Explain plans and the profiler · Docs

Explain a query

You can explain a query from three places:

the Explain tab of a collection tab;
IntelliShell, with .explain() or .explain("executionStats");
the aggregation editor.

MotionQL shows the winning plan as a tree, with the index used, the number of documents examined and the execution time.

What to look for

Stage	Meaning
`COLLSCAN`	The server read every document in the collection. Fine for small collections; slow for big ones
`IXSCAN`	The server walked an index to find matching documents
`FETCH`	The server loaded full documents after an index scan
`SORT`	The results were sorted in memory, because no index provides the requested order
`PROJECTION_COVERED`	The index alone answered the query; no documents were loaded

A healthy query usually returns about as many documents as it examines. If documents examined is far higher than documents returned, an index on the filtered fields will probably help.

The query bar warns you before you run: a COLLSCAN or in-memory sort badge appears on filtered or sorted queries that would scan or sort without an index. It's checked with a queryPlanner explain, which doesn't execute the query.

For a longer walkthrough, read Reading MongoDB explain plans: COLLSCAN vs IXSCAN.

Query profiler

The Query Profiler uses MongoDB's database profiler to find slow operations.

Open Query Profiler from the command palette.
Set the profiling level: off, slow operations only (above a threshold in milliseconds), or all operations.
Run your workload, then look at the summary of slow operations and the individual samples.
Clear the profiling data when you're done.

The profiler writes to system.profile and adds some load, so prefer "slow operations only" on busy servers.

Server monitoring and operations

Server Monitoring: live metrics from serverStatus (operations per second, connections, network in and out, memory, WiredTiger cache, active and queued reads and writes, uptime), dbStats for the open database, and the busiest collections from top. It polls every 2 to 30 seconds; pause and resume any time. Sections your user can't read (they need the clusterMonitor role), or that Atlas free and shared tiers refuse, show as unavailable with the reason.
Query builder → Optimization: runs explain("executionStats") on the query you're building and shows keys and documents examined, documents returned, server time, the winning plan and a COLLSCAN warning, with index suggestions. An index is created only after you type the collection name.
Running Operations: list current operations and kill one. Killing is a write, so it's blocked on read-only connections.
Topology: replica set members and their roles, and the shards of a sharded cluster.

AI help

With Allow AI on for the connection, the assistant can explain a plan in plain words and suggest indexes. It receives the shape of the plan, not your documents. See AI assistant.