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PGQL: a Property Graph Query Language Oskar van Rest Sungpack Hong Jinha Kim Oracle Labs Oracle Labs Oracle Labs [email protected] [email protected] [email protected] Xuming Meng Hassan Chafi Oracle Labs Oracle Labs [email protected] hassan.chafi@oracle.com ABSTRACT need has arisen for a well-designed query language for graphs that Graph-based approaches to data analysis have become more supports not only typical SQL-like queries over structured data, but widespread, which has given need for a query language for also graph-style queries for reachability analysis, path finding, and graphs. Such a graph query language needs not only SQL-like graph construction. It is important that such a query language is functionality for querying structured data, but also intrinsic general enough, but also has a right level between expressive power support for typical graph-style applications: reachability anal- and ability to process queries efficiently on large-scale data. ysis, path finding and graph construction. Various graph query languages have been proposed in the past. We propose a new query language for the popular Property Most notably, there is SPARQL, which is the standard query lan- Graph (PG) data model: the Property Graph Query Language guage for the RDF (Resource Description Framework) graph data (PGQL). PGQL is based on the paradigm of graph pattern model, and which has been adopted by several graph databases [1, matching, closely follows syntactic structures of SQL, and pro- 7, 12]. However, the RDF data model regularizes the graph repre- vides regular path queries with conditions on labels and prop- sentation as set of triples (or edges), which means that even con- erties to allow for reachability and path finding queries. Be- stant literals are encoded as graph vertices. Such artificial vertices sides intrinsic vertex, edge and path types, PGQL also has the make it hard to express graph queries in a natural way. A more nat- graph as intrinsic type and allows for graph construction and ural data model is the Property Graph (PG) data model, in which query composition. vertices and edges in a graph can be associated with arbitrary prop- erties as key-value pairs. The PG data model has been adopted by various graph databases [5, 8, 11, 4] and graph processing frame- CCS Concepts works [6, 2], and has formed the basis of recently proposed lan- •Information systems ! Query languages for non-relational guages for graph algorithms [22, 10, 23] and graph queries [3, 21]. engines; A notable query language for the PG data model is Cypher [3], which is, just like SPARQL, based on graph pattern matching, an Keywords elegant way of defining patterns in graphs. However, Cypher is missing some fundamental graph querying functionalities, namely, Graph query languages, graph-structured data, property graphs regular path queries and graph construction. To overcome the limitations of existing graph query languages, 1. INTRODUCTION we designed a new query language for the PG data model: the In recent years, both the data management community and the Property Graph Query Language (PGQL). PGQL combines graph data mining community have been paying a lot of attention to the pattern matching with SQL-like syntax and functionality and has graph-based approaches in which graphs are used as fundamental full-blown support for regular path queries and graph construction. representation for data analysis. The graph data model allows for Because its syntax is SQL-like, the language is intuitive to use for promising analyses over topology, in addition to analyses over data existing SQL users. Furthermore, PGQL queries return a “result stored in topology, which can be performed by standard relational set” with variables and bindings, just like in SQL. This means that mechanisms. queries can be naturally nested inside SQL queries. In PGQL, a Essential for typical data analyses is a query language to query graph pattern is not only composed of vertices and edges, but also data in a high-level and productive manner. SQL, having been of paths, such that users can perform reachability analyses or find widely adopted as the query language for relational databases, il- paths in a graph. Supported is the well-studied class of regular path lustrates very well the importance of query languages in data man- queries, but not just with conditions on labels of edges, but with agement. Now graph technology has become more widespread, the conditions on labels as well as properties of vertices and edges along paths. PGQL also has an intrinsic graph type to support Permission to make digital or hard copies of all or part of this work for graph transformation applications, allowing one or more graphs to personal or classroom use is granted without fee provided that copies