MongoDB:如何更改_id_索引的引擎类型(从B树到LSM树)？

Question

我们可以使用WiredTiger engine和type=lsm创建一个集合，但是在MongoDB文档中没有提到这个特性：

db.createCollection(
    "test",
    { storageEngine: { wiredTiger: {configString: "type=lsm"}}}
)

一旦插入了一些文档并添加了索引，看起来WiredTiger真的创建了LSM文件。

db.test.insert([
    { value: 1},
    { value: 2},
    { value: 3}
]) // Done in 16:04
db.test.createIndex(
    { value: 1 },
    { storageEngine: { wiredTiger: {configString: "type=lsm"}}}
) // Done in 19:59

$ ls -ltr
...
-rw-r--r--. 1 mongod mongod        16384 Jan 15 16:04 collection-0-1708338433081558809-000002.lsm
-rw-r--r--. 1 mongod mongod        16384 Jan 15 16:04 index-1-1708338433081558809.wt
-rw-r--r--. 1 mongod mongod        16384 Jan 15 19:59 index-3-1708338433081558809-000002.lsm

集合和索引树看起来像LSM- value_1，但是索引_id_看起来仍然像B-树。

如何更改索引_id的引擎类型

Answer 1

这不是你想听到的答案，但目前是不可能的。

_id是一种比较特殊的索引。来自https://github.com/mongodb/mongo/blob/73b456d5c059b17d1c7f0f8badb7c72391ee2173/src/mongo/db/catalog/index_key_validate.cpp#L74

所有索引的规范验证器：

static std::set<StringData> allowedFieldNames = {
    IndexDescriptor::k2dIndexBitsFieldName,
    IndexDescriptor::k2dIndexMaxFieldName,
    IndexDescriptor::k2dIndexMinFieldName,
    IndexDescriptor::k2dsphereCoarsestIndexedLevel,
    IndexDescriptor::k2dsphereFinestIndexedLevel,
    IndexDescriptor::k2dsphereVersionFieldName,
    IndexDescriptor::kBackgroundFieldName,
    IndexDescriptor::kCollationFieldName,
    IndexDescriptor::kDefaultLanguageFieldName,
    IndexDescriptor::kDropDuplicatesFieldName,
    IndexDescriptor::kExpireAfterSecondsFieldName,
    IndexDescriptor::kGeoHaystackBucketSize,
    IndexDescriptor::kIndexNameFieldName,
    IndexDescriptor::kIndexVersionFieldName,
    IndexDescriptor::kKeyPatternFieldName,
    IndexDescriptor::kLanguageOverrideFieldName,
    IndexDescriptor::kNamespaceFieldName,
    IndexDescriptor::kPartialFilterExprFieldName,
    IndexDescriptor::kPathProjectionFieldName,
    IndexDescriptor::kSparseFieldName,
    IndexDescriptor::kStorageEngineFieldName,
    IndexDescriptor::kTextVersionFieldName,
    IndexDescriptor::kUniqueFieldName,
    IndexDescriptor::kWeightsFieldName,
    // Index creation under legacy writeMode can result in an index spec with an _id field.
    "_id"};

_id索引允许的规范：

static const std::set<StringData> allowedIdIndexFieldNames = {
    IndexDescriptor::kCollationFieldName,
    IndexDescriptor::kIndexNameFieldName,
    IndexDescriptor::kIndexVersionFieldName,
    IndexDescriptor::kKeyPatternFieldName,
    IndexDescriptor::kNamespaceFieldName,
    // Index creation under legacy writeMode can result in an index spec with an _id field.
    "_id"};

如您所见，除了名称、排序规则、版本等之外，您可以灵活地更改任何内容。其中没有kStorageEngineFieldName。