Known limitations for AQL queries

The following hard-coded limitations exist for AQL queries:

• An AQL query cannot use more than 1000 result registers. One result register is needed for every named query variable and for internal/anonymous query variables, e.g. for intermediate results. Subqueries also require result registers.
• An AQL query cannot have more than 4000 execution nodes in its initial query execution plan. This number includes all execution nodes of the initial execution plan, even if some of them could be optimized away later by the query optimizer during plan optimization.
• An AQL query cannot use more than 2048 collections/shards.
• Expressions in AQL queries cannot have a nesting of more than 500 levels. As an example, the expression 1 + 2 + 3 + 4 is 3 levels deep (because it is interpreted and executed as 1 + (2 + (3 + 4))).
• When reading any data from JSON or VelocyPack input or when serializing any data to JSON or VelocyPack, there is a maximum recursion depth for nested arrays and objects, which is slightly below 200. Arrays or objects with higher nesting than this will cause Too deep nesting in Array/Object exceptions.

Please note that even queries that are still below these limits may not yield good performance, especially when they have to put together data from lots of different collections. Please also consider that large queries (in terms of intermediate result size or final result size) can use considerable amounts of memory and may hit the configurable memory limits for AQL queries.

The following other limitations are known for AQL queries:

• Subqueries that are used inside expressions are pulled out of these expressions and executed beforehand. That means that subqueries do not participate in lazy evaluation of operands, for example in the ternary operator. Also see evaluation of subqueries.
• It is not possible to use a collection in a read operation after it was used for a write operation in the same AQL query.
• In the cluster, all collections that are accessed dynamically by traversals working with collection sets (instead of named graphs) must be stated in the query’s initial WITH statement. To make the WITH statement required in single server as well (e.g. for testing a migration to cluster), please start the server with the option --query.require-with.