In MongoDB, the db.collection.unhideIndex() method unhides a hidden index.
A hidden index is one that’s hidden from the query planner. When you unhide an index, it is no longer hidden from the query planner, and it’s immediately available for use.
MongoDB introduced the db.collection.hideIndex() method in MongoDB 4.4.
This method hides an existing index from the query planner. This allows you to evaluate the potential impact of dropping an index without actually dropping the index.
If hiding it has a negative impact, you can use db.collection.unhideIndex() to unhide the index. This saves you from having to drop the index, then recreate it.
When using mongoimport to import files into MongoDB, you have the option of specifying a mode to use. These modes determine what happens if there’s already matching documents in the collection that you’re trying to import into.
By default, mongoimport uses insert mode, but there are other modes you can use. The mode you use will depend on what you’re trying to do.
Below is an overview of each mode along with examples.
In MongoDB the db.collection.countDocuments() method returns the count of documents that match the query for a collection or view.
The collection part is the name of the collection or view to perform the count operation on.
In MongoDB, the distinct aggregation command finds the distinct values for a specified field across a single collection.
It returns a document that contains an array of the distinct values, as well as an embedded document with query statistics and the query plan.
In MongoDB, the db.collection.distinct() method finds the distinct values for a specified field across a single collection or view and returns the results in an array.
Distinct values are those with redundant duplicates removed. Distinct values are unique values. For example, if you have 2 or 3 documents with the same value, the distinct command will return just one value.
db.collection.distinct() is a shell wrapper method for the distinct command.
In MongoDB the $sortByCount aggregation pipeline stage groups incoming documents based on the value of a specified expression, then computes the count of documents in each distinct group.
Each group is output in its own document, which consists of two fields:
_id field containing the distinct grouping value, andcount field containing the number of documents belonging to that grouping.The documents are sorted by count in descending order.
In MongoDB the $count aggregation operator passes a document to the next stage in the aggregation pipeline that contains a count of the number of documents input to the current stage.
In MongoDB the cursor.count() method counts the number of documents referenced by a cursor..
Note that it doesn’t actually perform the query. It simply counts and returns the number of results that would be returned by the query.
The cursor.count() method is a wrapper for the count command.