!CHAPTER Hash Indexes

!SUBSECTION Introduction to Hash Indexes

This is an introduction to ArangoDB's 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`. 

!SECTION Accessing Hash Indexes from the Shell

<!-- js/server/modules/@arangodb/arango-collection.js-->


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.

    @startDocuBlockInline ensureUniqueConstraint
    @EXAMPLE_ARANGOSH_OUTPUT{ensureUniqueConstraint}
    ~db._create("test");
    db.test.ensureIndex({ type: "hash", fields: [ "a", "b.c" ], unique: true });
    db.test.save({ a : 1, b : { c : 1 } });
    db.test.save({ a : 1, b : { c : 1 } }); // xpError(ERROR_ARANGO_UNIQUE_CONSTRAINT_VIOLATED)
    db.test.save({ a : 1, b : { c : null } });
    db.test.save({ a : 1 });  // xpError(ERROR_ARANGO_UNIQUE_CONSTRAINT_VIOLATED)
    ~db._drop("test");
    @END_EXAMPLE_ARANGOSH_OUTPUT
    @endDocuBlock ensureUniqueConstraint


<!-- js/server/modules/@arangodb/arango-collection.js-->


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.

    @startDocuBlockInline ensureHashIndex
    @EXAMPLE_ARANGOSH_OUTPUT{ensureHashIndex}
    ~db._create("test");
    db.test.ensureIndex({ type: "hash", fields: [ "a" ] });
    db.test.save({ a : 1 });
    db.test.save({ a : 1 });
    db.test.save({ a : null });
    ~db._drop("test");
    @END_EXAMPLE_ARANGOSH_OUTPUT
    @endDocuBlock ensureHashIndex