MongoDB的skip,limit,sort执行顺序，以及aggregate

执行顺序

.find().sort().skip().limit()中.sort().skip().limit()的各种排列组合。
当sort,skip,limit一起使用时，无论其位置变化，总是先sort再skip，最后limit。

如果要改变执行顺序，用aggregate

aggregate有管道流的性质， skip, s k i p , limit,$sort执行顺序跟位置一致，memos是collection。现在的顺序：match -> limit -> skip -> sort

db.memos.aggregate(
{ $match: { status: 'P' } },
{ $limit:5},
{ $skip:2},
{ $sort: { age : -1 } }
)

aggregate的用法

例子1

>>> 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}]

例子2

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