Explain

Explain shows how the server runs your query: which indexes it uses, how many keys and documents it examined, how many it returned and how long it took. The plan is drawn as a tree of stages. A collection scan (COLLSCAN) is flagged, so a query that reads every document stands out.

Explain a query

1. Set up the query in the collection view, or the pipeline in the aggregation editor.
2. Choose Explain to run the query on the server and measure it, or Plan only to see the chosen plan without running it.
3. The Explain view opens with the result. In the collection view, its icon sits with Tree, Table and JSON at the right end of the status bar, so you can switch back to the documents and return to the plan.

Read the result

The top of the view says either:

• “COLLSCAN: the query reads every document of the collection. An index on the filtered or sorted fields avoids that.” The collection scan stage is highlighted in the plan too.
• “Uses index …” with the names of the indexes the plan uses.

Below that are four figures:

Figure	Meaning
Returned	Documents the query returned.
Keys examined	Index keys the server read.
Docs examined	Documents the server read.
Time	Execution time in milliseconds.

With Plan only the query was not run, so there are no counts.

The plan is a tree of stages, such as IXSCAN or COLLSCAN. Each stage shows its index, the documents it returned and its time, with details such as keysExamined, docsExamined, keyPattern, filter, sortPattern and direction. Raw explain output shows the server’s full answer as JSON.

Fix a collection scan

If the plan shows COLLSCAN for a query you run often, create an index on the fields the query filters or sorts on in the index manager, then choose Explain again. The plan then shows an IXSCAN stage on the new index.

Related

• Indexes
• Collection view and query bar
• Aggregation editor