What is the time complexity of search query in Graph database (especially Neo4j) ?
I'm having relational data with me. I'm confused to use a Relational database or Graph database to store that data. So, I want to store the data based on the performance and time complexity of the queries for that particular database. But, I'm unable to find the performance and time complexity of the queries for Graph database.
Can anyone help me out ?
The answer isn't so simple because the time complexities typically depend upon what you're doing in the query (the results of the query planner), there isn't a one-size-fits-all time complexity for all queries.
I can speak some for Neo4j (disclaimer: I am a Neo4j employee).
I won't say much on Lucene index lookups in Neo4j, as these are typically performed only to find starting nodes by index, and represent a fraction of execution time of a query. Relationship traversals tend to be where the real differences show themselves.
Once starting nodes are found, Neo4j walks the graph through relationship traversal, which for Neo4j is basically pointer chasing through memory, which tends to be where native graph dbs outperform relational dbs: The cost of chasing pointers is constant per traversal.
For relational dbs (including graph layers built on top of relational dbs), relationship traversal is usually accomplished by various table join algorithms, which have their own time complexity, typically O(log n) when the join is backed by a B-tree index. This can be quite efficient, but we are in the age of big data and data lakes, so data is getting larger, and the efficiency of the join does grow based on the data in the tables being joined. And this is fine for smaller numbers of joins, but there are some queries that require many joins (sometimes we won't have an upper bound on when to stop joining), and we may want to traverse through many kinds of tables/nodes (and sometimes we may not care what kind they are), so we may need the capability to join to or through any table arbitrarily, which isn't easily expressed or performed in a relational db.
This makes native graph databases like Neo4j well-positioned for handling queries over connected data, especially with a non-trivial or growing number of relationship traversals (or if the traversals are unbounded, such as for reachabilty queries, shortest-path, and others). The cost for queries is associated with the part of the graph touched or walked by the query, not the total number of nodes in the database, so it helps when you can adequately constrain the query to touch the smallest possible subgraph in the db.
As far as your question of whether to use a relational or graph database, that depends upon the connectedness of your data and the queries you plan to run.
If you do decide upon a graph database, then you have choices here as well, and a different set of criteria, such as native vs non-native implementation (Neo4j is a native graph database and takes advantage of index-free adjacency for relationship traversals), whether you need ACID (Neo4 is an ACID database), and if a rich and expressive query language is desired (Cypher is Neo4j's query language, feel free to learn and compare against others).
For more in-depth info, here's a DZone article on Why Graph Databases Outperfrom RDBMS on Connected Data, and an article on Explaining Graph Databases to a Developer by Neo4j's Chief Scientist Jim Webber.
Actually, the most probable scenario is to use both Neo4j and some DBMS (relational or nosql like Mongo). Because it is too hard to store all dataset in Neo4j.
Speed-wise traversing nodes in DBMS is 10-100++ times slower than Neo4j. Also Neo4j has built-in shortestPath (and many other) methods.
Also, can mention hybrid solutions, like ArangoDB. It has graph engine + document-based engine. But under the hood it is two separate tables, so it is almost as inconvenient as Neo4j+DBMS.
It would actually depend upon the size of your data and the complexity.
In Graph databases like neo4j the time complexity is depends on the kind of query and on the planners(executors) behind the query. Graph database perticularly Neo4j performs easier JOINS which give us a clear view of data.
For more information please visit this reference blog by Neo4j.
And you can also refer to this question as it is similar to yours.
Hope this helps!
Related
In the book Neo4j in Action by Aleksa Vukotic and Nicki Watt, the authors say:
In our experience, it is less common for relationship indexes to be good solutions. We are not saying that relationship indexing is poor practice, but if you find yourself adding lots of relationship indexes, it is worth asking why.
It sounds that the authors do not recommend to index relationship in a graph database but no explanation is given thereafter. Does anyone know why?
I've voted for this question to be migrated to SO, and answering it while hoping it to be really migrated. I used Neo4j a couple of years. Although it has changed a lot since then, the principles of being a graph database won't alter much I believe. In my opinion, if you need a lot of indices to promptly query the relationships between the nodes, you could have designed your data model in some other way such that it focuses more on the graph nodes (just for example, relationships being your nodes, and nodes being your relationships as in line graph); because the querying mechanism (e.g. Cypher query) is generally optimised for the nodes.
First, it's important to understand the role of indexes in Neo4j, in that indexes are used to find starting points in the graph, after which relationship traversal and filtering are used to perform the remainder of the pattern matching and to complete the query.
The advice therefore is about the same as: "we do not recommend using relationships as starting points in the graph", and we find that true more often than not.
Usually when you need to do index lookups, you have certain "things" in mind as your starting places, and important things in graphs are typically represented by nodes. If we're asking "what employees are connected to this particular company" we're interested in starting quickly by finding that particular company and expanding out, not in finding all :EMPLOYED_BY relationships in the graph and filtering by the connected company, which would take far more time.
