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## 20.4. Indexes

In order to rapidly find nodes or relationship based on properties, Neo4j supports indexing. This is commonly used to find start nodes for traversals.

By default, the underlying index is powered by Apache Lucene, but it is also possible to use Neo4j with other index implementations.

You can create an arbitrary number of named indexes. Each index handles either nodes or relationships, and each index works by indexing key/value/object triplets, object being either a node or a relationship, depending on the index type.

### 20.4.1. Index management

Just like the rest of the API, all write operations to the index must be performed from within a transaction.

#### Creating an index

If an index already exists with the name you choose, that index will be returned, and the optional arguments you passed will be ignored.

    with db.transaction:
        # Create a relationship index
        rel_idx = db.relationship.indexes.create('my_rels')

        # Create a node index, passing optional
        # arguments to the index provider.
        # In this case, enable full-text indexing.
        node_idx = db.node.indexes.create('my_nodes', type='fulltext')

#### Retrieving a pre-existing index

    with db.transaction:
        node_idx = db.node.indexes.get('my_nodes')

        rel_idx = db.relationship.indexes.get('my_rels')

#### Deleting indexes

    with db.transaction:
        node_idx = db.node.indexes.get('my_nodes')
        node_idx.delete()

        rel_idx = db.relationship.indexes.get('my_rels')
        rel_idx.delete()

### 20.4.2. Indexing things

#### Adding nodes or relationships to an index

    with db.transaction:
        # Indexing nodes
        a_node = db.node()
        node_idx = db.node.indexes.create('my_nodes')

        # Add the node to the index
        node_idx['akey']['avalue'] = a_node

        # Indexing relationships
        a_relationship = a_node.knows(db.node())
        rel_idx = db.relationship.indexes.create('my_rels')

        # Add the relationship to the index
        rel_idx['akey']['avalue'] = a_relationship

#### Removing indexed items

Removing items from an index can be done at several levels of granularity. See the example below.

    # Remove specific key/value/item triplet
    del idx['akey']['avalue'][item]

    # Remove all instances under a certain
    # key
    del idx['akey'][item]

    # Remove all instances all together
    del idx[item]

### 20.4.3. Searching the index

You can retrieve indexed items in two ways. Either you do a direct lookup, or you perform a query. The direct lookup is the same across different index providers while the query syntax depends on what index provider you use. As mentioned previously, Lucene is the default and by far most common index provider. For querying Lucene you will want to use the lucene query language.

There is a python library for programatically generating Lucene queries, available at GitHub.

| Important
Unless you loop through the entire index result, you have to close the result when you are done with it. If you do not, the database does not know when it can release the resources the result is taking up.

#### Direct lookups

    hits = idx['akey']['avalue']
    for item in hits:
        pass

    # Always close index results when you are
    # done, to free up resources.
    hits.close()

#### Querying

    hits = idx.query('akey:avalue')
    for item in hits:
        pass

    # Always close index results when you are
    # done, to free up resources.
    hits.close()

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