Working with Indexes

Learn how to use different indexes efficiently by going through the ArangoDB Performance Course.

Index Identifiers and Handles

An index handle uniquely identifies an index in the database. It is a string and consists of the collection name and an index identifier separated by a /. The index identifier part is a numeric value that is auto-generated by ArangoDB.

A specific index of a collection can be accessed using its index handle or index identifier as follows:

db.collection.index("<index-handle>");
db.collection.index("<index-identifier>");
db._index("<index-handle>");

For example: Assume that the index handle, which is stored in the _id attribute of the index, is demo/362549736 and the index was created in a collection named demo. Then this index can be accessed as:

db.demo.index("demo/362549736");

Because the index handle is unique within the database, you can leave out the collection and use the shortcut:

db._index("demo/362549736");

An index may also be looked up by its name. Since names are only unique within a collection, rather than within the database, the lookup must also include the collection name.

db._index("demo/primary")
db.demo.index("primary")

Collection Methods

Listing all indexes of a collection

returns information about the indexes getIndexes()

Returns an array of all indexes defined for the collection. Since ArangoDB 3.4, indexes() is an alias for getIndexes().

Note that _key implicitly has an index assigned to it.

arangosh> db.test.ensureIndex({ type: "persistent", fields: [
........> "attribute", "secondAttribute.subAttribute"] });
arangosh> db.test.getIndexes();

{ 
  "cacheEnabled" : false, 
  "deduplicate" : true, 
  "estimates" : true, 
  "fields" : [ 
    "attribute", 
    "secondAttribute.subAttribute" 
  ], 
  "id" : "test/71925", 
  "isNewlyCreated" : true, 
  "name" : "idx_1733159536547594240", 
  "selectivityEstimate" : 1, 
  "sparse" : false, 
  "type" : "persistent", 
  "unique" : false, 
  "code" : 201 
}
[ 
  { 
    "fields" : [ 
      "_key" 
    ], 
    "id" : "test/0", 
    "name" : "primary", 
    "selectivityEstimate" : 1, 
    "sparse" : false, 
    "type" : "primary", 
    "unique" : true 
  }, 
  { 
    "cacheEnabled" : false, 
    "deduplicate" : true, 
    "estimates" : true, 
    "fields" : [ 
      "attribute" 
    ], 
    "id" : "test/71917", 
    "name" : "idx_1733159536546545665", 
    "selectivityEstimate" : 1, 
    "sparse" : false, 
    "type" : "persistent", 
    "unique" : true 
  }, 
  { 
    "cacheEnabled" : false, 
    "deduplicate" : true, 
    "estimates" : true, 
    "fields" : [ 
      "uniqueAttribute" 
    ], 
    "id" : "test/71921", 
    "name" : "idx_1733159536546545666", 
    "selectivityEstimate" : 1, 
    "sparse" : false, 
    "type" : "persistent", 
    "unique" : true 
  }, 
  { 
    "cacheEnabled" : false, 
    "deduplicate" : true, 
    "estimates" : true, 
    "fields" : [ 
      "attribute", 
      "secondAttribute.subAttribute" 
    ], 
    "id" : "test/71925", 
    "name" : "idx_1733159536547594240", 
    "selectivityEstimate" : 1, 
    "sparse" : false, 
    "type" : "persistent", 
    "unique" : false 
  } 
]

Creating an index

Ensures that an index exists:

collection.ensureIndex(index-description)

Ensures that an index according to the index-description exists. A new index will be created if none exists with the given description.

Calling this method returns an index object. Whether or not the index object existed before the call is indicated in the return attribute isNewlyCreated.

The index-description input value must contain at least a type attribute. Other attributes may be necessary, depending on the index type.

type can be one of the following values:

• persistent: persistent index
• fulltext: fulltext index (deprecated from ArangoDB 3.10 onwards)
• geo: geo index, with one or two attributes

fields is an array of attribute paths, containing the document attributes (or subattributes) to be indexed. Some indexes allow using only a single path, and others allow multiple. If multiple attributes are used, their order matters.

