BSON Types类型

Binary Data二进制数据

A BSON binary binData value is a byte array. A binData value has a subtype that indicates how to interpret the binary data. The following table shows the subtypes:BSON二进制binData值是一个字节数组。binData值有一个子类型，指示如何解释二进制数据。下表显示了子类型：

ObjectId

ObjectIds are small, likely unique, fast to generate, and ordered. ObjectId values are 12 bytes in length, consisting of:ObjectId很小，可能是唯一的，生成速度快，并且是有序的。ObjectId值的长度为12个字节，由以下部分组成：

• A 4-byte timestamp, representing the ObjectId's creation, measured in seconds since the Unix epoch.一个4字节的时间戳，表示ObjectId的创建，以Unix纪元以来的秒为单位。
• A 5-byte random value generated once per client-side process. This random value is unique to the machine and process. If the process restarts or the primary node of the process changes, this value is re-generated.每个客户端进程生成一次5字节的随机值。此随机值对于机器和过程是唯一的。如果流程重新启动或流程的主节点发生更改，则会重新生成此值。
• A 3-byte incrementing counter per client-side process, initialized to a random value. The counter resets when a process restarts.每个客户端进程有一个3字节的递增计数器，初始化为随机值。当进程重新启动时，计数器会重置。

For timestamp and counter values, the most significant bytes appear first in the byte sequence (big-endian). This is unlike other BSON values, where the least significant bytes appear first (little-endian).对于时间戳和计数器值，最重要的字节在字节序列中首先出现（大端序）。这与其他BSON值不同，在其他值中，最低有效字节首先出现（小字节序）。

If an integer value is used to create an ObjectId, the integer replaces the timestamp.如果使用整数值创建ObjectId，则该整数将替换时间戳。

In MongoDB, each document stored in a standard collection requires a unique _id field that acts as a primary key. 在MongoDB中，存储在标准集合中的每个文档都需要一个唯一的_id字段作为主键。If an inserted document omits the _id field, the MongoDB driver automatically generates an ObjectId for the _id field.如果插入的文档省略了_id字段，MongoDB驱动程序会自动为_id字段生成一个ObjectId。

This also applies to documents inserted through update operations with upsert: true.这也适用于通过upsert:true更新操作插入的文档。

MongoDB clients should add an _id field with a unique ObjectId. Using ObjectIds for the _id field provides the following additional benefits:MongoDB客户端应添加一个具有唯一ObjectId的_id字段。使用ObjectIds作为_id字段提供了以下额外好处：

• You can access ObjectId creation time in mongosh using the ObjectId.getTimestamp() method.您可以使用ObjectId.getTimestamp()方法访问mongosh中的ObjectId创建时间。

• ObjectIds are approximately ordered by creation time, but are not perfectly ordered. Sorting a collection on an _id field containing ObjectId values is roughly equivalent to sorting by creation time.ObjectId大致按创建时间排序，但并非完全有序。在包含ObjectId值的_id字段上对集合进行排序大致相当于按创建时间排序。

  Important

  While ObjectId values should increase over time, they are not necessarily monotonic. This is because they:虽然ObjectId值应该随着时间的推移而增加，但它们不一定是单调的。这是因为他们：

  • Only contain one second of temporal resolution, so ObjectId values created within the same second do not have a guaranteed ordering, and仅包含一秒的时间分辨率，因此在同一秒内创建的ObjectId值没有保证的顺序，并且
  • Are generated by clients, which may have differing system clocks.由可能具有不同系统时钟的客户端生成。

Use the ObjectId() methods to set and retrieve ObjectId values.使用ObjectId()方法设置和检索ObjectId值。

Starting in MongoDB 5.0, mongosh replaces the legacy mongo shell. 从MongoDB 5.0开始，mongosh取代了传统的mongo shell。The ObjectId() methods work differently in mongosh than in the legacy mongo shell. ObjectId()方法在mongosh中的工作方式与传统mongo shell不同。For more information on the legacy methods, see Legacy mongo Shell.有关遗留方法的更多信息，请参阅遗留mongoShell。

String字符串

BSON strings are UTF-8. In general, drivers for each programming language convert from the language's string format to UTF-8 when serializing and deserializing BSON. BSON字符串是UTF-8。一般来说，在序列化和反序列化BSON时，每种编程语言的驱动程序都会从语言的字符串格式转换为UTF-8。This makes it possible to store most international characters in BSON strings with ease. 这使得可以轻松地将大多数国际字符存储在BSON字符串中。[1] In addition, MongoDB $regex queries support UTF-8 in the regex string.此外，MongoDB$regex查询在正则表达式字符串中支持UTF-8。

