MongoDB后台索引大小

Question

我正在创建一个索引({ a : 1, b : -1 })，一次在前台，其他在后台进行。检查总索引大小。前台:82536720背景:154927024

有人能告诉我为什么会有这么大的尺寸差异吗？

Mongo > db.testing.ensureIndex({a:1,b:-1})
Mongo > db.testing.stats()
{
    "ns" : "test.testing",
    "count" : 2402158,
    "size" : 86479836,
    "avgObjSize" : 36.00089419596879,
    "totalIndexSize" : 160486704,
    "indexSizes" : {
        "_id_" : 77949984,
        "a_1_b_-1" : 82536720
    },
}
Mongo > db.testing.dropIndexes()
Mongo > db.testing.ensureIndex({a:1,b:-1},{background : true})
Mongo > db.testing.stats()
{
    "count" : 2402158,
    "size" : 86479836,
    "avgObjSize" : 36.00089419596879,
    "totalIndexSize" : 232877008,
    "indexSizes" : {
        "_id_" : 77949984,
        "a_1_b_-1" : 154927024
    },
}

Answer 1

MongoDB documentation说

Background index construction allows read and write operations to continue while building the index; however, these index builds take longer to complete and result in a larger index.

所以，不足为奇。基本上，mongo 使用与前台索引不同的机制来构建后台索引，这导致最终索引更大。