If an attribute path contains an [*] extension (e.g. friends[*].id), it means that the index attribute value is treated as an array and all array members are indexed separately. This is possible with persistent indexes.

storedValues: in indexes of type persistent, additional attributes can be stored in the index. These additional attributes cannot be used for index lookups or for sorting, but they can be used for projections. This allows an index to fully cover more queries and avoid extra document lookups. Non-existing attributes are stored as null values inside storedValues. The maximum number of attributes in storedValues is 32. It is not possible to create multiple indexes with the same fields attributes and uniqueness but different storedValues attributes. That means the value of storedValues is not considered in index creation calls when checking if an index is already present or needs to be created. In unique indexes, only the attributes in fields are checked for uniqueness, but the attributes in storedValues are not checked for their uniqueness.

name can be a string. Index names are subject to the same character restrictions as collection names. If omitted, a name will be auto-generated so that it is unique with respect to the collection, e.g. idx_832910498.

The purpose of user-defined index names is have easy-to-remember names to use in index hints in AQL queries. If no index hints are used, going with the auto-generated index names is fine.

sparse can be true or false.

For persistent the sparsity can be controlled, fulltext and geo are sparse by definition.

unique can be true or false and is supported by persistent. By default, all user-defined indexes are non-unique. Only the attributes in fields are checked for uniqueness. Any attributes in from storedValues are not checked for their uniqueness.

deduplicate can be true or false and is supported by array indexes of type persistent. It controls whether inserting duplicate index values from the same document into a unique array index will lead to a unique constraint error or not. The default value is true, so only a single instance of each non-unique index value will be inserted into the index per document. Trying to insert a value into the index that already exists in the index will always fail, regardless of the value of this attribute.

estimates can be true or false and is supported by indexes of type persistent. This attribute controls whether index selectivity estimates are maintained for the index. Not maintaining index selectivity estimates can have a slightly positive impact on write performance. The downside of turning off index selectivity estimates will be that the query optimizer will not be able to determine the usefulness of different competing indexes in AQL queries when there are multiple candidate indexes to choose from. The estimates attribute is optional and defaults to true if not set. It will have no effect on indexes other than persistent (with hash and skiplist being mere aliases for persistent nowadays).

cacheEnabled can be true or false and is supported by indexes of type persistent. The attribute controls whether an extra in-memory hash cache is created for the index. The hash cache can be used to speed up index lookups. The cache can only be used for queries that look up all index attributes via an equality lookup (==). The hash cache cannot be used for range scans, partial lookups or sorting. The cache will be populated lazily upon reading data from the index. Writing data into the collection or updating existing data will invalidate entries in the cache. The cache may have a negative effect on performance in case index values are updated more often than they are read. The maximum size of cache entries that can be stored is currently 4 MB, i.e. the cumulated size of all index entries for any index lookup value must be less than 4 MB. This limitation is there to avoid storing the index entries of “super nodes” in the cache. cacheEnabled defaults to false and should only be used for indexes that are known to benefit from an extra layer of caching.

Examples

arangosh> db.test.ensureIndex({ type: "persistent", fields: [ "a" ], sparse: true });
arangosh> db.test.ensureIndex({ type: "persistent", fields: [ "a", "b" ], unique: true });

{ 
  "cacheEnabled" : false, 
  "deduplicate" : true, 
  "estimates" : true, 
  "fields" : [ 
    "a" 
  ], 
  "id" : "test/71883", 
  "isNewlyCreated" : true, 
  "name" : "idx_1733159536536059905", 
  "selectivityEstimate" : 1, 
  "sparse" : true, 
  "type" : "persistent", 
  "unique" : false, 
  "code" : 201 
}
{ 
  "cacheEnabled" : false, 
  "deduplicate" : true, 
  "estimates" : true, 
  "fields" : [ 
    "a", 
    "b" 
  ], 
  "id" : "test/71887", 
  "isNewlyCreated" : true, 
  "name" : "idx_1733159536537108480", 
  "selectivityEstimate" : 1, 
  "sparse" : false, 
  "type" : "persistent", 
  "unique" : true, 
  "code" : 201 
}

