How does Mongo decide which index to use?
MongoDB uses multikey indexes to index the content stored in arrays. If you index a field that holds an array value, MongoDB creates separate index entries for every element of the array. These multikey indexes allow queries to select documents that contain arrays by matching on element or elements of the arrays.
Does MongoDB use execution plans?
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 true about index in MongoDB?
An index in MongoDB is a special data structure that holds the data of few fields of documents on which the index is created. Indexes improve the speed of search operations in database because instead of searching the whole document, the search is performed on the indexes that holds only few fields.
Where is MongoDB used?
MongoDB works on all types of computing platforms, both on-premise and in the cloud (both private, and public clouds such as AWS, Azure, and Google Cloud) MongoDB can be used from all major languages. MongoDB can be accessed from all major ETL and data management systems.
Can we create multiple index on same table in SQL?
It is possible for an index to have two or more columns. Multi column indexes are also known as compound or concatenated indexes. Let us look at a query that could use two different indexes on the table based on the WHERE clause restrictions. We first create these indexes.
How to include executionstats in MongoDB explain results?
The returned executionStats information details the execution of the winning plan. In order to include executionStats in the results, you must run the explain in either: allPlansExecution verbosity mode. Use allPlansExecution mode to include partial execution data captured during plan selection.
How does the winning plan work in MongoDB?
Details the completed execution of the winning plan as a tree of stages; i.e. a stage can have an inputStage or multiple inputStages. Each stage consists of execution information specific to the stage. The estimated amount of time in milliseconds for query execution.
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.
What does keysexamined mean in the MongoDB manual?
For query execution stages that scan an index (e.g. IXSCAN), keysExamined is the total number of in-bounds and out-of-bounds keys that are examined in the process of the index scan. If the index scan consists of a single contiguous range of keys, only in-bounds keys need to be examined.