cursor.skip()

On this page本页内容

Definition定义

cursor.skip(<offset>)
Important重要
mongosh Method

This is a mongosh method. This is not the documentation for Node.js or other programming language specific driver methods.

In most cases, mongosh methods work the same way as the legacy mongo shell methods. However, some legacy methods are unavailable in mongosh.

For the legacy mongo shell documentation, refer to the documentation for the corresponding MongoDB Server release:

For MongoDB API drivers, refer to the language specific MongoDB driver documentation.

Call the skip() method on a cursor to control where MongoDB begins returning results. 在游标上调用skip()方法以控制MongoDB开始返回结果的位置。This approach may be useful in implementing paginated results.这种方法在实现分页结果时可能很有用。

Note注意

You must apply skip() to the cursor before retrieving any documents from the database.在从数据库检索任何文档之前，必须将skip()应用于游标。

The skip() method has the following parameter:skip()方法具有以下参数：

Parameter参数Type类型Description描述
offsetnumberThe number of documents to skip in the results set.结果集中要跳过的文档数。

Behavior行为

Using skip() with sort()使用skip()和sort()

If using skip() with sort(), be sure to include at least one field in your sort that contains unique values, before passing results to skip().如果将skip()与sort()结合使用，请确保在将结果传递给skip()之前，在排序中至少包含一个包含唯一值的字段。

Sorting on fields that contain duplicate values may return an inconsistent sort order for those duplicate fields over multiple executions, especially when the collection is actively receiving writes.对包含重复值的字段进行排序可能会在多次执行中返回这些重复字段的不一致排序顺序，尤其是当集合正在积极接收写入时。

The easiest way to guarantee sort consistency is to include the _id field in your sort query.确保排序一致性的最简单方法是在排序查询中包含_id字段。

See Consistent sorting with the sort() method for more information.有关详细信息，请参阅使用sort()方法进行一致排序。

Pagination Example分页示例

Using 使用skip()

The following JavaScript function uses skip() to paginate a collection by its _id field:以下JavaScript函数使用skip()按集合的_id字段为其分页：

function printStudents(pageNumber, nPerPage) {
  print( "Page: " + pageNumber );
  db.students.find()
             .sort( { _id: 1 } )
             .skip( pageNumber > 0 ? ( ( pageNumber - 1 ) * nPerPage ) : 0 )
             .limit( nPerPage )
             .forEach( student => {
               print( student.name );
             } );
}

The skip() method requires the server to scan from the beginning of the input results set before beginning to return results. skip()方法要求服务器在开始返回结果之前从输入结果集的开头进行扫描。As the offset increases, skip() will become slower.随着偏移量的增加，skip()将变慢。

Using Range Queries使用范围查询

Range queries can use indexes to avoid scanning unwanted documents, typically yielding better performance as the offset grows compared to using skip() for pagination.范围查询可以使用索引来避免扫描不需要的文档，与使用skip()进行分页相比，当偏移量增加时，通常会产生更好的性能。

Descending Order降序

Use this procedure to implement pagination with range queries:使用以下步骤可以使用范围查询实现分页：

  • Choose a field such as _id which generally changes in a consistent direction over time and has a unique index to prevent duplicate values,选择一个字段（如_id），该字段通常随时间以一致的方向变化，并具有唯一索引以防止重复值，
  • Query for documents whose field is less than the start value using the $lt and sort() operators, and使用$lt和sort()运算符查询字段小于起始值的文档，以及
  • Store the last-seen field value for the next query.存储上次看到的字段值以供下次查询使用。

For example, the following function uses the above procedure to print pages of student names from a collection, sorted approximately in order of newest documents first using the _id field (that is, in descending order):例如，以下函数使用上述过程打印集合中的学生姓名页面，首先使用_id字段（即降序）大致按照最新文档的顺序排序：

function printStudents(startValue, nPerPage) {
  let endValue = null;
  db.students.find( { _id: { $lt: startValue } } )
             .sort( { _id: -1 } )
             .limit( nPerPage )
             .forEach( student => {
               print( student.name );
               endValue = student._id;
             } );
  return endValue;
}

You may then use the following code to print all student names using this pagination function, using MaxKey to start from the largest possible key:然后，您可以使用以下代码打印使用此分页功能的所有学生姓名，使用MaxKey从可能的最大键开始：

let currentKey = MaxKey;
while (currentKey !== null) {
  currentKey = printStudents(currentKey, 10);
}
Note注意

While ObjectId values should increase over time, they are not necessarily monotonic. 虽然ObjectId值应随时间增加，但它们不一定是单调的。This is because they: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仅包含一秒钟的时间分辨率，因此在同一秒钟内创建的ObjectId值没有保证的顺序，并且
  • Are generated by clients, which may have differing system clocks.由客户端生成，可能具有不同的系统时钟。

Ascending Order升序

Returning paginated results in ascending order is similar to the previous, but uses $gt with an ascending sort order:以升序返回分页结果与前一个类似，但使用$gt和升序排序：

function printStudents(startValue, nPerPage) {
  let endValue = null;
  db.students.find( { _id: { $gt: startValue } } )
             .sort( { _id: 1 } )
             .limit( nPerPage )
             .forEach( student => {
               print( student.name );
               endValue = student._id;
             } );
  return endValue;
}

Using this function is likewise similar, but with MinKey as the starting key:使用此功能类似，但以MinKey作为起始键：

let currentKey = MinKey;
while (currentKey !== null) {
  currentKey = printStudents(currentKey, 10);
}
←  cursor.size()cursor.sort() →