Detailed explanation of MongoDB fixed collection

Generally, the collection we create has no size. You can keep adding documents to it. This collection can grow dynamically. There is also a collection in MongoDB called a fixed collection. The size of this collection is fixed. I can Set the number of documents in the collection when creating it, assuming it is 100. When the number of documents in the collection reaches 100, if you insert another document into the collection, only the latest 100 documents will be retained, and the previous documents will be will be deleted. Generally, we can use fixed collections for log information, and other data that need to be deleted regularly can also use fixed collections. In this article, we will take a look at the use of this fixed collection.

This article will share with you a detailed explanation of MongoDB fixed collections.

1.Installing MongoDB on Linux

2.Basic MongoDB operations
3.MongoDB data types
4.MongoDB document update operations
5.MongoDB documents Query operation (1)
6.MongoDB document query operation (2)
7.MongoDB document query operation (3)
8.MongoDB view execution plan
9. First introduction to the index in MongoDB
10. Various types of indexes in MongoDB

Creation

The creation method of fixed collection is also relatively simple, as follows:

db.createCollection("my_collect",{capped:true,size:10000,max:100})

capped:true parameter representation This collection is a fixed-size collection, size represents the size of the collection in kb, and max represents the maximum number of documents in the collection. What we are doing here is equivalent to giving two constraints to the fixed collection. As long as any one of the constraints is met, the collection will start to delete older data. Once a fixed collection is successfully created, it cannot be modified. If you want to modify it, you can only delete it and start again. At this point we can try to add 120 simple pieces of data to the collection, and then we will find that the earliest 20 pieces of data disappear.

In addition to directly creating a fixed collection, we can also convert an ordinary collection into a fixed collection through the convertToCapped operation, as follows:

db.runCommand({convertToCapped:"sang_collect",size:10})

Natural sorting problem

Natural sorting It is to arrange the documents according to the order in which they are on the disk. Natural sorting in ordinary collections does not make much sense, because the position of the documents is always changing, while the documents in the fixed collection are saved in the order in which they are inserted. Natural order is the insertion order of documents, so we can use natural ordering to sort documents from old to new, as follows:

db.sang_collect.find().sort({$natural:1})

You can also sort from new to old:

db.sang_collect.find().sort({$natural:-1})

in a fixed collection Other operations are basically the same as ordinary collections and will not be described here.

The above content is a detailed explanation of MongoDB fixed collections. I hope it can help everyone.

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