mongodb多键索引似乎在排序时未使用

让我们假设我有tx_collection，它具有如下3个文档

Let's assume that I have tx_collection which has 3 documents like below

{
    "block_number": 1,
    "value": 122
    "transfers": [
        {
            "from": "foo1", 
            "to": "bar1", 
            "amount": 111
        },
        {
            "from": "foo3", 
            "to": "bar3", 
            "amount": 11
        },
    ]
},
{
    "block_number": 2,
    "value": 88
    "transfers": [
        {
            "from": "foo11", 
            "to": "bar11", 
            "amount": 33
        },
        {
            "from": "foo22", 
            "to": "bar22", 
            "amount": 55
        },
    ]
},
{
    "block_number": 3,
    "value": 233
    "transfers": [
        {
            "from": "foo1", 
            "to": "bar1", 
            "amount": 33
        },
        {
            "from": "foo3", 
            "to": "bar3", 
            "amount": 200
        },
    ]
}

对于性能问题，我在transfers.amount

当我按transfers.amount排序时，

db.getCollection('tx_transaction').find({}).sort({"transfers.amount":-1})

我希望文档的顺序是按子字段transfers.amount的最大值排序的，例如

what I expected order of documents is sorted by max value of subfield transfers.amount like

{
    "block_number": 3,
    "value": 233
    "transfers": [
        {
            "from": "foo1", 
            "to": "bar1", 
            "amount": 33
        },
        {
            "from": "foo3", 
            "to": "bar3", 
            "amount": 200
        },
    ]
},
{
    "block_number": 1,
    "value": 122
    "transfers": [
        {
            "from": "foo1", 
            "to": "bar1", 
            "amount": 111
        },
        {
            "from": "foo3", 
            "to": "bar3", 
            "amount": 11
        },
    ]
},
{
    "block_number": 2,
    "value": 88
    "transfers": [
        {
            "from": "foo11", 
            "to": "bar11", 
            "amount": 33
        },
        {
            "from": "foo22", 
            "to": "bar22", 
            "amount": 55
        },
    ]
}

由于只有3个文档，因此排序效果很好.排序顺序是我期望的3号块-> 1号块-> 2号块

The sort works well since there are only 3 documents. Sorted order is block number 3 -> block number 1 -> block_number 2 which I expected

按摩就像

"errmsg" : "Executor error during find command: OperationFailed: Sort operation used more than the maximum 33554432 bytes of RAM. Add an index, or specify a smaller limit.",

似乎在排序时不使用多键索引.

您知道为什么会引发此错误消息吗?

It seems that multikey index is not used when sort.

do you have any idea why this error message is thrown?

JFYI.

• 我的mongodb版本是3.6.3
• tx_collection被分片