In MongoDB, you can use the $push operator to append a value to an array.
You can use various modifiers to specify the value’s position in the array, the order of the elements in the array, append multiple values, etc.
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$unionWith – MongoDB’s Equivalent of UNION ALL
If you’re familiar with SQL, you might know about the UNION clause, which concatenates the results of two queries into a single result set. In particular, UNION ALL includes duplicates.
In MongoDB, we can use the $unionWith aggregation pipeline stage to achieve the same effect that UNION ALL produces. The $unionWith stage performs a union of two collections – it combines pipeline results from two collections into a single result set. And it includes duplicates.
MongoDB dropIndexes Command
MongoDB has an administration command called dropIndexes that drops one or more indexes (except the index on the _id field) from the specified collection.
MongoDB dropIndexes()
In MongoDB, the dropIndexes() method allows you to drop one or more indexes from a collection.
To drop a single index, pass the name of the index or its definition/specification document. If it’s a text index, you can only specify the index name.
To drop multiple indexes, pass the index names in an array.
To drop all indexes (except for the _id index), don’t pass any arguments.
MongoDB dropIndex()
There are several ways to drop an index in MongoDB, one of which is to use the dropIndex() method.
It’s pretty simple to use – just pass the name of the index or its definition/specification document. If it’s a text index, you can only specify the index name.
How unhideIndex() Works in MongoDB
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.
How hideIndex() Works in MongoDB
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.
The Import Modes of mongoimport
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.
MongoDB countDocuments()
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.
MongoDB distinct Command
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.