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cursor.skip()

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Definition定义

cursor.skip(<offset>)
Important

mongosh Method

This page documents a mongosh method. This is not the documentation for a language-specific driver, such as Node.js.

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. This approach may be useful in implementing paginated results.在游标上调用skip()方法来控制MongoDB从哪里开始返回结果。这种方法在实现分页结果时可能很有用。

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()将skip()与sort()结合使用

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()一起使用，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为这些重复字段返回不一致的排序顺序，尤其是当集合正在主动接收写入时。

The easiest way to guarantee sort consistency is to include the _id field in your sort query.保证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。

Using skip() with limit()将skip()与limit()一起使用

When you chain skip() and limit(), the method chaining order does not affect the results. The server always applies the skip operation based on the sort order before it applies the limit on how many documents to return.当您连缀skip()和limit()时，方法连缀顺序不会影响结果。服务器在应用要返回的文档数量限制之前，总是根据排序顺序应用跳过操作。

The following code example shows different chaining orders for skip() and limit() that always produce the same query results for the same data set:以下代码示例显示了skip()和limit()的不同连缀顺序，它们总是为同一数据集生成相同的查询结果：

db.myColl.find().sort({_id: 1}).skip(3).limit(6);

db.myColl.find().sort({_id: 1}).limit(6).skip(3);

Pagination Example分页示例

Using skip()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()通过其_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始进行扫描。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，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:使用以下过程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,选择诸如_id之类的字段，该字段通常随时间以一致的方向变化并且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.为下一个查询存储上次看到的字段值。

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，这些页面首先使用_id字段（即按降序）按最新文档的顺序大致排序：

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的姓名，使用MaxKey从尽可能大的键开始：

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

While ObjectId values should increase over time, they are not necessarily monotonic. 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只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，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 $gt with an ascending sort order:按升序返回分页结果与前面类似，但使用$gt并按升序排序：

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，但使用MinKey作为起始键：

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