19_pattern_query

Example 19 — Pattern query

What it demonstrates

The fluent graph/query API: build a labelled property graph with a declared schema, freeze it into a CSR snapshot, then run MATCH-style pattern queries that combine label and property predicates and a one-hop expansion. For every matched node the example reads back the age and dept properties with lpg.Graph.GetNodeProperty, so the output shows not just which nodes matched but why they matched.

Domain / scenario

A tiny org graph of five people. Each Person node carries an age (int64) and a dept (string); some are also tagged Admin. Three directed edges model a "reports-to / follows" relation:

alice  Person Admin  age=30  dept=eng   -> bob, carol
bob    Person        age=25  dept=eng
carol  Person        age=30  dept=ops
dave   Person Admin  age=42  dept=eng   -> erin
erin   Person        age=28  dept=ops

The four queries each exercise a different combination of predicates:

• (n:Person:Admin) — two label predicates intersected (alice, dave).
• (n:Person) WHERE n.age = 30 — label plus an int64 property predicate (alice, carol).
• (n:Admin)-->(b) — one-hop expansion: the targets reachable in a single step from an Admin node (bob, carol, erin).
• (n:Person {dept: 'ops'}) — label plus a string property predicate (carol, erin).

Every result is sorted by node key, so the output is byte-stable regardless of the engine's internal iteration order.

How to run

go run ./examples/19_pattern_query

Expected output

MATCH (n:Person:Admin) RETURN n.name, n.age, n.dept
  - alice  age=30  dept=eng
  - dave   age=42  dept=eng

MATCH (n:Person) WHERE n.age = 30 RETURN n.name, n.age, n.dept
  - alice  age=30  dept=eng
  - carol  age=30  dept=ops

MATCH (n:Admin)-->(b) RETURN b.name, b.age, b.dept  (one hop out)
  - bob    age=25  dept=eng
  - carol  age=30  dept=ops
  - erin   age=28  dept=ops

MATCH (n:Person {dept: 'ops'}) RETURN n.name, n.age, n.dept
  - carol  age=30  dept=ops
  - erin   age=28  dept=ops

Key APIs

• graph/lpg.New — build the mutable labelled property graph.
• graph/lpg/schema.New / Schema.RegisterLabel / Schema.RegisterProperty — declare the Person/Admin labels and the age/dept property keys with their kinds.
• graph/lpg.Graph.SetNodeLabel / SetNodeProperty — populate labels and typed property values.
• graph/lpg.Graph.GetNodeProperty — read a matched node's age/dept back, with PropertyValue.Int64 / PropertyValue.String to unwrap the typed value.
• graph/csr.BuildFromAdjList — freeze the builder into an immutable CSR snapshot used as the query surface.
• graph/query.New / Engine.Match / Pattern.Vertex / Pattern.Out / Pattern.Collect — express and run the pattern queries.
• graph/query.WithLabel / WithProperty — the label and property predicates that seed each pattern.

Further reading

• graph/query — the fluent pattern-query package
• graph/lpg — the labelled property graph and its property model
• graph/lpg/schema — schema declaration for labels and property keys
• graph/csr — the immutable CSR snapshot used as the query surface
• Example 02 — property graph — introduces the labelled property graph model
• docs/examples-standard.md — the standard every example follows