Add limits for AQL query complexity (#726)

Author: jsteemann

3.9/aql/extending-conventions.md

@@ -37,7 +37,7 @@ purely functional and thus free of side effects and state, and state modificatio
 
 {% hint 'warning' %}
 Modification of global variables is unsupported, as is reading or changing
-the data of any collection from inside an AQL user function.
+the data of any collection or running queries from inside an AQL user function.
 {% endhint %}
 
 User function code is late-bound, and may thus not rely on any variables

3.9/aql/extending.md

@@ -59,6 +59,10 @@ To overcome these mentioned limitations, you may want to increase the
 (at the expense of increased memory usage), and the
 [number of available server threads](../programs-arangod-server.html#server-threads).
 
+In addition, modification of global JavaScript variables from inside UDFs is 
+unsupported, as is reading or changing the data of any collection or running
+queries from inside an AQL user function.
+
 ### Deployment Details
 
 Internally, UDFs are stored in a system collection named `_aqlfunctions`

3.9/aql/fundamentals-limitations.md

@@ -5,11 +5,13 @@ description: Known limitations that apply in AQL queries
 Known limitations for AQL queries
 =================================
 
-The following limitations are known 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.
+- An AQL query cannot have more than _4000_ execution nodes in its initial 
+  query execution plan.
 - An AQL query cannot use more than _2048_ collections/shards.
 - 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.
@@ -18,6 +20,18 @@ The following limitations are known for AQL queries:
   [`DOCUMENT()` function](functions-miscellaneous.html#document) or
   [traversals working with collection sets](graphs-traversals.html#working-with-collection-sets)
   (instead of named graphs).
+- 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))`).
+
+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

3.9/release-notes-upgrading-changes39.md

@@ -19,6 +19,26 @@ now expected to be present on all targets that run ArangoDB 3.9 executables.
 If a target does not support AVX instructions, it may fail with SIGILL at
 runtime.
 
+AQL
+---
+
+The following complexity limits have been added in 3.9 for AQL queries, 
+Additional complexity limits have been added for AQL queries, in order to 
+prevent programmatically generated large queries from causing trouble 
+(too deep recursion, enormous memory usage, long query optimization 
+and distribution passes etc.).
+
+The following limits have been added:
+
+- a recursion limit for AQL query expressions. An expression can now be
+  up to 500 levels deep. An example expression is `1 + 2 + 3 + 4`, which
+  is 3 levels deep `1 + (2 + (3 + 4))`.
+  The recursion of expressions is limited to 500 levels.
+- a limit for the number of execution nodes in the initial query 
+  execution plan. The number of execution nodes is limited to 4,000.
+
+Also see [Known limitations for AQL queries](aql/fundamentals-limitations.html).
+
 Startup options
 ---------------
 

3.9/release-notes.md

@@ -38,7 +38,7 @@ For a detailed list of changes to the ArangoDB core programs and tools,
 please refer to the version specific changelogs:
 
 - Changelogs 3.x:
-  [3.8](https://raw.githubusercontent.com/arangodb/arangodb/3.9/CHANGELOG){:target="_blank"},
+  [3.9](https://raw.githubusercontent.com/arangodb/arangodb/3.9/CHANGELOG){:target="_blank"},
   [3.8](https://raw.githubusercontent.com/arangodb/arangodb/3.8/CHANGELOG){:target="_blank"},
   [3.7](https://raw.githubusercontent.com/arangodb/arangodb/3.7/CHANGELOG){:target="_blank"},
   [3.6](https://raw.githubusercontent.com/arangodb/arangodb/3.6/CHANGELOG){:target="_blank"},
@@ -72,7 +72,7 @@ Also see [Version Specific Upgrade Information](upgrading-version-specific.html)
 in the _Upgrading_ chapter.
 
 - Incompatible changes in 3.x:
-  [3.8](release-notes-upgrading-changes39.html),
+  [3.9](release-notes-upgrading-changes39.html),
   [3.8](release-notes-upgrading-changes38.html),
   [3.7](release-notes-upgrading-changes37.html),
   [3.6](release-notes-upgrading-changes36.html),