Dropping an index via a collection handle

Drops an index:

collection.dropIndex(index)

Drops the index. If the index does not exist, then false is returned. If the index existed and was dropped, then true is returned. Note that you cannot drop some special indexes (e.g. the primary index of a collection or the edge index of an edge collection).

collection.dropIndex(index-handle)

Same as above. Instead of an index an index handle can be given.

arangosh> db.example.ensureIndex({ type: "persistent", fields: ["a", "b"] });
arangosh> var indexInfo = db.example.getIndexes();
arangosh> indexInfo;
arangosh> db.example.dropIndex(indexInfo[0])
arangosh> db.example.dropIndex(indexInfo[1].id)
arangosh> indexInfo = db.example.getIndexes();

{ 
  "cacheEnabled" : false, 
  "deduplicate" : true, 
  "estimates" : true, 
  "fields" : [ 
    "a", 
    "b" 
  ], 
  "id" : "example/71756", 
  "isNewlyCreated" : true, 
  "name" : "idx_1733159536493068289", 
  "selectivityEstimate" : 1, 
  "sparse" : false, 
  "type" : "persistent", 
  "unique" : false, 
  "code" : 201 
}
[ 
  { 
    "fields" : [ 
      "_key" 
    ], 
    "id" : "example/0", 
    "name" : "primary", 
    "selectivityEstimate" : 1, 
    "sparse" : false, 
    "type" : "primary", 
    "unique" : true 
  }, 
  { 
    "cacheEnabled" : false, 
    "deduplicate" : true, 
    "estimates" : true, 
    "fields" : [ 
      "a", 
      "b" 
    ], 
    "id" : "example/71756", 
    "name" : "idx_1733159536493068289", 
    "selectivityEstimate" : 1, 
    "sparse" : false, 
    "type" : "persistent", 
    "unique" : false 
  } 
]
false
true
[ 
  { 
    "fields" : [ 
      "_key" 
    ], 
    "id" : "example/0", 
    "name" : "primary", 
    "selectivityEstimate" : 1, 
    "sparse" : false, 
    "type" : "primary", 
    "unique" : true 
  } 
]

Load Indexes into Memory

Loads all indexes of this collection into memory:

collection.loadIndexesIntoMemory()

This function tries to cache all index entries of this collection into the main memory. Therefore it iterates over all indexes of the collection and stores the indexed values, not the entire document data, in memory. All lookups that could be found in the cache are much faster than lookups not stored in the cache so you get a nice performance boost. It is also guaranteed that the cache is consistent with the stored data.

This function honors memory limits. If the indexes you want to load are smaller than your memory limit this function guarantees that most index values are cached. If the index is larger than your memory limit this function will fill up values up to this limit and for the time being there is no way to control which indexes of the collection should have priority over others.

arangosh> db.example.loadIndexesIntoMemory();

{ 
  "result" : true 
}

Database Methods

Fetching an index by handle

Finds an index:

db._index(index-handle)

Returns the index with index-handle or null if no such index exists.

arangosh> db.example.ensureIndex({ type: "persistent", fields: [ "a", "b" ] });
arangosh> var indexInfo = db.example.getIndexes().map(function(x) { return x.id; });
arangosh> indexInfo;
arangosh> db._index(indexInfo[0])
arangosh> db._index(indexInfo[1])

{ 
  "cacheEnabled" : false, 
  "deduplicate" : true, 
  "estimates" : true, 
  "fields" : [ 
    "a", 
    "b" 
  ], 
  "id" : "example/66799", 
  "isNewlyCreated" : true, 
  "name" : "idx_1733159490746843137", 
  "selectivityEstimate" : 1, 
  "sparse" : false, 
  "type" : "persistent", 
  "unique" : false, 
  "code" : 201 
}
[ 
  "example/0", 
  "example/66799" 
]
{ 
  "fields" : [ 
    "_key" 
  ], 
  "id" : "example/0", 
  "name" : "primary", 
  "sparse" : false, 
  "type" : "primary", 
  "unique" : true, 
  "code" : 200 
}
{ 
  "cacheEnabled" : false, 
  "deduplicate" : true, 
  "estimates" : true, 
  "fields" : [ 
    "a", 
    "b" 
  ], 
  "id" : "example/66799", 
  "name" : "idx_1733159490746843137", 
  "sparse" : false, 
  "type" : "persistent", 
  "unique" : false, 
  "code" : 200 
}

