Hash Indexes

Introduction to Hash Indexes

It is possible to define a hash index on one or more attributes (or paths) of a document. This hash index is then used in queries to locate documents in O(1) operations. If the hash index is unique, then no two documents are allowed to have the same set of attribute values.

Creating a new document or updating a document will fail if the uniqueness is violated. If the index is declared sparse, a document will be excluded from the index and no uniqueness checks will be performed if any index attribute value is not set or has a value of null.

Accessing Hash Indexes from the Shell

Unique Hash Indexes

Ensures that a unique constraint exists: collection.ensureIndex({ type: "hash", fields: [ "field1", ..., "fieldn" ], unique: true })

Creates a unique hash index on all documents using field1, ... fieldn as attribute paths. At least one attribute path has to be given. The index will be non-sparse by default.

All documents in the collection must differ in terms of the indexed attributes. Creating a new document or updating an existing document will will fail if the attribute uniqueness is violated.

To create a sparse unique index, set the sparse attribute to true:

collection.ensureIndex({ type: "hash", fields: [ "field1", ..., "fieldn" ], unique: true, sparse: true })

In case that the index was successfully created, the index identifier is returned.

Non-existing attributes will default to null. In a sparse index all documents will be excluded from the index for which all specified index attributes are null. Such documents will not be taken into account for uniqueness checks.

In a non-sparse index, all documents regardless of null - attributes will be indexed and will be taken into account for uniqueness checks.

In case that the index was successfully created, an object with the index details, including the index-identifier, is returned.

arangosh> db.test.ensureIndex({ type: "hash", fields: [ "a", "b.c" ], unique: true });
arangosh> db.test.save({ a : 1, b : { c : 1 } });
arangosh> db.test.save({ a : 1, b : { c : 1 } });
arangosh> db.test.save({ a : 1, b : { c : null } });
arangosh> db.test.save({ a : 1 });
show execution results

Non-unique Hash Indexes

Ensures that a non-unique hash index exists: collection.ensureIndex({ type: "hash", fields: [ "field1", ..., "fieldn" ] })

Creates a non-unique hash index on all documents using field1, ... fieldn as attribute paths. At least one attribute path has to be given. The index will be non-sparse by default.

To create a sparse unique index, set the sparse attribute to true:

collection.ensureIndex({ type: "hash", fields: [ "field1", ..., "fieldn" ], sparse: true })

In case that the index was successfully created, an object with the index details, including the index-identifier, is returned.

arangosh> db.test.ensureIndex({ type: "hash", fields: [ "a" ] });
arangosh> db.test.save({ a : 1 });
arangosh> db.test.save({ a : 1 });
arangosh> db.test.save({ a : null });
show execution results

Hash Array Indexes

Ensures that a hash array index exists (non-unique): collection.ensureIndex({ type: "hash", fields: [ "field1[*]", ..., "fieldn[*]" ] })

Creates a non-unique hash array index for the individual elements of the array attributes field1[*], ... fieldn[*] found in the documents. At least one attribute path has to be given. The index always treats the indexed arrays as sparse.

It is possible to combine array indexing with standard indexing: collection.ensureIndex({ type: "hash", fields: [ "field1[*]", "field2" ] })

In case that the index was successfully created, an object with the index details, including the index-identifier, is returned.

arangosh> db.test.ensureIndex({ type: "hash", fields: [ "a[*]" ] });
arangosh> db.test.save({ a : [ 1, 2 ] });
arangosh> db.test.save({ a : [ 1, 3 ] });
arangosh> db.test.save({ a : null });
show execution results

Ensure uniqueness of relations in edge collections

It is possible to create secondary indexes using the edge attributes _from and _to, starting with ArangoDB 3.0. A combined index over both fields together with the unique option enabled can be used to prevent duplicate relations from being created.

For example, a document collection verts might contain vertices with the document handles verts/A, verts/B and verts/C. Relations between these documents can be stored in an edge collection edges for instance. Now, you may want to make sure that the vertex verts/A is never linked to verts/B by an edge more than once. This can be achieved by adding a unique, non-sparse hash index for the fields _from and _to:

db.edges.ensureIndex({ type: "hash", fields: [ "_from", "_to" ], unique: true });

Creating an edge { _from: "verts/A", _to: "verts/B" } in edges will be accepted, but only once. Another attempt to store an edge with the relation A → B will be rejected by the server with a unique constraint violated error. This includes updates to the _from and _to fields.

Note that adding a relation B → A is still possible, so is A → A and B → B, because they are all different relations in a directed graph. Each one can only occur once however.

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