Comparison of Relational Databases and Graph Databases

Question

Can someone explain to me the advantages and disadvantages for a relation database such as MySQL compared to a graph database such as Neo4j?

In SQL you have multipl

Answer 1

While the relational model can easily represent the data that is contained in a graph model, we face two significant problems in practice:

1. SQL lacks the syntax to easily perform graph traversal, especially traversals where the depth is unknown or unbounded. For instance, using SQL to determine friends of your friends is easy enough, but it is hard to solve the “degrees of separation” problem.
2. Performance degrades quickly as we traverse the graph. Each level of traversal adds significantly to query response time.

Reference: Next Generation Databases

Answer 2

Graph databases are worth investigating for the use cases that they excel in, but I have had some reason to question some assertions in the responses above. In particular:

A relational database is much faster when operating on huge numbers of records (dan1111's first bullet point)

Graph databases are much faster than relational databases for connected data - a strength of the underlying model. A consequence of this is that query latency in a graph database is proportional to how much of the graph you choose to explore in a query, and is not proportional to the amount of data stored, thus defusing the join bomb. (Jim Webber's first bullet point)

In other words the more complex our queries and relationships get the more we benefit from a graph versus a relational database. (Uli Bethke's 2nd paragraph)

While these assertions may well have merit, I have yet to find a way to get my specific use case to align with them. Reference: Graph Database or Relational Database Common Table Extensions: Comparing acyclic graph query performance

Answer 3

The key difference between a graph and relational database is that relational databases work with sets while graph databases work with paths.

This manifests itself in unexpected and unhelpful ways for a RDBMS user. For example when trying to emulate path operations (e.g. friends of friends) by recursively joining in a relational database, query latency grows unpredictably and massively as does memory usage, not to mention that it tortures SQL to express those kinds of operations. More data means slower in a set-based database, even if you can delay the pain through judicious indexing.

As Dan1111 hinted at, most graph databases don't suffer this kind of join pain because they express relationships at a fundamental level. That is, relationships physically exist on disk and they are named, directed, and can be themselves decorated with properties (this is called the property graph model, see: https://github.com/tinkerpop/blueprints/wiki/Property-Graph-Model). This means if you chose to, you could look at the relationships on disk and see how they "join" entities. Relationships are therefore first-class entities in a graph database and are semantically far stronger than those implied relationships reified at runtime in a relational store.

So why should you care? For two reasons:

1. Graph databases are much faster than relational databases for connected data - a strength of the underlying model. A consequence of this is that query latency in a graph database is proportional to how much of the graph you choose to explore in a query, and is not proportional to the amount of data stored, thus defusing the join bomb.
2. Graph databases make modelling and querying much more pleasant meaning faster development and fewer WTF moments. For example expressing friend-of-friend for a typical social network in Neo4j's Cypher query language is just MATCH (me)-[:FRIEND]->()-[:FRIEND]->(foaf) RETURN foaf.

Answer 4

Relational Databases are much more efficient in storing tabular data. Despite the word “relational” in their name, relational databases are much less effective at storing or expressing relationships between stored data elements. The term 'relational' in relational databases relates more to relating columns within a table, not relating information in different tables. Relationships between columns exist to support set operations. So as Database grows in millions or billions records it becomes extremely slow to retrieve data from relational databases.

Unlike a relational database, a graph database is structured entirely around data relationships. Graph databases treat relationships not as a schema structure but as data, like other values. It is very fast to retrieve data from graph databases. From a relational database standpoint, you could think of this as pre-materializing JOINs once at insertion time instead of computing them for every query. Because the data is structured entirely around data relationships, real-time query performance can be achieved no matter how large or connected the dataset gets. The graph databases take more storage space compared to relational database.

Answer 5

Dan1111 has already given an answer flagged as correct. A couple of additional points are worth noting in passing.

First, in almost every implementation of graph databases, the records are "pinned" because there are an unknown number of pointers pointing at the record in its current location. This means that a record cannot be shuffled to a new location without either leaving a forwarding address at the old location or breaking an unknown number of pointers.

Theoretically, one could shuffle all the records at once and figure out a way to locate and repair all the pointers. In practice this is an operation that could take weeks on a large graph database, during which time the database would have to be off the air. It's just not feasible.

By contrast, in a relational database, records can be reshuffled on a fairly large scale, and the only thing that has to be done is to rebuild any indexes that have been affected. This is a fairly large operation, but nowhere near as large as the equivalent for a graph database.

The second point worth noting in passing is that the world wide web can be seen as a gigantic graph database. Web pages contain hyperlinks, and hyperlinks reference, among other things, other web pages. The reference is via URLs, which function like pointers.

When a web page is moved to a different URL without leaving a forwarding address at the old URL, an unknown number of hyperlinks will become broken. These broken links then give rise to the dreaded, "Error 404: page not found" message that interrupts the pleasure of so many surfers.

Answer 6

There actually is conceptual reasoning behind both styles. Wikipedia on the relational model and graph databases gives good overviews of this.

The primary difference is that in a graph database, the relationships are stored at the individual record level, while in a relational database, the structure is defined at a higher level (the table definitions).

This has important ramifications:

• A relational database is much faster when operating on huge numbers of records. In a graph database, each record has to be examined individually during a query in order to determine the structure of the data, while this is known ahead of time in a relational database.
• Relational databases use less storage space, because they don't have to store all of those relationships.

Storing all of the relationships at the individual-record level only makes sense if there is going to be a lot of variation in the relationships; otherwise you are just duplicating the same things over and over. This means that graph databases are well-suited to irregular, complex structures. But in the real world, most databases require regular, relatively simple structures. This is why relational databases predominate.