mongodb 排序时似乎没有使用多键索引

Question

假设我有 tx_collection，其中有 3 个文档，如下所示

{
    "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
        },
    ]
}

为了性能问题，我在transfers.amount

上创建了多键索引

当我按 transfers.amount、

排序时

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

我期望的文档顺序是按子字段的最大值排序的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
        },
    ]
}

排序效果很好，因为只有 3 个文档。排序顺序是块号 3 -> 块号 1 -> block_number 2 我期望

我的问题是，当有 1900 万个文档时，它会抛出错误消息

按摩很赞

"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.",

排序的时候好像没有用到多键索引

您知道为什么会抛出此错误消息吗？

仅供参考。

我的mongodb版本是3.6.3
tx_collection 已分片

Answer 1

从 MongoDB 3.6 和更新版本开始，我认为这是预期的，正如 Use Indexes to Sort Query Results 中提到的那样：

As a result of changes to sorting behavior on array fields in MongoDB 3.6, when sorting on an array indexed with a multikey index the query plan includes a blocking SORT stage. The new sorting behavior may negatively impact performance.

In a blocking SORT, all input must be consumed by the sort step before it can produce output. In a non-blocking, or indexed sort, the sort step scans the index to produce results in the requested order.

换句话说，“阻塞排序”意味着SORT_KEY_GENERATOR阶段的存在，即内存排序的阶段。这是从 pre-3.6 MongoDB 更改而来的，因为 SERVER-19402 解决了数组字段排序的不一致问题。

有一张票可以改善这种情况：SERVER-31898。不幸的是，目前还没有针对此行为的解决方法。

mongodb 排序时似乎没有使用多键索引

mongodb multikey index seems like not being used when sort

sorting

indexing

mongodb

multikey

我的问题是，当有 1900 万个文档时，它会抛出错误消息

排序的时候好像没有用到多键索引