cursor.skip()

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Definition定义

cursor.skip(<offset>)

mongo Shell Method

This page documents the mongo shell method, and does not refer to the MongoDB Node.js driver (or any other driver) method. 本页记录了mongo shell方法，未提及MongoDB Node.js驱动程序（或任何其他驱动程序）方法。For corresponding MongoDB driver API, refer to your specific MongoDB driver documentation instead.有关相应的MongoDB驱动程序API，请参阅特定的MongoDB驱动程序文档。

Call the skip() method on a cursor to control where MongoDB begins returning results. 对游标调用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.这种方法可能有助于实现分页结果。

Note

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，必须对游标应用skip()。

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
offset number The 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()将skip()与sort()一起使用

If using skip() with sort(), be sure that you are performing a stable sort before passing results to skip(). 如果将skip()与sort()一起使用，请确保在将结果传递给skip()之前执行了稳定的排序。A stable sort ensures that the sort order of returned documents remains the same across multiple executions of the same sort; especially important when used with the skip() method.稳定的排序可确保在多次执行相同排序时，返回文档的排序顺序保持不变；与skip()方法一起使用时尤其重要。

See Stable sorting with the sort() method for more information.有关更多信息，请参阅使用sort()方法进行稳定排序。

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字段对集合进行分页：

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开始扫描。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()分页相比，随着偏移量的增加，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，并具有唯一的索引以防止重复值，
  • 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.为下一次查询存储最后一次看到的字段值。

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）按最新文档的大致顺序排序：

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使用此分页功能打印所有学生姓名，使用MaxKey从可能的最大键开始：

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

Note

While ObjectId values should increase over time, they are not necessarily monotonic. 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。This is because they:这是因为：

  • 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，这些客户端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 $gt with an ascending sort order:按升序返回分页结果与上一个类似，但使用升序排序的$gt：

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);
}