Often we find that those who encounter this restriction, and need this kind of fast lookup of relationships anyway, may need to rethink their model. Often when there is a need to lookup relationships as starting places in the graph, it is an indication that the thing represented by a relationship is important enough that it really should be a node in the graph (with its own relationships to the previously connected nodes), so this becomes a "modeling smell" that drives refactoring changes to the model. Often this kind of change feels more natural after, and affords more capability for the thing as a node that wasn't available when it was being modeled as a relationship (for example, the ability to apply multiple labels to it, or to connect it via relationships to more nodes than just the original two).
All that said, there will be cases where a relationship really does just need to be a relationship (either for business reasons, or because it truly is most practical modeling-wise for it to be kept as a relationship), and using those relationships as starting points in the graph make sense.
With the fulltext schema indexes introduced in Neo4j 3.5, we added the capability to add relationship indexes by relationship type(s) and property(or properties). So the capability is there, if needed, after you've ruled out refactoring of your model.
Most of the reasons for using a graph database seem to be that relational databases are slow when making graph like queries.
However, if I am using GraphQL with a data loader, all my queries are flattened and combined using the data loader, so you end up making simpler SELECT * FROM X type queries instead of doing any heavy joins. I might even be using a No-SQL database which is usually pretty fast at these kinds of flat queries.
If this is the case, is there a use case for Graph databases anymore when combined with GraphQL? Neo4j seems to be promoting GraphQL. I'd like to understand the advantages if any.
GraphQL doesn't negate the need for graph databases at all, the connection is very powerful and makes GraphQL more performant and powerful.
You mentioned:
However, if I am using GraphQL with a data loader, all my queries are flattened and combined using the data loader, so you end up making simpler SELECT * FROM X type queries instead of doing any heavy joins.
This is a curious point, because if you do a lot of SELECT * FROM X and the data is connected by a graph loader, you're still doing the joins, you're just doing them in software outside of the database, at another layer, by another means. If even that software layer isn't joining anything, then what you gain by not doing joins in the database you're losing by executing many queries against the database, plus the overhead of the additional layer. Look into the performance profile of sequencing a series of those individual "easy selects". By not doing those joins, you may have lost 30 years value of computer science research...rather than letting the RDMBS optimize the query execution path, the software layer above it is forcing a particular path by choosing which selects to execute in which order, at which time.
It stands to reason that if you don't have to go through any layer of formalism transformation (relational -> graph) you're going to be in a better position. Because that formalism translation is a cost you must pay every time, every query, no exceptions. This is sort of equivalent to the obvious observation that XML databases are going to be better at executing XPath expressions than relational databases that have some XPath abstraction on top. The computer science of this is straightforward; purpose-built data structures for the task typically outperform generic data structures adapted to a new task.
I recommend Jim Webber's article on the motivations for a native graph database if you want to go deeper on why the storage format and query processing approach matters.
What if it's not a native graph database? If you have a graph abstraction on top of an RDBMS, and then you use GraphQL to do graph queries against that, then you've shifted where and how the graph traversal happens, but you still can't get around the fact that the underlying data structure (tables) isn't optimized for that, and you're incurring extra overhead in translation.
So for all of these reasons, a native graph database + GraphQL is going to be the most performant option, and as a result I'd conclude that GraphQL doesn't make graph databases unnecessary, it's the opposite, it shows where they shine.
They're like chocolate and peanut butter. Both great, but really fantastic together. :)
Yes GraphQL allows you to make some kind of graph queries, you can start from one entity, and then explore its neighborhood, and so on.
But, if you need performances in graph queries, you need to have a native graph database.
With GraphQL you give a lot of power to the end-user. He can make a deep GraphQL query.
If you have an SQL database, you will have two choices:
to compute a big SQL query with a lot of joins (really bad idea)
make a lot of SQL queries to retrieve the neighborhood of the neighborhood, ...
If you have a native graph database, it will be just one query with good performance! It's a graph traversal, and native graph database are made for this.
Moreover, if you use GraphQL, you consider your data model as a graph. So to store it as graph seems obvious and gives you less headache :)
I recommend you to read this post: The Motivation for Native Graph Databases
Answer for Graph Loader
With Graph loader you will do a lot of small queries (it's the second choice on my above answer) but wait no, ... there is a cache record.
Graph loaders just do batch and cache.
For comparaison:
you need to add another library and implement the logic (more code)
you need to manage the cache. There is a lot of documentation about this topic. (more memory and complexity)
due to SELECT * in loaders, you will always get more data than needed Example: I only want the id and name of a user not his email, birthday, ... (less performant)
...
The answer from FrobberOfBits is very good. There are many reasons to add (or avoid) using GraphQL, whether or not a graph database is involved. I wanted to add a small consideration against putting GraphQL in front of a graph. Of course, this is just one of what ought to be many other considerations involved with making a decision.
If the starting point is a relational database, then GraphQL (in front of that datbase) can provide a lot of flexibility to the caller – great for apps, clients, etc. to interact with data. But in order to do that, GraphQL needs to be aligned closely with the database behind it, and specifically the database schema. The database schema is sort of "projected out" to apps, clients, etc. in GraphQL.