are be constructed and returned from a query. not made or distributed for profit or commercial advantage and that copies Specifically, our contributions are as follows: (i) the design of bear this notice and the full citation on the first page. To copy otherwise, to PGQL, an intuitive high-level pattern-matching query language for republish, to post on servers or to redistribute to lists, requires prior special the PG data model, which closely follows SQL’s syntactic struc- permission and/or a fee. tures and provides SQL-like functionality as part of the solution Proceedings of the Fourth International Workshop on Graph Data Manage- (ii) ment Experience and Systems (GRADES 2016), June 24, 2016, Redwood (Section 3), the design of a syntax for regular path queries for Shores, USA PG graphs with conditions on labels and properties of vertices and Copyright 2016 ACM XXXXXXXXXXXX ...$15.00. edges along paths (Section 4), (iii) an elegant solution to graph 1 SELECT friend.name, friend.age construction inside a query language, allowing for graphs to be re- 2 //'Andy' 12(query solutions in comment) 3 FROM snGraph turned from a query and for graph queries to be composed (Sec- 4 WHERE tion 5). 5 (x WITH name = 'Paul') -[:likes]-> (friend), 6 (y WITH name = 'Amber') -/:likes*1..2/-> (friend), 7 x.age >= 2 * friend.age 2. RELATED WORK 8 ORDER BY friend.name There are two types of languages for graphs. First, there are the Figure 1: Example PGQL query, returning the friends of Paul that graph pattern-matching query languages that are similar to PGQL. are also friends, or friends of friends, of Amber. Only friends that Most notably, SPARQL [9] is the standard query language for the are at least twice as young are returned. RDF model. While SPARQL supports reachability by means of regular path queries (RPQs), actual paths that satisfy an RPQ can- not be returned from a query. Furthermore, conditions that compare 3. PATTERN MATCHING QUERIES vertices and edges along paths cannot be specified. Earlier propos- PGQL is based on the paradigm of graph pattern matching, which als [17, 16, 19, 27], which were also based on a labeled graph, is an elegant way of defining pattern in graphs (Section 3.1). It suffered from similar limitations. Cypher [3], the query language combines this with SQL-like syntax and also provides SQL-like of the graph database Neo4j, was the first pattern-matching query functionality as part of the solution (Section 3.2). language to target the PG model. While it has an intrinsic path type, path query support is based on finding all simple trails (i.e. paths 3.1 Graph Pattern Matching with non-repeating edges). However, evaluation of such queries A PGQL query is composed of the three clauses SELECT, FROM often requires iteration over the entire set of trails, which is an op- and WHERE, which are followed by optional solution modifier clauses eration that is exponential in the size of the graph and therefore such as ORDER BY, GROUP BY, and LIMIT. The FROM clause can also unsuitable for large graphs. Furthermore, Cypher does not have be omitted when there is only one graph instance. Additionally, an intrinsic graph type to support graph construction applications. PGQL includes special operators for graph pattern matching: ver- GraphQL [21] is a pattern-matching query language based on a PG- tex matching, edge matching and path matching. These matching like model. It supports recursion in such a way that not only paths operators are placed in the WHERE clause along with predicates over but even entire graph patterns can be the unit of repetition. How- vertices and edges. ever, the output of a GraphQL query is a set of graphs rather than Figure 1 shows an example PGQL query which finds patterns a result set with variables and bindings. In practice we are often from a data graph named snGraph (line 3). In the WHERE clause, interested in the data (i.e. properties) inside vertices and edges and the query matches a vertex x that has a property name with value such a lanugage does not allow you to extract such information. ’Paul’. The vertex x has an edge with the label ’likes’. The PQL [24] is a pattern-matching query language for biological net- destination vertex of this edge is referred to as vertex friend (line works with SQL-like syntax. Its WHERE clause supports reachability 5). Similarly, the query matches another vertex y with its name be- patterns, while actual paths are constructed in the SELECT clause ing ’Amber’. The vertex y is also connected to the vertex friend using pairs of bound variables from the WHERE clause. but through a path; the path is composed only of edges with label Second, there are the more general purpose imperative graph ’likes’ and the (hop-)length of the path is between 1 and 2 in- languages.
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