explain-plan-model

Ampersand model abstraction for MongoDB explain plans.

Description

Parses the JSON output of the explain cursor option in MongoDB, which provides useful information about the query planning and execution stats.

Currently, this model only works on regular queries and not for the Aggregation Framework.

Example

Here is an example with the node.js driver to populate an explain plan model for a given query:

var MongoClient = require('mongodb').MongoClient;
var ExplainPlanModel = require('mongodb-explain-plan-model');

MongoClient.connect('mongodb://localhost:27018/mongodb', function(err, db) {
  db.collection('fanclub')
    .find({age: {$gte: 33, $lte: 40}})
    .explain(function(err2, explain) {
      var model = new ExplainPlanModel(explain, {parse: true});

      console.log(model.executionSuccess);      // ==> true
      console.log(model.isCovered);             // ==> false
      console.log(model.isCollectionScan);      // ==> false
      console.log(model.usedIndex);             // ==> "age_1"
      console.log(model.inMemorySort);          // ==> false
      console.log(model.nReturned);             // ==> 191665
      console.log(model.executionTimeMillis);   // ==> 146
      // ...

      db.close();
    });
});

Stages

To access the raw information of any execution stage, you can use the .findStageByName() method. This walks the tree of stage (depth-first, pre-order) and returns the first stage matching the name (or null if the stage is not present).

For example, to get information about the IXSCAN stage, use:

var ixscan = model.findStageByName('IXSCAN');
if (ixscan) {
  console.log('IXSCAN took', ixscan.executionTimeMillisEstimate, 'ms.');
} else {
  console.log('no IXSCAN stage found.');
}

Supported Versions

This model works best with explain plan output from MongoDB 3.0 and higher, but it does have a legacy mode and tries to infer the values from older formats. However, depending on the explain plan, some fields may be null for older formats.

If the explain plan is from an older (2.6 or below) version, the legacyMode flag is set to true.