However, if the starting point is a native graph database (Neo4j, etc.) there's a world of schema flexibility available to you because it's a graph. No more database migrations, schema updates, etc. If you have new things to model in the data, just go ahead and do it. This is a really, really powerful aspect of graphs. If you were to put GraphQL in front of a graph database, you also introduce the schema concept – GraphQL needs to be shown what is / isn't allowed in the data. While your graph database would allow you to continue evolving your data model as product needs change and evolve, your GraphQL interactions would need to be updated along the way to "know" about what new things are possible. So there's a cost of less flexibility, and something else to maintain over time.
It might be great to use a graph + GraphQL, or it might be great to just use a graph by itself. Of course, like all things, this is a question of trade-offs.
We have a system that is a hybrid of Neo and Postgres. We tried to store data using the technology that was the most appropriate.
Our Users table ended up in Postgres, as did the supporting RBAC-related tables.
Users can be associated with certain Neo nodes. When we want to know about a node's user, our models have to fetch from Postgres - there is no Neo query that can fetch the User, of course.
This made perfect sense to us when we did it. Now we have 6 months of Neo under our belts and I am getting the idea we made a mistake.
I remember us saying, "There could be a thousand users!" It never occurred to us at the time that we'd be managing millions of Neo nodes... but it's coming. A thousand of anything isn't a problem.
With more understanding, it's clear to me that the user/RBAC was a Neo slam-dunk.
Please offer me some guidance on when to use a relational database versus Neo.
This is a very generic question. As you already pointed out it really depends on your use-cases and context. In general both databases are general purpose but shine for certain applications.
All JOIN heavy, tree, graph, path-matching and schema-free requirements will be easier and faster with Neo4j.
I wouldn't use Neo4j for:
binary data
very high write volumes (> 100k to 1M updates / s)
many global number crunching queries in real-time
Disclaimer: I work for Neo4j :)
Neo4j is a great tool for mapping relational data, but I am curious what under what conditions it would not be a good tool to use.
In which use cases would using neo4j be a bad idea?
You might want to check out this slide deck and in particular slides 18-22.
Your question could have a lot of details to it, but let me try to focus on the big pieces. Graph databases are naturally indexed by relationships. So graph databases will be good when you need to traverse a lot of relationships. Graphs themselves are very flexible, so they'll be good when the inter-connections between your data need to change from time to time, or when the data about your core objects that's important to store needs to change. Graphs are a very natural method of modeling some (but not all) data sources, things like peer to peer networks, road maps, organizational structures, etc.
Graphs tend to not be good at managing huge lists of things. For example, if you were going to build a customer transaction database with analytics (where you need 1 million customers, 50 million transactions, and all you do is post transactions all day long) then it's probably not a good fit. RDBMS is great at that, notice how that use case doesn't exploit relationships really.
Make sure to read those two links I provided, they have much more discussion.
For maintenance reasons, any service aggregating data feeds has until now been well advised to keep their sources independent.
If I want to explore relationships between different feeds, this can be done at application level, using data tracking (for example) user preferences amongst the other feeds.
Graph databases are about managing relationship complexity, but this complexity is in many cases a design choice. Putting all your kids in one bathtub is fine until you drop the soap..
I´ve been looking for a triple store for my project. In this project i want to store my data according to certain ontologies (OWL).
From my research i ended up with two tecnologies Neo4J and BigData that seems to fit well in this case.
I want to know if any of this two is more apropriated to use with RDF, RDFS, OWL and SPARQL Queries.
Neo4j can be used to store as entity-relationship-entity form. In case of Bigdata, you should not be upload your whole data into Neo4j because it will become very heavy and process will be very much slow. You should use complimentary db for storing actual data and store ids and some params into Neo4j for Graph traversal to perform sort of Graph Analytics. Neo4j is mainly build up for Graph Analytics that its power or you have to use Graph engine e.g GraphX (Spark).
Thanks,
You might want to try out the SparQL plugin for Neo4j, see here for a HTTP based test, and this Berlin Dataset Test for embedded usage.
Neo4J is a specific technology, while big data is more a generic term. I think what you're asking about OLAP and OLTP. As data gets bigger, there are differences between use cases for RDF style graph databases, which are often used for OLAP (On-line Analytical Processing) style analytics. In short, OLAP is designed for analytics that look across an big data set, while OLTP is more aimed at INSERT/DELETEs (on potentially big data).
OLAP-based traversals tend to process the entire graph, while OLTP based traversals tend to process smaller data sets by starting with one or a handful of vertices and traversing from there.
For example, let’s say you wanted to calculate the average age of friends of one particular user. Great use case for OLTP, since the query data set is small. However, if you wanted to calculate the average age of everyone on the database, OLAP is the preferred technology.
OLAP is optimal for deep analysis of a lot of data, while OLTP is better suited for fast running queries and a lot of INSERTs. If you’re trying to achieve a SLA where the analytics must complete within a certain timeframe, consider the type of analytics and which one is better suited. Or maybe you need both.