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Index and query analyzer for MongoDB: compares MongoDB log files and index entries to make index recommendations
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Dex, the Index Bot

Dex is a MongoDB performance tuning tool that compares logged queries to the available indexes in the queried collection(s) and generates index suggestions based on simple heuristics. To use Dex, you provide--at a minimum--a path to your MongoDB log file and a connection URI for your database.

Dex uses the URI you provide as a helpful way to determine when an index is recommended. Dex does not take existing indexes into account when actually constructing its ideal recommendation.

Currently, Dex only recommends complete indexes, not partial indexes. Dex ignores partial indexes that may be used by the query in favor of a better index, if one is not found. Dex recommends partially-ordered indexes according to a rule of thumb:

Your index field order should first answer: 1) Equivalent value checks 2) Sort clauses 3) Range value checks ($in, $nin, $lt/gt, $lte/gte, etc.)

Note that your data cardinality may warrant a different order than the suggested indexes.



Run Dex on a log file and provide a URI (with auth credentials, if any) to the corresponding database.

> dex -f my/mongod/data/path/mongodb.log mongodb://myUser:myPass@myHost:12345/myDb


Dex also supports filtering this analysis by specific collections and databases. Note that when you intend to analyze multiple databases you must provide a connection URI for the admin database.

> dex -f my/mongod/data/path/mongodb.log -n "myFirstDb.collectionOne" mongodb://myUser:myPass@myHost:12345/myFirstDb

> dex -f my/mongod/data/path/mongodb.log -n "*.collectionOne" mongodb://myUser:myPass@myHost:12345/admin

> dex -f my/mongod/data/path/mongodb.log -n "myFirstDb.*" -n "mySecondDb.*" mongodb://myUser:myPass@myHost:12345/admin

Help Contents

Usage: dex [<options>] uri

Scans a provided MongoDB log file and uses the provided URI to compare queries
found in the logfile to the indexes available in the database, recommending
indexes for those queries which are not indexed. Recommended for MongoDB version
2.0.4 or later.

  -h, --help            show this help message and exit
                        path to a MongoDB log file. If provided, the file will
                        be searched for queries.
  -p, --profile         flag to examine the MongoDB system.profile collection.
                        If set, the profile collection will be searched for
                        queries. NOT YET SUPPORTED.
                        a MongoDB namespace (db.collection). Can be provided
                        multiple times. This option creates a filter, and
                        queries not in the provided namespace(s) will not be
                        analyzed. Format: -n ('db.collection' | '*' | 'db.*' |
                        'collection'). '*.*' and '*.collection' are redundant
                        but also supported. An asterisk is shorthand for 'all'
                        --actual regexes are not supported. Note that -n '*'
                        is equivalent to not providing a -n argument.
  -v, --verbose         enables provision of additional output information.


For logfile input, provide logfiles generated by mongod 2.0.4 or later.


  • pyyaml
  • pymongo
  • dargparse


> pip install dex


To run Dex's unit test suite, you must bring up a mongodb server on 27017. Dex will use create the dex_test db and drop it when the tests are complete.

> python -m dex.test.test



By default, Dex outputs each unique recommendation. A recommendation includes a db name, index command, and the fields from the query that prompted the recommendation. Dex concludes a run with a list of run statistics, including:

  • Total lines read - Total number of lines in the input file
  • Understood query lines - Number of lines successfully parsed by the LogParser. For the average case, this line is expected to be somewhat low compared to Total Lines Read.
  • Unique recommendations - Number of unique recommendations found for all understood query lines
  • Lines impacted by recommendations - Total number of understood lines that generated recommendations.

Runtime Output to STDERR

Dex provides runtime output as it processes. In default mode, Dex outputs each unique recommendation it generates, and concludes with run statistics. Each recommendation includes the following fields:

  • namespace - The db.collection name.
  • index - A json string describing the recommended index.
  • shellCommand - A helpful cut-and-paste command to create the index from the MongoDB shell.


    "index": "{'simpleIndexedField': 1, 'simpleUnindexedFieldThree': 1}", 
    "namespace": "dex_test.test_collection" 
    "shellCommand": "db.test_collection.ensureIndex({'simpleIndexedField': 1, 'simpleUnindexedFieldThree': 1}, {'background': true})"
Total lines read: 7
Understood query lines: 7
Unique recommendations: 5
Lines impacted by recommendations: 5

Final Output to STDOUT

Dex returns a JSON document containing runtime statistics and each unique recommendation.


    "linesPassed": 7, 
    "linesRecommended": 5, 
    "results": [
            "index-json": "{'simpleUnindexedField': 1}", 
            "shell-command": "db.test_collection.ensureIndex({'simpleUnindexedField': 1}, {'background': true})", 
            "namespace": "dex_test.test_collection" 
    "uniqueRecommendations": 5, 
    "linesProcessed": 7


When -v/--verbose is specified, Dex outputs the full query report for each unique recommendation, including:

  • namespace - The db.collection name.
  • rawFields - Raw fields extracted from the query
  • queryAnalysis - Provides basic information about the query, such as fieldCount), as well as the analyzed type of each field. 'suppported' is False when the query contains an UNSUPPORTED field.
  • indexAnalysis - Shows which indexes cover the query (either in full or partially), and whether or not the analysis shows room for improvement. For each index, the following information is available:

    idealOrder - Is the index sorted according to Dex's internal heuristic.

    queryFieldsCovered - Number of query fields the index services.

    coverage - Indexes with a coverage of 'none' are not output. 'partial' coverage indicates that (0 < query-fields-covered < total-query-fields). 'full' coverage indicates that (query-fields-covered == total-query-fields).

    supported - Indicates that the index is not of an unsupported type (such as 2d)

    index - The mongodb-provided document describing the index.

  • parsed - The raw query as read by Dex's LogParser.

  • recommendation - The recommendation itself (as described in Default Output above)


    "indexAnalysis": {
        "needsRecommendation": true, 
        "fullIndexes": [], 
        "partialIndexes": [
                "index": {
                    "key": [
                    "v": 1
                "supported": true, 
                "coverage": "partial", 
                "idealOrder": true, 
                "queryFieldsCovered": 1
    "parsed": {
        "ns": "dex_test.test_collection", 
        "query": {
            "simpleIndexedField": "value"
        "orderby": {
            "simpleUnindexedFieldThree": -1
        "findandmodify": "test_collection", 
        "update": {
            "$set": {
                "something": "somethingelse"
    "recommendation": {
        "index": "{'simpleIndexedField': 1, 'simpleUnindexedFieldThree': 1}", 
        "namespace": "dex_test.test_collection",
        "shellCommand": "db.test_collection.ensureIndex({'simpleIndexedField': 1, 'simpleUnindexedFieldThree': 1}, {'background': true})"
    "namespace": "dex_test.test_collection",
    "queryAnalysis": {
        "fieldCount": 2, 
        "supported": true, 
        "analyzedFields": [
                "fieldName": "simpleIndexedField", 
                "fieldType": "EQUIV"
                "fieldName": "simpleUnindexedFieldThree", 
                "fieldType": "SORT", 
                "seq": 0
Total lines read: 7
Understood query lines: 7
Unique recommendations: 5
Lines impacted by recommendations: 5

Future Usage

In the future, Dex will support using the system.profile collection instead of (or in addition to) a log file. Example:

> dex -p -n "myFirstDb.*" -n "mySecondDb.*" mongodb://myUser:myPass@myHost:12345/admin



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