Arangodb document arrays vs key/value collection

Question

Are there limits to how many array values can be in a document other than document size? Arangodb can index into the arrays since version 2.8 so that's not a reason to go to a key/value collection format.

Eg group document with member array:

{'_key': group1, members: [1, 2, 3, ...]} 

Is there a limit to how large the array members can be? Is it better to break this out in a key/value {group: group1, member: 1} collection for performance reasons?

Answer 1

There is no artificial limit in place for the number of array values or object keys in ArangoDB.

However, there are a few practical limits that you may want to consider:

• the more array/object members you use in a document, the bigger the document will grow byte-wise. The performance of reading and writing individual documents obviously depends on the document size, so the bigger the documents are, the slower this will get and the more memory each individual document will consume during querying. This will especially hurt with the RocksDB storage engine, as due to the level design of RocksDB each document revision may need to be shoved through the various levels of the LSM tree and thus needs to be copied/written several times.
• searching for specifying object keys inside documents normally uses a binary search, so its performance degrades logarithmically with the number of object keys. However, the performance of the full iteration of all object keys or all array values will grow linearly with the number of members.
• when using huge documents from ArangoDB's JavaScript functionality, eg when using ArangoDB's Foxx microservice framework, the documents need to be converted to plain JavaScript objects & arrays. The V8 JavaScript implementation that is used by ArangoDB should behave well for small and medium-sized objects/arrays, but it has its problems with huge values. Apart from that it may also limit the number of object keys/array members internally.
• peeking into the middle of an array from an AQL query will normally not use any index. The same is true when querying for arbitrary object keys. For object keys there is the possibility to create an index on dedicated keys, but obviously the keys need to be known in advance.

All that said, you may still want to make sure that objects/arrays do no get excessively big, because otherwise performance and memory usage may degrade.