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How to find the slowest queries in MongoDB?
We can show the source, nicely formatted, of the top ten slowest queries. This query lists the source of the top slowest queries in the order of slowest first. For initial development work this sort of query is very useful, particularly because each document stores performance information along with queries.
What does the secondaries do in MongoDB 4.2?
The secondaries log only the slow oplog entries; increasing the logLevel does not log all oplog entries. Starting in MongoDB 4.2, the profiler entries and the diagnostic log messages (i.e. mongod/mongos log messages) for read/write operations include:
What does slowopsamplerate do in MongoDB profiler?
See Profiler Overhead for more information on potential performance degradation. When logLevel is set to 0, MongoDB records slow operations to the diagnostic log at a rate determined by slowOpSampleRate.
What happens to the diagnostic log in MongoDB?
At higher logLevel settings, all operations appear in the diagnostic log regardless of their latency with the following exception: the logging of slow oplog entry messages by the secondaries. The secondaries log only the slow oplog entries; increasing the logLevel does not log all oplog entries.
Is there a way to tune the performance of MongoDB?
MongoDB has a very sensible range of information that is available for tuning the performance of queries and aggregations. When tuning queries, it generally pays to pick off the worst-offending ones first and to tackle the most obvious problems first.
What does the explain facility do in MongoDB?
MongoDB provides an ‘explain’ facility that gives you a tool for troubleshooting queries, which comes in three flavors: All three can be used to return information on query plans and the execution statistics of those query plans.
Where does the MongoDB profiler store the queries?
When it collects queries, it does so from the entire instance, which means all your databases and connections on that instance. It can be turned on or off from applications or from the shell. The profiler stores the queries in db.system.profile. This can be queried just like any other database, so you can find a lot of information quickly.
How does the query planner work in MongoDB?
MongoDB has a query planner that selects an existing cached plan or creates one for the query. To process a query, the query planner searches the query plan cache to see whether a suitable query plan exists in cache that can be reused.