PGQL: a Property Graph Query Language

PGQL is an SQL-like query language for the property graph data model. See:

• PGQL website: http://pgql-lang.org/
• Specification: http://pgql-lang.org/spec/1.2/.

PGQL Parser and Static Query Validator

The 'master' branch of this repository contains a parser for PGQL with the following features:

• Easy-to-understand IR: Given a query string, the parser returns an easy-to-understand intermediate representation (IR) of the query as a set of Java objects

  • see GraphQuery.java

• Query validation: built-in to the parser is a static query validator that provides meaningful caret-style (e.g. ^^^) error messages:

  Example 1

  SELECT n.name, o.name
    FROM g
   MATCH (n) -[e]-> (m)

  Error(s) in line 1:
  
      SELECT n.name, o.name
                     ^
      Unresolved variable
  

  Example 2

  SELECT AVG(n.age), n
    FROM g
   MATCH (n:Person)

  Error(s) in line 1:
  
      SELECT AVG(n.age), n
                         ^
      Aggregation expected here since SELECT has other aggregation
  

• Pretty printing: invoking GraphQuery.toString() will "pretty print" the graph query, which turns unformatted queries into formatted ones:

  SELECT n.name FROM g MATCH
  (n:Person) WHERE n.name = 'Anthony'
  OR n.name = 'James'

  SELECT n.name
    FROM g MATCH (n:person)
   WHERE n.name = 'Anthony'
      OR n.name = 'James'

• Code completion: given a (partial) query string and a cursor position, the parser can suggest a set of code completions, including built-in functions, labels and properties. These completions can be used in e.g. a web editor. By providing the parser with metadata about the graph (existing properties and labels), the completions will also include label and property suggestions.

Build and Install the Parser

PGQL's parser can be built on Linux, macOS or Window.

First install JDK 1.8 or higher and Maven 3.5.4 or higher. Then, follow these instructions:

On Linux or macOS:

• Build and install to your local Maven repository by running sh install.sh

On Windows:

• Open System Properties and add a new variable JAVA_OPTS with value -Xms512m -Xmx1024m -Xss16m
• Build and install to your local Maven repository by running install_on_windows.bat

Getting Started

After you have installed the parser like explained above, parse two example queries:

• On Linux or macOS execute cd example; sh run.sh
• On Windows execute:

  cd example
  mvn clean package exec:java -Dexec.mainClass="oracle.pgql.lang.example.Main" -Dexec.cleanupDaemonThreads=false
  

public class Main {

  public static void main(String[] args) throws PgqlException {

    try (Pgql pgql = new Pgql()) {

      // parse query and print graph query
      PgqlResult result1 = pgql.parse("SELECT n FROM g MATCH (n:Person) -[e:likes]-> (m:Person) WHERE n.name = 'Dave'");
      System.out.println(result1.getGraphQuery());

      // parse query with errors and print error messages
      PgqlResult result2 = pgql.parse("SELECT x, y, FROM g MATCH (n) -[e]-> (m)");
      System.out.println(result2.getErrorMessages());

    }
  }
}

The AST returned by the parser is a GraphQuery object. This would be the input to your query planner.

Documentation

See PGQL 1.2 Specification.

Development

• Download Eclipse with Spoofax 2.5.1 pre-installed here
• Import the following projects into Eclipse (File>Import...>Maven>Existing Maven Projects>Browse...):
  • graph-query-ir: Java representation of graph queries
  • pqgl-spoofax: Spoofax implementation of PGQL (parser + error checks)
  • pgql-lang: translation of Spoofax AST into graph-query-ir
• Source code for PGQL's website can be found on the gh-pages branch of this repository

Contributions

PGQL is an open source project. See Contributing for details.

Oracle gratefully acknowledges the contributions to PGQL made by the community.