arangodb/Documentation/Books/AQL/ExecutionAndPerformance/ExplainingQueries.md

Explaining queries

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

Explaining a query can be achieved by calling the HTTP REST API. A query can also be explained from the ArangoShell using ArangoStatement's explain method.

By default, the query optimizer will return what it considers to be the optimal plan. The optimal plan will be returned in the plan attribute of the result. If explain is called with option allPlans set to true, all plans will be returned in the plans attribute instead. The result object will also contain an attribute warnings, 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. The list of available node types can be found here
• estimatedCost: the total estimated cost for the plan. If there are multiple plans, the optimizer will choose the plan with the lowest total cost.
• collections: an array of collections used in the query
• rules: an array of rules the optimizer applied. The list of rules can be found here
• 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:

@startDocuBlockInline 07_workWithAQL_statementsExplain
@EXAMPLE_ARANGOSH_OUTPUT{07_workWithAQL_statementsExplain}
|var stmt = db._createStatement(
 "FOR user IN _users RETURN user");
stmt.explain();
@END_EXAMPLE_ARANGOSH_OUTPUT
@endDocuBlock 07_workWithAQL_statementsExplain

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

@startDocuBlockInline 08_workWithAQL_statementsPlans
@EXAMPLE_ARANGOSH_OUTPUT{08_workWithAQL_statementsPlans}
|var formatPlan = function (plan) {
|    return { estimatedCost: plan.estimatedCost,
|        nodes: plan.nodes.map(function(node) {
            return node.type; }) }; };
formatPlan(stmt.explain().plan);
@END_EXAMPLE_ARANGOSH_OUTPUT
@endDocuBlock 08_workWithAQL_statementsPlans

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

@startDocuBlockInline 09_workWithAQL_statementsPlansBind
@EXAMPLE_ARANGOSH_OUTPUT{09_workWithAQL_statementsPlansBind}
|var stmt = db._createStatement(
| `FOR doc IN @@collection FILTER doc.user == @user RETURN doc`
);
stmt.bind({ "@collection" : "_users", "user" : "root" });
stmt.explain();
@END_EXAMPLE_ARANGOSH_OUTPUT
@endDocuBlock 09_workWithAQL_statementsPlansBind

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:

@startDocuBlockInline 10_workWithAQL_statementsPlansOptimizer0
@EXAMPLE_ARANGOSH_OUTPUT{10_workWithAQL_statementsPlansOptimizer0}
|var stmt = db._createStatement(
  "FOR user IN _users FILTER user.user == 'root' RETURN user");
stmt.explain({ allPlans: true }).plans.length;
@END_EXAMPLE_ARANGOSH_OUTPUT
@endDocuBlock 10_workWithAQL_statementsPlansOptimizer0

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

@startDocuBlockInline 10_workWithAQL_statementsPlansOptimizer1
@EXAMPLE_ARANGOSH_OUTPUT{10_workWithAQL_statementsPlansOptimizer1}
~var stmt = db._createStatement("FOR user IN _users FILTER user.user == 'root' RETURN user");
|stmt.explain({ allPlans: true }).plans.map(
    function(plan) { return formatPlan(plan); });
@END_EXAMPLE_ARANGOSH_OUTPUT
@endDocuBlock 10_workWithAQL_statementsPlansOptimizer1

explain will also accept 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 will match all optimizer rules.

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

@startDocuBlockInline 10_workWithAQL_statementsPlansOptimizer2
@EXAMPLE_ARANGOSH_OUTPUT{10_workWithAQL_statementsPlansOptimizer2}
~var stmt = db._createStatement("FOR user IN _users FILTER user.user == 'root' RETURN user");
|stmt.explain({ optimizer: {
   rules: [ "-all", "+remove-redundant-calculations" ] } });
@END_EXAMPLE_ARANGOSH_OUTPUT
@endDocuBlock 10_workWithAQL_statementsPlansOptimizer2

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:

@startDocuBlockInline 10_workWithAQL_statementsPlansOptimizer3
@EXAMPLE_ARANGOSH_OUTPUT{10_workWithAQL_statementsPlansOptimizer3}
var query = "FOR doc IN mycollection FILTER doc.value > 42 RETURN doc";
require("@arangodb/aql/explainer").explain(query, {colors:false});
@END_EXAMPLE_ARANGOSH_OUTPUT
@endDocuBlock 10_workWithAQL_statementsPlansOptimizer3

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