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Explain AQL Queries

You can explain and profile AQL queries to inspect the execution plans and to understand the performance characteristics, as well as create debug packages for reporting issues

If it is unclear how a given query will perform, clients can retrieve a query’s execution plan from the AQL query optimizer without actually executing the query. Getting the query execution plan from the optimizer is called explaining.

An explain throws an error if the given query is syntactically invalid. Otherwise, it returns the execution plan and some information about what optimizations could be applied to the query. The query is not executed.

You can explain a query using the HTTP REST API or via arangosh.

Inspecting query plans

The explain() method of an ArangoStatement (db._createStatement(...).explain()) creates very verbose output. To get a human-readable output of the query plan, you can use db._explain(). You can use it as follows (without disabling syntax highlighting with { colors: false }):

db._explain("LET s = SLEEP(0.25) LET t = SLEEP(0.5) RETURN 1", {}, {colors: false});
Show output

The plan contains all execution nodes that are used during a query. These nodes represent different stages in a query. Each stage gets the input from the stage directly above (its dependencies). The plan shows you the estimated number of items (results) for each query stage (under Est.). Each query stage roughly equates to a line in your original query, which you can see under Comment.

Profiling queries

Sometimes when you have a complex query it can be unclear on what time is spent during the execution, even for intermediate ArangoDB users.

By profiling a query it gets executed with special instrumentation code enabled. It gives you all the usual information like when explaining a query, but additionally you get the query profile, runtime statistics and per-node statistics.

To use this in an interactive fashion in the shell, you can call db._profileQuery(), or use the web interface. You can use db._profileQuery() as follows (without disabling syntax highlighting with { colors: false }):

db._profileQuery("LET s = SLEEP(0.25) LET t = SLEEP(0.5) RETURN 1", {}, {colors: false});
Show output

For more information, see Profiling Queries.

Execution plans in detail

By default, the query optimizer returns what it considers to be the optimal plan. The optimal plan is returned in the plan attribute of the result. If explain is called with the allPlans option set to true, all plans are returned in the plans attribute.

The result object also contains a warnings attribute, which is an array of warnings that occurred during optimization or execution plan creation.

Each plan in the result is an object with the following attributes:

  • nodes: the array of execution nodes of the plan. See the list of execution nodes
  • estimatedCost: the total estimated cost for the plan. If there are multiple plans, the optimizer chooses the plan with the lowest total cost.
  • collections: an array of collections used in the query
  • rules: an array of rules the optimizer applied. See the list of optimizer rules
  • variables: array of variables used in the query (note: this may contain internal variables created by the optimizer)

Here is an example for retrieving the execution plan of a simple query:

var stmt = db._createStatement("FOR user IN _users RETURN user");
Show output

As the output of explain() is very detailed, it is recommended to use some scripting to make the output less verbose:

var formatPlan = function (plan) {
  return { estimatedCost: plan.estimatedCost,
    nodes: {
      return node.type; }) }; };
Show output

If a query contains bind parameters, they must be added to the statement before explain() is called:

var stmt = db._createStatement(
  `FOR doc IN @@collection FILTER doc.user == @user RETURN doc`
stmt.bind({ "@collection" : "_users", "user" : "root" });
Show output

In some cases, the AQL optimizer creates multiple plans for a single query. By default only the plan with the lowest total estimated cost is kept, and the other plans are discarded. To retrieve all plans the optimizer has generated, explain can be called with the option allPlans set to true.

In the following example, the optimizer has created two plans:

var stmt = db._createStatement(
  "FOR user IN _users FILTER user.user == 'root' RETURN user");
stmt.explain({ allPlans: true }).plans.length;

To see a slightly more compact version of the plan, the following transformation can be applied:

stmt.explain({ allPlans: true })
  function(plan) { return formatPlan(plan); });
Show output

explain() also accepts the following additional options:

  • maxPlans: limits the maximum number of plans that are created by the AQL query optimizer
  • optimizer:
    • rules: an array of to-be-included or to-be-excluded optimizer rules can be put into this attribute, telling the optimizer to include or exclude specific rules. To disable a rule, prefix its name with a -, to enable a rule, prefix it with a +. There is also a pseudo-rule all, which matches all optimizer rules.

The following example disables all optimizer rules but remove-redundant-calculations:

stmt.explain({ optimizer: {
  rules: [ "-all", "+remove-redundant-calculations" ] } });
Show output

The contents of an execution plan are meant to be machine-readable. To get a human-readable version of a query’s execution plan, the following commands can be used (without disabling syntax highlighting with { colors: false }):

var query = "FOR doc IN mycollection FILTER doc.value > 42 RETURN doc";
require("@arangodb/aql/explainer").explain(query, {colors:false});
Show output

The above command prints the query’s execution plan in the ArangoShell directly, focusing on the most important information.

Gathering debug information about a query

If an explain provides no suitable insight into why a query does not perform as expected, it may be reported to the ArangoDB support. In order to make this as easy as possible, there is a built-in command in ArangoShell for packaging the query, its bind parameters, and all data required to execute the query elsewhere.

require("@arangodb/aql/explainer").debugDump(filepath, query[, bindVars[, options]])

You can specify the following parameters:

  • filepath (string): A file path to save the debug package to
  • query (string): An AQL query
  • bindVars (object, optional): The bind parameters for the query
  • options (object, optional): Options for the query, with two additionally supported settings compared to db._query():
    • examples (number, optional): How many sample documents of your collection data to include. Default: 0
    • anonymize (boolean, optional): Whether all string attribute values of the sample documents shall be substituted with strings like XXX.

The command stores all data in a file with a configurable filename:

var query = "FOR doc IN mycollection FILTER doc.value > 42 RETURN doc";
require("@arangodb/aql/explainer").debugDump("/tmp/query-debug-info", query);
Show output

Entitled users can send the generated file to the ArangoDB support to facilitate reproduction and debugging.

You can also create debug packages using the web interface, see Query debug packages.

If a query contains bind parameters, you need to specify them along with the query string:

var query = "FOR doc IN @@collection FILTER doc.value > @value RETURN doc";
var bindVars = { value: 42, "@collection": "mycollection" };
require("@arangodb/aql/explainer").debugDump("/tmp/query-debug-info", query, bindVars);
Show output

It is also possible to include example documents from the underlying collection in order to make reproduction even easier. Example documents can be sent as they are, or in an anonymized form. The number of example documents can be specified in the examples options attribute, and should generally be kept low. The anonymize option replaces the contents of string attributes in the examples with XXX. However, it does not replace any other types of data (e.g. numeric values) or attribute names. Attribute names in the examples are always preserved because they may be indexed and used in queries:

var query = "FOR doc IN @@collection FILTER doc.value > @value RETURN doc";
var bind = { value: 42, "@collection": "mycollection" };
var options = { examples: 10, anonymize: true };
require("@arangodb/aql/explainer").debugDump("/tmp/query-debug-info", query, bind, options);
Show output

To get a human-readable output from a debug package JSON file, you can use the inspectDump() method:

require("@arangodb/aql/explainer").inspectDump(inFilepath[, outFilepath])

You can specify the following parameters:

  • inFilepath (string): The path to the debug package JSON file
  • outFilepath (string, optional): A path to store the formatted output in a file instead of printing to the shell