Indexes and constraints

Indexes

Indexes speed up MATCH patterns that filter on a property. Without an index, gr scans every node with the given label. With an index, it narrows the search in O(log n).

Property index

CREATE INDEX FOR (n:Person) ON (n.name)

gr creates a range index that supports equality (= "Alice"), range (> 25, < 30), prefix (STARTS WITH "A"), and IN [...] predicates.

Named index

CREATE INDEX idx_person_name FOR (n:Person) ON (n.name)

Providing a name makes the index easier to reference in DROP INDEX and SHOW INDEXES.

Composite index

CREATE INDEX FOR (n:Person) ON (n.name, n.age)

A composite index covers queries that filter on both columns together, or on the leading column alone.

Relationship property index

CREATE INDEX FOR ()-[r:KNOWS]-() ON (r.since)

Indexes relationship properties the same way as node properties.

Drop an index

DROP INDEX idx_person_name
DROP INDEX IF EXISTS idx_person_name

Show indexes

SHOW INDEXES

Returns: name, type, labelsOrTypes, properties, state (ONLINE or POPULATING).

Indexes and MERGE

MERGE on an indexed property does a single index look-up instead of a full scan. Always create an index before running MERGE on a property that could match many nodes.

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Constraints

Constraints enforce invariants at write time. A write that violates a constraint returns *engine.ConstraintError and the transaction rolls back.

Uniqueness constraint

CREATE CONSTRAINT FOR (n:Person) REQUIRE n.email IS UNIQUE

Every Person node must have a distinct email value. Two Persons without an email property are allowed (nulls are not compared for uniqueness).

gr automatically creates a supporting index for the constrained property.

Node key constraint

A node key combines uniqueness and existence:

CREATE CONSTRAINT FOR (n:Person) REQUIRE (n.id) IS NODE KEY

Every Person node must have an id property, and no two Person nodes can share the same id.

Composite node key:

CREATE CONSTRAINT FOR (n:Person) REQUIRE (n.firstName, n.lastName) IS NODE KEY

Property existence constraint

CREATE CONSTRAINT FOR (n:Person) REQUIRE n.name IS NOT NULL

Every Person node must have a name property.

Relationship property existence

CREATE CONSTRAINT FOR ()-[r:KNOWS]-() REQUIRE r.since IS NOT NULL

Every KNOWS relationship must have a since property.

Named constraint

CREATE CONSTRAINT person_email_unique FOR (n:Person) REQUIRE n.email IS UNIQUE

Drop a constraint

DROP CONSTRAINT person_email_unique
DROP CONSTRAINT IF EXISTS person_email_unique

Show constraints

SHOW CONSTRAINTS

Returns: name, type, labelsOrTypes, properties.

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Handling constraint errors in Go

import (
    "errors"
    "github.com/tamnd/gr/engine"
)

_, err = db.Exec(ctx, `CREATE (:Person {email:"a@b.com"})`, nil)
var ce *engine.ConstraintError
if errors.As(err, &ce) {
    fmt.Printf("constraint %q violated: %v\n", ce.Constraint, ce.Message)
}

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Schema catalog

List all labels, relationship types, and property keys:

SHOW LABELS
SHOW RELATIONSHIP TYPES
SHOW PROPERTY KEYS

From the CLI:

gr run graph.gr "SHOW INDEXES"
gr run graph.gr "SHOW CONSTRAINTS"
gr run graph.gr "SHOW LABELS"

Inside the shell:

gr> :schema