memgraph/docs/user_technical/reference_guide/05_indexing.md

Indexing

An index stores additional information on certain types of data, so that retrieving said data becomes more efficient. Downsides of indexing are:

• requiring extra storage for each index and
• slowing down writes to the database.

Carefully choosing which data to index can tremendously improve data retrieval efficiency, and thus make index downsides negligible.

Memgraph automatically indexes labeled data. This improves queries which fetch nodes by label:

MATCH (n :Label) ... RETURN n

Indexing can also be applied to data with a specific combination of label and property. These are not automatically created, instead a user needs to create them explicitly. Creation is done using a special CREATE INDEX ON :Label(property) language construct.

For example, to index nodes which is labeled as :Person and has a property named age:

CREATE INDEX ON :Person(age)

After the index is created, retrieving those nodes will become more efficient. For example, the following query will retrieve all nodes which have an age property, instead of fetching each :Person node and checking whether the property exists.

MATCH (n :Person {age: 42}) RETURN n

Using index based retrieval also works when filtering labels and properties with WHERE. For example, the same effect as in the previous example can be done with:

MATCH (n) WHERE n:Person AND n.age = 42 RETURN n

Since the filter inside WHERE can contain any kind of an expression, the expression can be complicated enough so that the index does not get used. We are continuously improving the recognition of index usage opportunities from a WHERE expression. If there is any suspicion that an index may not be used, we recommend putting properties and labels inside the MATCH pattern.

Currently, once an index is created it cannot be deleted. This feature will be implemented very soon. The expected syntax for removing an index will be DROP INDEX ON :Label(property).