MongoDB 聚合结果大小限制

The aggregate command can return either a cursor or store the results in a collection. When returning a cursor or storing the results in a collection, each document in the result set is subject to the BSON Document Size limit, currently 16 megabytes; if any single document that exceeds the BSON Document Size limit, the command will produce an error. The limit only applies to the returned documents; during the pipeline processing, the documents may exceed this size. The db.collection.aggregate() method returns a cursor by default.

each document in the result set is subject to the BSON Document Size limit, currently 16 megabytes

我想知道这个 result set 是否就是 aggregate 返回的 result。如果是，那么 result set 中的单个元素的大小不能超过 16MB，否则整个 result set 的大小总和不能超过 16MB。

结论是 result 中的单个文件不能超过限制。

使用两个 10 MB 的文件进行模拟：

from pymongo import MongoClient
from unittest import TestCase

class TestAggregateSizeLimit(TestCase):

    def setUp(self):
        self.client = MongoClient()
        self.coll = self.client['test-database']['test-collection']

        with open('10mb.txt', 'r') as f:
            content = f.read()

        self.coll.insert_one({
            'filename': 1,
            'content': content
        })
        self.coll.insert_one({
            'filename': 2,
            'content': content
        })

    def tearDown(self):
        self.client.drop_database('test-database')

    def test_two_aggregate_result(self):
        result = list(self.coll.aggregate(
            [
                {'$sort': {'_id': 1}},
                {'$group': {'_id': '$filename', 'content': {'$first': '$content'}}}
            ]
        ))

        if result:
            print('多个文件总和超过 16 MB，但是单个文件没有超过 16MB，没有问题')
        else:
            print('多个文件总和超过 16 MB，但是单个文件没有超过 16MB，有问题')

    def test_one_aggregate_result(self):
        try:
            list(self.coll.aggregate(
                [
                    {'$group': {'_id': None, 'content': {'$push': '$content'}}}
                ]
            ))
        except Exception as e:
            # pymongo==2.8 报错 “$cmd failed: aggregation result exceeds maximum document size (16MB)”
            # pymongo==3.7.0 报错 “BSONObj size: 20971635 (0x1400073) is invalid. Size must be between 0 and 16793600(16MB) ”
            print(e)
            print('结果中的单个文件超过 16MB，有问题')
        else:
            print('结果中的单个文件超过 16MB，没有问题')

完整代码见 https://github.com/Jay54520/playground/tree/master/mongodb_size_limit

另外，在搜索过程中发现有人说 allowDiskUse 可以解除这个限制，这个是错误的。allowDiskUse 用于避免 pipeline 的 stage 的内存使用超过 100 MB 而报错，而上面的限制是针对单个文件而言。

Pipeline stages have a limit of 100 megabytes of RAM. If a stage exceeds this limit, MongoDB will produce an error. To allow for the handling of large datasets, use the allowDiskUse option to enable aggregation pipeline stages to write data to temporary files.[2]

参考

1. https://docs.mongodb.com/manual/core/aggregation-pipeline-limits/#result-size-restrictions
2. https://docs.mongodb.com/manual/core/aggregation-pipeline-limits/#memory-restrictions