Global indexes can be used in graph traversal queries for the first traversal step
reached after theV() step, and are used to trim down the number of
vertices that are initially fetched. Remember that a search index must be used if
two or more properties are used for global indexing. In general, the traversal step
involves a vertex label and can include a property key and a particular property
value. In a traversal, the step following g.V() is generally the
step in which an index will be consulted. If a mid-traversal V()
step is called, then an additional indexed step can be consulted to narrow the list
of vertices that will be traversed.
Note: Graph traversals will only use indexes if the both the vertex
label and property key are specified. If both are not specified, indexing will not
be used and a full graph scan for the property key can result. If full graph scan is
disabled, a query will fail, as shown in this example where a property is specified,
but a vertex label is not
specified:
g.V().has('name','Julia Child')
Could not find an index to answer query clause and graph.allow_scan is disabled:
((label = FridgeSensor & name WITHIN [Julia Child]) | (label = author & name WITHIN [Julia Child]) |
(label = book & name WITHIN [Julia Child]) | (label = ingredient & name WITHIN [Julia Child]) |
(label = meal & name WITHIN [Julia Child]) | (label = recipe & name WITHIN [Julia Child]) |
(label = reviewer & name WITHIN [Julia Child]))
Edge indexes and property indexes (vertex-centric indexes) can be used to narrow the
query after a global index has found the starting vertex. They allow definition of
the edges that will be followed or the meta-properties that will be used to further
restrict the query.
Procedure
Global index
The graph traversal shown uses an index to discover certain person vertices to
start the query.
This graph traversal uses an index, if the index exists, because the
traversal step has('person', 'name', 'Emeril Lagasse')
identifies the vertex label and the property key indexed. After finding the
initial vertex to traverse from, the outgoing created edges
are walked and the adjacent vertices are listed by name.
This graph traversal shows the importance of using the vertex label in
combination with the property key, as two different elements, persons and
recipes, use the same property key name.
Checking for the use of indexing can be accomplished with the
profile()
method:
gremlin> g.V().has('person', 'name', 'Emeril Lagasse').out('created').values('name').profile()
==>Traversal Metrics
Step Count Traversers Time (ms) % Dur
=============================================================================================================
DsegGraphStep(vertex,[],(label = person & name ... 1 1 8.427 27.42
query-optimizer 0.792
\_condition=((label = person & name = Emeril Lagasse) & (true))
query-setup 0.028
\_isFitted=true
\_isSorted=false
\_isScan=false
index-query 6.514
\_indexType=Materialized
\_usesCache=false
\_statement=SELECT "personId" FROM "dse60"."person_p_byName" WHERE "name" = ? LIMIT ?; with params (java.
lang.String) Emeril Lagasse, (java.lang.Integer) 50000
\_options=Options{consistency=Optional[ONE], serialConsistency=Optional.empty, fallbackConsistency=Option
al.empty, pagingState=null, pageSize=-1, user=Optional.empty, waitForSchemaAgreement=true, asyn
c=true}
...
Note the index-query used in the first step
DsegGraphStep identifies the index type as
materialized. If an index was not used, index-query would
be missing from the profile output.
Edge index
An edge index can narrow the query, such as this one that finds all the
outgoing edges for reviews that John Doe wrote that have a rating of
greater or equal to 3 stars:
Using profile() on the query shows that a global index
query was used in the initial step, and the output shown here shows that in the
second step, the ratedByStars edge index was used to cut the
latency of the query.
Tip: The local() step can be used to affect how an edge index narrows a
query.
Property index
A property index can narrow the query, such as this one that finds the
countries that Julia Child lived in, starting in the year 1961 (in
this case, only one country):
Using
profile() on the query shows that a global index query was
used in the initial step, and the output shown here shows that in the second
SELECT step, the byStartYear property index was used to cut the
latency of the query.
Tip: The local() step can also be handy for use with property
indexes.
