Predicate indexing

Filtering on a predicate by applying a function requires an index.

When filtering by applying a function, Dgraph uses the index to make the search through a potentially large dataset efficient.

All scalar types can be indexed.

Types int, float, bool and geo have only a default index each: with tokenizers named int, float, bool and geo.

Types string and dateTime have a number of indices.

String Indices

The indices available for strings are as follows.

Dgraph function	Required index / tokenizer	Notes
`eq`	`hash`, `exact`, `term`, or `fulltext`	The most performant index for `eq` is `hash`. Only use `term` or `fulltext` if you also require term or full-text search. If you’re already using `term`, there is no need to use `hash` or `exact` as well.
`le`, `ge`, `lt`, `gt`	`exact`	Allows faster sorting.
`allofterms`, `anyofterms`	`term`	Allows searching by a term in a sentence.
`alloftext`, `anyoftext`	`fulltext`	Matching with language specific stemming and stopwords.
`regexp`	`trigram`	Regular expression matching. Can also be used for equality checking.

Warning Incorrect index choice can impose performance penalties and an increased transaction conflict rate. Use only the minimum number of and simplest indexes that your application needs.

DateTime Indices

The indices available for dateTime are as follows.

Index name / Tokenizer	Part of date indexed
`year`	index on year (default)
`month`	index on year and month
`day`	index on year, month and day
`hour`	index on year, month, day and hour

The choices of dateTime index allow selecting the precision of the index. Applications, such as the movies examples in these docs, that require searching over dates but have relatively few nodes per year may prefer the year tokenizer; applications that are dependent on fine grained date searches, such as real-time sensor readings, may prefer the hour index.

All the dateTime indices are sortable.

Sortable Indices

Not all the indices establish a total order among the values that they index. Sortable indices allow inequality functions and sorting.

Indexes int and float are sortable.
string index exact is sortable.
All dateTime indices are sortable.

For example, given an edge name of string type, to sort by name or perform inequality filtering on names, the exact index must have been specified. In which case a schema query would return at least the following tokenizers.

{
  "predicate": "name",
  "type": "string",
  "index": true,
  "tokenizer": [
    "exact"
  ]
}

Count index

For predicates with the @count Dgraph indexes the number of edges out of each node. This enables fast queries of the form:

{
  q(func: gt(count(pred), threshold)) {
    ...
  }
}

List Type

Predicate with scalar types can also store a list of values if specified in the schema. The scalar type needs to be enclosed within [] to indicate that its a list type.

occupations: [string] .
score: [int] .

A set operation adds to the list of values. The order of the stored values is non-deterministic.
A delete operation deletes the value from the list.
Querying for these predicates would return the list in an array.
Indexes can be applied on predicates which have a list type and you can use Functions on them.
Sorting is not allowed using these predicates.
These lists are like an unordered set. For example: ["e1", "e1", "e2"] may get stored as ["e2", "e1"], i.e., duplicate values will not be stored and order may not be preserved.

Filtering on list

Dgraph supports filtering based on the list. Filtering works similarly to how it works on edges and has the same available functions.

For example, @filter(eq(occupations, "Teacher")) at the root of the query or the parent edge will display all the occupations from a list of each node in an array but will only include nodes which have Teacher as one of the occupations. However, filtering on value edge is not supported.

Reverse Edges

A graph edge is unidirectional. For node-node edges, sometimes modeling requires reverse edges. If only some subject-predicate-object triples have a reverse, these must be manually added. But if a predicate always has a reverse, Dgraph computes the reverse edges if @reverse is specified in the schema.

The reverse edge of anEdge is ~anEdge.

For existing data, Dgraph computes all reverse edges. For data added after the schema mutation, Dgraph computes and stores the reverse edge for each added triple.

type Person {
  name string
}
type Car {
  regnbr string
  owner Person
}
owner uid @reverse .
regnbr string @index(exact) .
name string @index(exact) .

This makes it possible to query Persons and their cars by using:

q(func type(Person)) {
  name
  ~owner { name }
}

To get a different key than ~owner in the result, the query can be written with the wanted label (cars in this case):

q(func type(Person)) {
  name
  cars: ~owner { name }
}

This also works if there are multiple “owners” of a car:

owner [uid] @reverse .

In both cases the owner edge should be set on the Car:

_:p1 <name> "Mary" .
_:p1 <dgraph.type> "Person" .
_:c1 <regnbr> "ABC123" .
_:c1 <dgraph.type> "Car" .
_:c1 <owner> _:p1

Querying Schema

A schema query queries for the whole schema:

schema {}

Note Unlike regular queries, the schema query is not surrounded by curly braces. Also, schema queries and regular queries cannot be combined.

You can query for particular schema fields in the query body.

schema {
  type
  index
  reverse
  tokenizer
  list
  count
  upsert
  lang
}

You can also query for particular predicates:

schema(pred: [name, friend]) {
  type
  index
  reverse
  tokenizer
  list
  count
  upsert
  lang
}

Note If ACL is enabled, then the schema query returns only the predicates for which the logged-in ACL user has read access.

Types can also be queried. Below are some example queries.

schema(type: Movie) {}
schema(type: [Person, Animal]) {}

Note that type queries do not contain anything between the curly braces. The output will be the entire definition of the requested types.