Dropping an index via a database handle

Drops an index:

db._dropIndex(index)

Drops the index. If the index does not exist, then false is returned. If the index existed and was dropped, then true is returned.

db._dropIndex(index-handle)

Drops the index with index-handle.

arangosh> db.example.ensureIndex({ type: "persistent", fields: [ "a", "b" ] });
arangosh> var indexInfo = db.example.getIndexes();
arangosh> indexInfo;
arangosh> db._dropIndex(indexInfo[0])
arangosh> db._dropIndex(indexInfo[1].id)
arangosh> indexInfo = db.example.getIndexes();

{ 
  "cacheEnabled" : false, 
  "deduplicate" : true, 
  "estimates" : true, 
  "fields" : [ 
    "a", 
    "b" 
  ], 
  "id" : "example/72399", 
  "isNewlyCreated" : true, 
  "name" : "idx_1733159536657694721", 
  "selectivityEstimate" : 1, 
  "sparse" : false, 
  "type" : "persistent", 
  "unique" : false, 
  "code" : 201 
}
[ 
  { 
    "fields" : [ 
      "_key" 
    ], 
    "id" : "example/0", 
    "name" : "primary", 
    "selectivityEstimate" : 1, 
    "sparse" : false, 
    "type" : "primary", 
    "unique" : true 
  }, 
  { 
    "cacheEnabled" : false, 
    "deduplicate" : true, 
    "estimates" : true, 
    "fields" : [ 
      "a", 
      "b" 
    ], 
    "id" : "example/72399", 
    "name" : "idx_1733159536657694721", 
    "selectivityEstimate" : 1, 
    "sparse" : false, 
    "type" : "persistent", 
    "unique" : false 
  } 
]
false
true
[ 
  { 
    "fields" : [ 
      "_key" 
    ], 
    "id" : "example/0", 
    "name" : "primary", 
    "selectivityEstimate" : 1, 
    "sparse" : false, 
    "type" : "primary", 
    "unique" : true 
  } 
]

Revalidating whether an index is used

So you’ve created an index, and since its maintenance isn’t for free, you definitely want to know whether your query can utilize it.

You can use explain to verify that a certain index is used:

arangosh> var explain = require("@arangodb/aql/explainer").explain;
arangosh> db.example.ensureIndex({ type: "persistent", fields: [ "a", "b" ] });
arangosh> explain("FOR doc IN example FILTER doc.a < 23 RETURN doc", {colors: false});

{ 
  "cacheEnabled" : false, 
  "deduplicate" : true, 
  "estimates" : true, 
  "fields" : [ 
    "a", 
    "b" 
  ], 
  "id" : "example/66813", 
  "isNewlyCreated" : true, 
  "name" : "idx_1733159490751037441", 
  "selectivityEstimate" : 1, 
  "sparse" : false, 
  "type" : "persistent", 
  "unique" : false, 
  "code" : 201 
}
Query String (47 chars, cacheable: true):
 FOR doc IN example FILTER doc.a < 23 RETURN doc

Execution plan:
 Id   NodeType        Est.   Comment
  1   SingletonNode      1   * ROOT
  6   IndexNode          0     - FOR doc IN example   /* persistent index scan */    
  5   ReturnNode         0       - RETURN doc

Indexes used:
 By   Name                      Type         Collection   Unique   Sparse   Cache   Selectivity   Fields         Ranges
  6   idx_1733159490751037441   persistent   example      false    false    false      100.00 %   [ `a`, `b` ]   (doc.`a` < 23)

Optimization rules applied:
 Id   RuleName
  1   use-indexes
  2   remove-filter-covered-by-index
  3   remove-unnecessary-calculations-2

41 rule(s) executed, 1 plan(s) created

(If you omit colors: false you will get nice colors in ArangoShell.)