<br />

This page describes how to retrieve query execution information when you execute a query.

## Use Query Explain

You can use Query Explain to understand how your queries are being executed.
This provides details that you can use to [optimize your queries](https://firebase.google.com/docs/firestore/enterprise/optimize-query-performance).

You can use Query Explain through the Google Cloud console or the `explain` command.  

##### Console

Execute a query in the Query Editor and open the **Explanation** tab:

1. In the Google Cloud console, go to the **Databases** page.

   [Go to Databases](https://console.cloud.google.com/firestore/databases)
2. From the list of databases, select a Cloud Firestore database. The Google Cloud console opens the **Firestore Explorer** for that database.
3. Enter a query in the query editor and click **Run**.
4. Click the **Explanation** tab to view the query analysis output.

   ![Query Explain tab in the console](https://firebase.google.com/static/docs/firestore/enterprise/images/firestore-query-explain-console.png)

##### MongoDB API

Query Explain in the MongoDB API is supported through the
[`explain`](https://www.mongodb.com/docs/manual/reference/command/explain/)
command which you can use in tools such as Mongo Shell and Compass.


The `explain` command is supported with the `aggregate`,
`find`, `distinct`, and `count`
commands, for example:  

```text
db.collection.explain('executionStats').find(...)
```


You can also use the `explain()` method, for example:  

```text
db.collection.find({QUERY}).explain('executionStats')
```

###### Limitations

Note the following limitations and differences:

- Query Explain does not support commands which return a cursor. For example,
  invoking explain by calling the following command directly is not supported:

  ```verilog
  db.collection.aggregate(..., explain: true)
  ```
- Query Explain is only supported on the
  `find`, `aggregate`, `count`, and `distinct`
  commands.

- The `Verbosity` and `Comment` options of Query Explain
  are not supported through the MongoDB API. The behaviour matches the
  `executionStats` option. The `allPlansExecution` and
  `queryPlanner` options are ignored if provided.

  If no verbosity is provided, the shell uses the `queryPlanner`
  verbosity and filters out execution stats. You must use the
  `executionStats` or `allPlansExecution` verbosity
  to see the full output.

## Analysis

The output of Query Explain contains two main components-the Summary Statistics and Execution Tree.
Consider this query as an example:  

    db.order.aggregate(
     [
       { "$match": { "user_id": 1234 } },
       { "$sort": { "date_placed": 1 } }
     ]
    )

## Summary Statistics

The top of the explained output contains a summary of the execution statistics.
Use these statistics to determine if a query has high latency or cost. It also
contains memory statistics which let you know how close your query is
to [memory limits](https://firebase.google.com/docs/firestore/enterprise/quotas).  

    Billing Metrics:
    read units: 1

    Execution Metrics:
    request peak memory usage: 4.00 KiB (4,096 B)
    results returned: 1

## Execution Tree

The execution tree describes the query execution as a series of nodes. The
bottom nodes (leaf nodes) retrieve data from the storage layer which traverses
up the tree to generate a query response.

For details about each execution node,
refer to the [Execution reference](https://firebase.google.com/docs/firestore/enterprise/query-explain-reference).

For details on how to use this information to optimize your queries,
see [Optimize query execution](https://firebase.google.com/docs/firestore/enterprise/optimize-query-performance).

The following is an example of an execution tree:  

    • Compute
    |  $out_1: map_set($record_1, "__id__", $__id___1, "__key__", $__key___1, "__row_id__", $__row_id___1, "__$0__", $__$0___2)
    |  is query result: true
    |
    |  Execution:
    |   records returned: 1
    |
    └── • Compute
        |  $__$0___2: UNSET
        |
        |  Execution:
        |   records returned: 1
        |
        └── • Compute
            |  $__key___1: UNSET
            |  $__row_id___1: UNSET
            |
            |  Execution:
            |   records returned: 1
            |
            └── • Compute
                |  $__id___1: _id($record_1.__key__)
                |
                |  Execution:
                |   records returned: 1
                |
                └── • MajorSort
                    |  fields: [$v_5 ASC]
                    |  output: [$record_1]
                    |
                    |  Execution:
                    |   records returned: 1
                    |   peak memory usage: 4.00 KiB (4,096 B)
                    |
                    └── • Compute
                        |  $v_5: array_get($v_4, 0L)
                        |
                        |  Execution:
                        |   records returned: 1
                        |
                        └── • Compute
                            |  $v_4: sortPaths(array($record_1.date_placed), [date_placed ASC])
                            |
                            |  Execution:
                            |   records returned: 1
                            |
                            └── • Filter
                                |  expression: $eq($user_id_1, 1,234)
                                |
                                |  Execution:
                                |   records returned: 1
                                |
                                └── • TableScan
                                       source: **/my_collection
                                       order: STABLE
                                       properties: * - { __create_time__, __update_time__ }
                                       output record: $record_1
                                       output bindings: {$user_id_1=user_id}
                                       variables: [$record_1, $user_id_1]

                                       Execution:
                                        records returned: 1
                                        records scanned: 1

## What's next

- To learn about the execution tree nodes, see the [Query execution reference](https://firebase.google.com/docs/firestore/enterprise/query-explain-reference).
- To learn how to optimize your queries, see [Optimize query execution](https://firebase.google.com/docs/firestore/enterprise/optimize-query-performance).