Overview

To execute a UQL query on an HDC graph, use the syntax:

exec{
  <query>
} on <hdcGraphName>

HDC graphs support queries that retrieve data from the database with enhanced efficiency. However, remember that any query attempting to write data will fail on HDC graphs.

Example Graph

To create the graph, execute each of the following UQL queries sequentially in an empty graphset:

create().node_schema("entity").edge_schema("link")
create().edge_property(@link, "weight", float)
insert().into(@entity).nodes([{_id:"A"},{_id:"B"},{_id:"C"},{_id:"D"}])
insert().into(@link).edges([{_from:"A", _to:"B", weight:1},{_from:"A", _to:"C", weight:1.5},{_from:"A", _to:"D", weight:0.5},{_from:"B", _to:"C", weight:2},{_from:"C", _to:"D", weight:0.5}])

Queries on HDC Graphs

To create an HDC graph hdcGraph of the entire graph:

hdc.graph.create("hdcGraph", {
  nodes: {"*": ["*"]},
  edges: {"*": ["*"]},
  direction: "undirected",
  load_id: true,
  update: "static",
  query: "query",
  default: false
}).to("hdc-server-1")

To run the ab() query on hdcGraph:

exec{
  ab().src({_id == "A"}).dest({_id == "C"}).depth(2) as p
  return p
} on hdcGraph

Result: p

To run the khop() query on hdcGraph:

exec{
  khop().src({_id == "A"}).depth(1).direction(right) as n
  return count(n)
} on hdcGraph

Result:

Limited Graph Traversal Direction

If an HDC graph is created with the direction option set to in or out, graph traversal is restricted to incoming or outgoing edges, respectively. Queries attempting to traverse in the missing direction throws errors or yields empty results.

To create an HDC graph hdcGraph_in_edges of the graph with nodes and incoming edges:

hdc.graph.create("hdcGraph_in_edges", {
  nodes: {"*": ["*"]},
  edges: {"*": ["*"]},
  direction: "in",
  load_id: true,
  update: "static",
  query: "query",
  default: false
}).to("hdc-server-1")

This khop() query attempts to follow the outgoing (right) edges on hdcGraph_in_edges, no result will be returned.

exec{
  khop().src({_id == "A"}).depth(1).direction(right) as n
  return count(n)
} on hdcGraph_in_edges

Result:

Exclusion of Node IDs

If an HDC graph is created with the load_id option set to false, it does not contain the _id values for nodes. Queries referencing _id throws errors or yields empty results.

To create an HDC graph hdcGraph_no_id of the graph without nodes' _id values:

hdc.graph.create("hdcGraph_no_id", {
  nodes: {"*": ["*"]},
  edges: {"*": ["*"]},
  direction: "undirected",
  load_id: false,
  update: "static",
  query: "query",
  default: false
}).to("hdc-server-1")

The ab() query utilizes _id to filter nodes on hdcGraph_no_id, error occurs as the HDC graph lacks nodes' _id:

exec{
  ab().src({_id == "A"}).dest({_id == "C"}).depth(2) as p
  return p{*}
} on hdcGraph_no_id

Exclusion of Properties

If an HDC graph is created without certain properties, queries referencing those properties throws errors or yields empty results.

To create an HDC graph hdcGraph_no_weight of the graph containing nodes and only system properties of @link edges:

hdc.graph.create("hdcGraph_no_weight", {
  nodes: {"*": ["*"]},
  edges: {"link": []},
  direction: "undirected",
  load_id: true,
  update: "static",
  query: "query",
  default: false
}).to("hdc-server-1")

The ab() query finds shortest path between two nodes on hdcGraph_no_weight, weighted by the edge property @link.weight, error occurs as the weight property is missing:

exec{
  ab().src({_id == "A"}).dest({_id == "C"}).depth(2).shortest(@link.weight) as p
  return p
} on hdcGraph_no_weight

Unsupported Data Modifications

The following query results in an error indicating that node insertion is not supported on HDC graphs:

exec{
  insert().into(@entity).nodes([{_id: "E"}])
} on hdcGraph