How MongoDB choose winning plan?
MongoDB runs the query optimizer to choose the winning plan and executes the winning plan to completion. In “allPlansExecution” mode, MongoDB returns statistics describing the execution of the winning plan as well as statistics for the other candidate plans captured during plan selection.
What is explain () in MongoDB?
Definition. explain. The explain command provides information on the execution of the following commands: aggregate , count , distinct , find , findAndModify , delete , mapReduce , and update . Although MongoDB provides the explain command, the preferred method for running explain is to use the db.
Where does query optimizer find information?
Once the query is optimized, the resulting plan is used by the Execution Engine to retrieve the desired data. The generated execution plan may be stored in memory, in the plan cache (known as the procedure cache in previous versions of SQL Server) in order that it might be reused if the same query is executed again.
How does the query optimizer work in MongoDB?
MongoDB uses its query optimizer to evaluate a number of candidate plans, and then takes what it believes is the best plan for this particular query. The winning query plan is typically what we’re looking to understand when trying to see if we can improve slow query performance.
How does the explain command work in MongoDB?
To return information on query plans and execution statistics of the query plans, MongoDB provides: the explain command. The explain results present the query plans as a tree of stages. Each stage passes its results (i.e. documents or index keys) to the parent node.
How is a candidate plan selected in MongoDB?
Candidate plans are evaluated and a winning plan is selected. The selected plan’s works value is compared to the Inactive entry’s. If the selected plan’s works value is: The selected plan replaces the placeholder Inactive entry and has an Active state.
What is the EXPLAIN PLAN tab in MongoDB?
MongoDB Compass provides an Explain Plan tab, which displays statistics about the performance of a query. These statistics can be useful in measuring if and how a query uses an index.