MongoDB的skip,limit,sort执行顺序，以及aggregate_jason_cuijiahui的博客

如果要改变执行顺序，用aggregate

aggregate有管道流的性质，
db.memos.aggregate(
{ $match: { status: 'P' } },
{ $limit:5},
{ $skip:2},
{ $sort: { age : -1 } }
aggregate的用法 
>>> from pymongo import MongoClient
>>> db = MongoClient().aggregation_example
>>> result = db.things.insert_many([{"x": 1, "tags": ["dog", "cat"]},
...                                 {"x": 2, "tags": ["cat"]},
...                                 {"x": 2, "tags": ["mouse", "cat", "dog"]},
...                                 {"x": 3, "tags": []}])
>>> result.inserted_ids
[ObjectId('...'), ObjectId('...'), ObjectId('...'), ObjectId('...')] 
>>> from bson.son import SON
>>> pipeline = [
...     {"$unwind": "$tags"},
...     {"$group": {"_id": "$tags", "count": {"$sum": 1}}},
...     {"$sort": SON([("count", -1), ("_id", -1)])}
... ]
>>> import pprint
>>> pprint.pprint(list(db.things.aggregate(pipeline)))
[{u'_id': u'cat', u'count': 3},
 {u'_id': u'dog', u'count': 2},
 {u'_id': u'mouse', u'count': 1}] 
document: 
"_id" : ObjectId("57506d74c469888f0d631be6"),
"LOC" : "User001",
"COL" : [ 
        "date" : "25/03/2016",
        "number" : "Folio009",
        "amount" : 100
        "date" : "25/04/2016",
        "number" : "Folio010",
        "amount" : 100
I assume you have a valid connection to MongoDB in Python. 
 The following code snippet will return a MongoDB cursor in result. 
pipeline = [
    {"$unwind": "$COL"},
    {"$group": {"_id": "$LOC", "sum": {"$sum": "$COL.amount"}}}
cursor = collection.aggregate(pipeline) 
Now you can convert cursor to list 
result = list(cursor) 
and if you print result’s value, you’ll get exactly the same result as in your Shell query. 
[{u'sum': 200.0, u'_id': u'User001'}] 
优化： 
 https://docs.mongodb.com/manual/reference/operator/aggregation/sort/ 
 https://blog.csdn.net/suyu_yuan/article/details/51766483
                    执行顺序： https://blog.csdn.net/thewindkee/article/details/54670750优化： https://docs.mongodb.com/manual/reference/operator/aggregation/sort/ https://blog.csdn.net/suyu_yuan/article/details/51766483...
				SORT AGGREGATE做为sort的option之一比较特殊，他并不做sort
SORT AGGREGATE作用于所有的data set上，用于aggregate function，例如sum, count, avg, min, max
select max(x)
from test;
Execution 
-----------------
				转自：http://docs.mongoing.com/manual-zh/core/aggregation-pipeline-optimization.html
1、db.memos.find({})
查询memos文档结果；
2、db.memos.aggregate({$skip:3})
跳过查询结果前三行；
3、db.memos.aggregate({$limit:3})
				Oracle10g：
create table t_count as select * from dba_objects;
create index t_count_i on t_count(object_id):
select count(*) from t_count;
select count(object_id) from t_count;
select co
				文章目录一。概念二。集合示例准备三。各个操作符的用法1.$lookup2.$match3.$unwind4.$project5.$limit6.$skip7.$group8.$sort
管道的概念
管道在Unix和Linux中一般用于将当前命令的输出结果作为下一个命令的参数。
MongoDB的聚合管道将MongoDB文档在一个管道处理完毕后将结果传递给下一个管道处理。管道操作是可以重复的。
MongoDB中聚合(aggregate)主要用于处理数据(诸如统计平均值，求和等)，并返回计算
最近在学习mongoDB的使用，本文来介绍一下其中aggregate的具体使用
先来看一个分组的例子，本例中$group是一个管道操作符，获得的结果可以接着输出到下一个管道，而内部的$sum是一个表达式操作符。
用$group 举个例子
				文章目录hash内存hashexternal hashsort内存sortexternal sortshufflehash shufflesort shuffleaggregatehash aggregatesort aggregate分布式aggregatejoinnested loop joinhash joinsort merge join分布式join总结
hash算法是指对于key计算出hash值，基于hash值识别相同key，从而进行归类、聚合等操作。
内存hash
类似于JAVA H