Timestamps时间戳

BSON has a special timestamp type for internal MongoDB use and is not associated with the regular Date type. This internal timestamp type is a 64 bit value where:BSON有一个特殊的时间戳类型供MongoDB内部使用，与常规Date类型无关。此内部时间戳类型是一个64位值，其中：

• the most significant 32 bits are a time_t value (seconds since the Unix epoch)最重要的32位是time_t值（自Unix纪元以来的秒数）
• the least significant 32 bits are an incrementing ordinal for operations within a given second.最低有效32位是给定秒内操作的递增ordinal。

While the BSON format is little-endian, and therefore stores the least significant bits first, the mongod instance always compares the time_t value before the ordinal value on all platforms, regardless of endianness.虽然BSON格式是小字节序，因此首先存储最低有效位，但mongod实例在所有平台上总是比较ordinal（序数）值之前的time_t值，而不管字节序如何。

In replication, the oplog has a ts field. The values in this field reflect the operation time, which uses a BSON timestamp value.在复制中，oplog有一个ts字段。此字段中的值反映了使用BSON时间戳值的操作时间。

Within a single mongod instance, timestamp values in the oplog are always unique.在单个mongod实例中，oplog中的时间戳值始终是唯一的。

Date

BSON Date is a 64-bit integer that represents the number of milliseconds since the Unix epoch (Jan 1, 1970). This results in a representable date range of about 290 million years into the past and future.BSON Date是一个64位整数，表示自Unix纪元（1970年1月1日）以来的毫秒数。这导致了过去和未来约2.9亿年的可代表日期范围。

The official BSON specification refers to the BSON Date type as the UTC datetime.官方的BSON规范将BSON日期类型称为UTC日期时间。

BSON Date type is signed. [2] Negative values represent dates before 1970.BSON日期类型已签名。[2]负值表示1970年之前的日期。

To construct a Date in mongosh, you can use the new Date() or ISODate() constructor.要在mongosh中构造Date，可以使用新的Date()或ISODate()构造函数。

var mydate1 = new Date()

mydate1

ISODate("2020-05-11T20:14:14.796Z")

var mydate2 = ISODate()

mydate2

ISODate("2020-05-11T20:14:14.796Z")

mydate1.toString()

Mon May 11 2020 13:14:14 GMT-0700 (Pacific Daylight Time)

mydate1.getMonth()

4

decimal128 BSON Data TypeBSON数据类型

decimal128 is a 128-bit decimal representation for storing very large or very precise numbers, whenever rounding decimals is important. 是一种128位十进制表示，用于存储非常大或非常精确的数字，只要舍入小数很重要。It was created in August 2009 as part of the IEEE 754-2008 revision of floating points. When you need high precision when working with BSON data types, you should use decimal128.它创建于2009年8月，是IEEE 754-2008浮点修订版的一部分。当您在处理BSON数据类型时需要高精度时，您应该使用decimal128。

decimal128 supports 34 decimal digits of precision, or significand along with an exponent range of -6143 to +6144. decimal128支持34位小数精度或有效位，以及-6143到+6144的指数范围。The significand is not normalized in the decimal128 standard, allowing for multiple possible representations: 10 x 10^-1 = 1 x 10^0 = .1 x 10^1 = .01 x 10^2, etc. 有效位未按照decimal128标准进行归一化，允许多种可能的表示：10 x 10^-1 = 1 x 10^0 = .1 x 10^1 = .01 x 10^2，以此类推。Having the ability to store maximum and minimum values in the order of 10^6144 and 10^-6143, respectively, allows for a lot of precision.能够分别以10^6144和10^-6143的顺序存储最大值和最小值，可以获得很高的精度。

Decimal128("9823.1297")

Decimal128.fromStringWithRounding("1234.99999999999")

> 0.1
0.1
> 0.2
0.2
> 0.1 * 0.2
0.020000000000000004
> 0.1 + 0.1
0.010000000000000002

class Main {
   public static void main(String[] args) {
      System.out.println("0.1 * 0.2:");
      System.out.println(0.1 * 0.2);
   }
}

0.1 * 0.2:
0.020000000000000004