mongodb的聚合函数的$skip + $limit 方法运用和顺序优化。

最新推荐文章于 2024-02-07 16:39:44 发布

suyu_yuan

最新推荐文章于 2024-02-07 16:39:44 发布

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本文介绍MongoDB中聚合管道的优化技巧，特别是针对$skip和$limit操作符的使用顺序进行优化，以提高查询效率。

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转自：http://docs.mongoing.com/manual-zh/core/aggregation-pipeline-optimization.html

`$skip` + `$limit` 顺序优化

如果你的管道中， $skip 后面跟着 $limit ，优化器会把 $limit 移到 $skip 前面，这个时候，$limit 的值会加上 $skip 的个数。

例如，如果管道由以下部分组成：

 
  { $skip: 10 },
{ $limit: 5 }

During the optimization phase, the optimizer transforms the sequence to the following:

 
  { $limit: 15 },
{ $skip: 10 }

对于类似 $sort + $limit 合并，例如 $sort + $skip + $limit ，优化器允许你做很多优化。详情请查看 $sort + $limit 合并，也可以在 $sort + $skip + $limit 顺序中查看例子。

对于在分片集合上的聚合，优化器可以减少从每个分片返回的文档个数。

1、db.memos.find({})
查询memos文档结果；

2、db.memos.aggregate({$skip:3})
跳过查询结果前三行；

3、db.memos.aggregate({$limit:3})

获取查询结果前三行；

1、db.memos.find({})如下：

{
"_id" : 1.0,
"name" : "sue",
"age" : 19.0,
"type" : 1.0,
"status" : "P",
"favorites" : {
"artist" : "Picasso",
"food" : "pizza"
},
"finished" : [
17.0,
3.0
],
"badges" : [
"blue",
"black"
],
"points" : [
{
"points" : 85.0,
"bonus" : 20.0
},
{
"points" : 85.0,
"bonus" : 10.0
}
]
}
{
"_id" : 6.0,
"name" : "abc",
"age" : 43.0,
"type" : 1.0,
"status" : "A",
"favorites" : {
"food" : "pizza",
"artist" : "Picasso"
},
"finished" : [
18.0,
12.0
],
"badges" : [
"black",
"blue"
],
"points" : [
{
"points" : 78.0,
"bonus" : 8.0
},
{
"points" : 57.0,
"bonus" : 7.0
}
]
}
{
"_id" : 7.0,
"name" : "abc",
"age" : 43.0,
"type" : 1.0,
"status" : "A",
"favorites" : {
"food" : "pizza",
"artist" : "Picasso"
},
"finished" : [
21.0,
14.0
],
"badges" : [
"black",
"blue"
],
"points" : [
{
"points" : 78.0,
"bonus" : 8.0
},
{
"points" : 57.0,
"bonus" : 7.0
}
]
}
{
"_id" : 8.0,
"name" : "sue",
"age" : 49.0,
"type" : 1.0,
"status" : "P",
"favorites" : {
"artist" : "Picasso",
"food" : "pizza"
},
"finished" : [
17.0,
3.0
],
"badges" : [
"blue",
"black"
],
"points" : [
{
"points" : 85.0,
"bonus" : 20.0
},
{
"points" : 85.0,
"bonus" : 10.0
}
]
}
{
"_id" : 9.0,
"name" : "sue",
"age" : 19.0,
"type" : 1.0,
"status" : "P",
"favorites" : {
"artist" : "Picasso",
"food" : "pizza"
},
"finished" : [
17.0,
3.0
],
"badges" : [
"blue",
"black"
],
"points" : [
{
"points" : 85.0,
"bonus" : 20.0
},
{
"points" : 85.0,
"bonus" : 10.0
}
]
}
{
"_id" : 10.0,
"name" : "abc",
"age" : 43.0,
"type" : 1.0,
"status" : "A",
"favorites" : {
"food" : "pizza",
"artist" : "Picasso"
},
"finished" : [
18.0,
12.0
],
"badges" : [
"black",
"blue"
],
"points" : [
{
"points" : 78.0,
"bonus" : 8.0
},
{
"points" : 57.0,
"bonus" : 7.0
}
]
}
{
"_id" : 11.0,
"name" : "abc",
"age" : 43.0,
"type" : 1.0,
"status" : "A",
"favorites" : {
"food" : "pizza",
"artist" : "Picasso"
},
"finished" : [
21.0,
14.0
],
"badges" : [
"black",
"blue"
],
"points" : [
{
"points" : 78.0,
"bonus" : 8.0
},
{
"points" : 57.0,
"bonus" : 7.0
}
]
}
{
"_id" : 12.0,
"name" : "sue",
"age" : 49.0,
"type" : 1.0,
"status" : "P",
"favorites" : {
"artist" : "Picasso",
"food" : "pizza"
},
"finished" : [
17.0,
3.0
],
"badges" : [
"blue",
"black"
],
"points" : [
{
"points" : 85.0,
"bonus" : 20.0
},
{
"points" : 85.0,
"bonus" : 10.0
}
]
}