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This article will explain in detail how to achieve a variety of query operations in MongoDB, the content of the article is of high quality, so the editor will share it with you for reference. I hope you will have a certain understanding of the relevant knowledge after reading this article.

1. Find operation

Find is used in MongoDB to query, and all and part of the query can be implemented by specifying the first parameter of find.

1. Query all

The empty query document {} matches the entire contents of the collection. If you do not specify a query document, the default is {}.

2. Partial query

3. Screening of keys

The filtering of keys is that only the key values of interest are returned when querying, which is achieved by specifying the second parameter of find. This can not only save the amount of data transmitted, but also save the time and memory consumption of decoding documents by the client.

When querying, the value of the query document that the database is concerned with must be constant.

II. Query conditions

1. Compare queries

Lt,$lte,$gt,$gte,$ne and =,! = are one-to-one correspondence, and they can be combined to find values in a range.

2. Associated query

$in is used to query multiple values of a key, and $nin returns documents that do not match all the criteria in the filtered array. Combine $in with $not to achieve the same effect as $nin.

Or is used to do or queries on multiple keys.

III. Specific types of queries

1. Null query

Null can match not only itself, but also documents where the key does not exist.

2. Regular expression

3. Array query

$all: matches the array through multiple elements.

$size: query an array of specified length.

$slice: returns a subset of the array.

4. Embedded document query

4. $where query

The $where query is the advanced query part of MongoDB, and you can execute any JavaScript as part of the query, which is a complement to other query methods.

The $where query needs to convert each document from a BSON to a JavaScript object and then run it through an expression of $where, which does not take advantage of the index, so the query is much slower than a regular query. If you must, you can use regular queries as pre-filters, and if you can use indexes, you can use indexes to filter based on non-$where clauses, and finally use $where to tune the results. Another way is to simplify by mapping-MapReduce.

Vernier

Cursors are useful things, and MongoDB databases use cursors to return the execution results of find. The client uses cursors to effectively control the final results, such as paging and sorting.

1. Limit,skip and sort

Limit: limit the number of results, limit the upper limit.

Skip: skip the first n documents matched and return the remaining documents. Skip can cause performance problems when skipping too many documents, which is recommended to avoid.

Sort: sorts documents by the specified key, with 1 in ascending order and-1 in descending order.

2. Advanced query options

Wrapper query: use sort,limit or skip for further control over the final result.

Useful configuration options:

$maxscan:integer, which specifies the maximum number of documents scanned by the query.

$min:document, the start condition of the query.

$max:document, the end condition of the query.

Hint:document, which specifies which index the server uses for the query.

$explain:boolean to get the details of query execution (indexes used, number of results, time-consuming, etc.) instead of actually executing the query.

Snapshot:boolean, to ensure that the result of the query is a consistent snapshot at the moment the query is executed, to avoid inconsistent reads.

A wrapper query wraps the query conditions into a larger query document, such as when executing a query like this:

Db.foo.find ({"name": "bar"}) .sort ("x": 1)

Instead of sending {"name": "bar"} directly to the database, shell converts the query from {"name": "bar"} to {"$query": {"name": "bar"}, "$orderby": {"x": 1}}.

3. Inside Vernier

There are two ways to look at cursors: client-side cursors and database cursors represented by client-side cursors (server-side).

On the server side, cursors consume memory and other resources, so they need to be released as soon as possible under reasonable circumstances. The server-side situation that causes the cursor to terminate is as follows:

1. Automatically clear the cursor when it completes the iteration of the matching result.

2. When the client is no longer in scope, the driver will send a special message to the server to destroy the cursor.

3. The timeout is destroyed. You can use the immortal function to close the cursor timeout. To do this, you must close the cursor after iterating over the result.

On how to achieve a variety of query operations in MongoDB to share here, I hope that the above content can be of some help to you, can learn more knowledge. If you think the article is good, you can share it for more people to see.

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