Query

JanusGraph’s standard global graph querying mechanism supports boolean queries for vertices or edges. In other words, an element either matches the query or it does not. There are no partial matches or result scoring.

Some indexing backends additionally support fuzzy search queries. For those queries, a score is computed for each match to indicate the "goodness" of the match and results are returned in the order of their score. Fuzzy search is particularly useful when dealing with full-text search queries where matching more words is considered to be better.

Since fuzzy search implementations and scoring algorithms differ significantly between indexing backends, JanusGraph does not support fuzzy search natively. However, JanusGraph provides a direct index query mechanism that allows search queries to be directly send to the indexing backend for evaluation (for those backends that support it).

Use Graph.indexQuery() to compose a query that is executed directly against an indexing backend. This query builder expects two parameters:

  1. The name of the indexing backend to query. This must be the name configured in JanusGraph’s configuration and used in the property key indexing definitions

  2. The query string

The builder allows configuration of the maximum number of elements to be returned via its limit(int) method. The builder’s offset(int) controls number of initial matches in the result set to skip. To retrieve all vertex or edges matching the given query in the specified indexing backend, invoke vertices() or edges(), respectively. It is not possible to query for both vertices and edges at the same time. These methods return an Iterable over Result objects. A result object contains the matched handle, retrievable via getElement(), and the associated score - getScore().

Consider the following example: 

ManagementSystem mgmt = graph.openManagement();
PropertyKey text = mgmt.makePropertyKey("text").dataType(String.class).make();
mgmt.buildIndex("vertexByText", Vertex.class).addKey(text).buildMixedIndex("search");
mgmt.commit();
// ... Load vertices ...
for (Result<Vertex> result : graph.indexQuery("vertexByText", "v.text:(farm uncle berry)").vertices()) {
   System.out.println(result.getElement() + ": " + result.getScore());
}

Query String

The query string is handed directly to the indexing backend for processing and hence the query string syntax depends on what is supported by the indexing backend. For vertex queries, JanusGraph will analyze the query string for property key references starting with "v." and replace those by a handle to the indexing field that corresponds to the property key. Likewise, for edge queries, JanusGraph will replace property key references starting with "e.". Hence, to refer to a property of a vertex, use "v.[KEY_NAME]" in the query string. Likewise, for edges write "e.[KEY_NAME]".

Elasticsearch and Lucene support the Lucene query syntax. Refer to the Lucene documentation or the Elasticsearch documentation for more information. The query used in the example above follows the Lucene query syntax.

graph.indexQuery("vertexByText", "v.text:(farm uncle berry)").vertices()

This query matches all vertices where the text contains any of the three words (grouped by parentheses) and score matches higher the more words are matched in the text.

In addition Elasticsearch supports wildcard queries, use "v.*" or "e.*" in the query string to query if any of the properties on the element match.

Query Totals

It is sometimes useful to know how many total results were returned from a query without having to retrieve all results. Fortunately, Elasticsearch and Solr provide a shortcut that does not involve retrieving and ranking all documents. This shortcut is exposed through the ".vertexTotals()", ".edgeTotals()", and ".propertyTotals()" methods.

The totals can be retrieved using the same query syntax as the indexQuery builder, but size is overwritten to be 0. 

graph.indexQuery("vertexByText", "v.text:(farm uncle berry)").vertexTotals()

Gotchas

Property Key Names

Names of property keys that contain non-alphanumeric characters must be placed in quotation marks to ensure that the query is parsed correctly. 

graph.indexQuery("vertexByText", "v.\"first_name\":john").vertices()

Some property key names may be transformed by the JanusGraph indexing backend implementation. For instance, an indexing backend that does not permit spaces in field names may transform "My Field Name" to "My•Field•Name", or an indexing backend like Solr may append type information to the name, transforming "myBooleanField" to "myBooleanField_b". These transformations happen in the index backend’s implementation of IndexProvider, in the "mapKey2Field" method. Indexing backends may reserve special characters (such as •) and prohibit indexing of fields that contain them. For this reason it is recommended to avoid spaces and special characters in property names.

In general, making direct index queries depends on implementation details of JanusGraph indexing backends that are normally hidden from users, so it’s best to verify a query empirically against the indexing backend in use.

Element Identifier Collision

The strings "v.", "e.", and "p." are used to identify a vertex, edge or property element respectively in a query. If the field name or the query value contains the same sequence of characters, this can cause a collision in the query string and parsing errors as in the following example: 

graph.indexQuery("vertexByText", "v.name:v.john").vertices() //DOES NOT WORK!

To avoid such identifier collisions, use the setElementIdentifier method to define a unique element identifier string that does not occur in any other parts of the query: 

graph.indexQuery("vertexByText", "$v$name:v.john").setElementIdentifier("$v$").vertices()

Mixed Index Availability Delay

When a query traverses a mixed index immediately after data is inserted the changes may not be visible. In Elasticsearch the configuration option that determines this delay is index refresh interval. In Solr the primary configuration option is max time.