MongoDB Learning: Database commands and fixed collections

How we want to delete a collection can take the db.test.drop () Way, in fact, behind this, this function is running the drop command. The same effect can be achieved with RunCommand.

Let's start by creating a new collection to use as a test:

> Use Maple1

Switched to DB Maple1

> Db.maple1.insert ({' name ': ' Zhanghongfeng '})

Writeresult ({"ninserted": 1})

Delete this collection, the results of the feedback ok:1 represents the Success

> Db.runcommand ({"Drop": "Maple1"})

{"ns": "Maple1.maple1", "Nindexeswas": 1, "OK": 1}

If the operation fails, there will be a reason why the errmsg prompt failed .

> Db.runcommand ({"Drop": "Maple1"})

{

"OK": 0,

"ErrMsg": "NS not Found",

"Code": 26,

"codename": "Namespacenotfound"

}

If you want to see all the commands running db.listcommands () in the Shell , you can get all the commands. Below is a list of the most common commands for MONGODB.

1 Buildinfo: returns The version number of the MONGODB server and the host's operating system

> Db.runcommand ({"Buildinfo": 1})

{

"Version": "3.4.7",

"Gitversion": "CF38C1B8A0A8DCA4A11737581BEAFEF4FE120BCD",

"Modules": [],

"Allocator": "Tcmalloc",

"Javascriptengine": "Mozjs",

"SysInfo": "Deprecated",

"Versionarray": [

3,

4,

7,

0

],

"OpenSSL": {

"Running": "OpenSSL 1.0.2g 1 Mar 2016",

"Compiled": "OpenSSL 1.0.2g 1 Mar 2016"

},

"Buildenvironment": {

"Distmod": "",

"Distarch": "x86_64",

"CC": "CC:CC (Ubuntu 7.1.0-13ubuntu1) 7.1.0",

"Ccflags": "-fno-omit-frame-pointer-fno-strict-aliasing-ggdb-pthread-wall-wsign-compare-wno-unknown-pragmas- Wno-nonnull-compare-wno-error=c++1z-compat-wno-error=noexcept-type-wno-error=format-truncation-wno-error= Int-in-bool-context-wno-overflow-winvalid-pch-werror-o2-wno-unused-local-typedefs-wno-unused-function- Wno-deprecated-declarations-wno-unused-const-variable-wno-unused-but-set-variable-wno-missing-braces- FSTACK-PROTECTOR-STRONG-FNO-BUILTIN-MEMCMP ",

"Cxx": "g++: g++ (Ubuntu 7.1.0-13ubuntu1) 7.1.0",

"Cxxflags": "-g-o2-fdebug-prefix-map=/build/mongodb-5gk9fl/mongodb-3.4.7=." -fstack-protector-strong-wformat-werror=format-security-woverloaded-virtual-wno-maybe-uninitialized-std=c++11 " ,

"Linkflags": "-wl,-bsymbolic-functions-wl,-z,relro-pthread-wl,-z,now-rdynamic-wl,--fatal-warnings- FSTACK-PROTECTOR-STRONG-FUSE-LD=GOLD-WL,--build-id-wl,-z,noexecstack-wl,--Warn-execstack-wl,-z,relro ",

"Target_arch": "x86_64",

"Target_os": "Linux"

},

"Bits": 64,

"Debug": false,

"Maxbsonobjectsize": 16777216,

"Storageengines": [

"Devnull",

"Ephemeralfortest",

"Mmapv1",

"Wiredtiger"

],

"OK": 1

}

2 Collstats: returns statistics for the specified collection, including data size, allocated storage space, and index size

> Db.runcommand ({"Collstats": "Maple"})

{

"NS": "Maple.maple",

"Size": 234,

"Count": 4,

"Avgobjsize": 58,

"Storagesize": 36864,

"Capped": false,

"Wiredtiger": {

"Metadata": {

"FormatVersion": 1

},

3 DISTINCT: lists all the different values for the specified key of the document in the specified collection that satisfy the query criteria

> Db.runcommand ({"distinct": "Maple", "key": "Name"})

{"Values": ["Zhanghongfeng_maple"], "OK": 1}

4 Drop and dropdatabase: Delete all data from the collection and all data for the current database

5 GetLastError: View the error message or other status information for the last operation performed on this collection.

6 IsMaster: Check whether this server is the primary or the server

> Db.runcommand ({"IsMaster": 1})

{

"IsMaster": true,

"Maxbsonobjectsize": 16777216,

"Maxmessagesizebytes": 48000000,

"Maxwritebatchsize": 1000,

"LocalTime": Isodate ("2017-12-27t13:45:21.228z"),

"Maxwireversion": 5,

"Minwireversion": 0,

"ReadOnly": false,

"OK": 1

}

7 Listcommands: Returns all commands that can be run on the server

8 listdatabases: list all databases on the server

9 Ping: detects if the link to the server is normal

> Db.runcommand ({"Ping": 1})

{"OK": 1}

10

Fixed collection:

As the name implies, a fixed collection is a collection that is able to fix the size of the collection and the number of documents. The fixed collection is created using createcollection. Size, as shown below , represents the sizes of the collection, andMax represents the maximum number of documents

> db.createcollection ("Fixedcollection", {capped:true,size:1000,max:2})

{"OK": 1}

Insert 2 documents

>db.fixedcollection.insert ({"Name": "Zhanghongfeng", "Age": 33})

> Db.fixedcollection.insert ({"Name": "Maple", "Age": 32})

Show 2 documents inserted

> Db.fixedcollection.find ()

{"_id": ObjectId ("5a43a73ba889535e61dc29a8"), "name": "Zhanghongfeng", "Age": 33}

{"_id": ObjectId ("5a43a77ba889535e61dc29a9"), "name": "Maple", "Age": 32}

Insert a document again

> Db.fixedcollection.insert ({"Name": "ZHF", "Age": 32})

Writeresult ({"ninserted": 1})

By querying, you can see that the oldest inserted document, Name:zhanghongfeng , was replaced when a document was inserted. This is because we specify a maximum number of 2 documents in a fixed collection . As a result, the newly inserted document replaces the previous document, replacing the earliest established document with the replacement method.

> Db.fixedcollection.find ()

{"_id": ObjectId ("5a43a77ba889535e61dc29a9"), "name": "Maple", "Age": 32}

{"_id": ObjectId ("5a43a78ca889535e61dc29aa"), "name": "ZHF", "Age": 32}

Fixed collections are arranged in the same way as ring queues, by specifying the order in which queries are found , $natural: -1 follows the document's insertion time from the early order of arrival, that is, starting from the end of the queue. $natural: 1 is starting from the head of the queue

> Db.fixedcollection.find (). Sort ({"$natural":-1})

{"_id": ObjectId ("5a43a78ca889535e61dc29aa"), "name": "ZHF", "Age": 32}

{"_id": ObjectId ("5a43a77ba889535e61dc29a9"), "name": "Maple", "Age": 32}

> Db.fixedcollection.find (). Sort ({"$natural": 1})

{"_id": ObjectId ("5a43a77ba889535e61dc29a9"), "name": "Maple", "Age": 32}

{"_id": ObjectId ("5a43a78ca889535e61dc29aa"), "name": "ZHF", "Age": 32}

MongoDB Learning: Database commands and fixed collections