A few weeks ago, Emil Eifrem, CEO of Neo Technology gave a webinar introduction to graph databases. I watched it as a lead up to my own presentation on graph databases and Neo4j.
Right around the 54 minute mark, Emil talks about a very interesting experiment showing the performance difference between a relational database and a graph database for a certain type of problem called "arbitrary path query", specifically, given 1,000 users with an average of 50 "friend" relationships each, determine if one person is connected to another in 4 or fewer hops.
Against a popular open-source relational database, the query took around 2,000 ms. For a graph database, the same determination took 2 ms. So the graph database was 1,000 times faster for this particular use case.
Not satisfied with that, they then decided to run the same experiment with 1,000,000 users. The graph database took 2 ms. They stopped the relational database after several days of waiting for results.
I showed this clip at my presentation to a few people who stuck around afterwards, and we tried to figure out why the graph database had the same performance with 1,000 times the data, while the relational database became unusably slow. The answer has to do with the way in which each type of database searches information.
Relational databases search all of the data looking for anything that meets the search criteria. The larger the set of data, the longer it takes to find matches, because the database has to examine everything in the collection.
Here's an example: let's assume there is a table with "friend" relationships:
> SELECT * FROM friends; +-------------+--------------+ | user_id | friend_id | +-------------+--------------+ | 1 | 2 | | 1 | 3 | | 1 | 4 | | 2 | 5 | | 2 | 6 | | 2 | 7 | | 3 | 8 | | 3 | 9 | | 3 | 10 | +-------------+--------------+In order to see if user "9" is connected to user "2", the database has to find all the friends of user "9", and see if user "2" is in that list. If not, find all of their friends, and then see if user "2" is in that list. The database has to scan the entire table each time. This means that if you double the number of rows in the table, you've doubled the amount of data to search, and thus doubled the amount of time it takes to find what you are looking for. (Even with indexing, it still has to find the values in the index tree, which involves traversing that tree. The index tree grows larger with each new record, meaning the time it takes to traverse grows larger as well. And for each search, you always start at the root of the tree.)
Each new batch of friends to look at requires an entirely new scan of the table/index. So more records leads to more search time.
Conversely, a graph database looks only at records that are directly connected to other records. If it is given a limit on how many "hops" it is allowed to make, it can ignore everything more than that number of hops away.
You could add another ring of records around the outside, or add a thousand more rings of records outside, but the search would never need to look at them because they are more steps away than the limit. The only way to increase the number of records searched (and thereby decrease performance) is to add more records within the 2-step limit, or add more relationships between the existing records.
So the reason why having 1,000 vs. 1,000,000 records causes such a stark difference between a relational and a graph database is that relational database performance decreases in relation to the number of records in the table, while graph database performance decreases in relation to the number of connections between the records.