我有一个mongos设置，包含2个分片和以下数据：
for (var i = 1; i <= 1000; i++) {
db.items.insert({item: "a", i_type: "x", i_id: i, price: i * 50});
db.items.insert({item: "a", i_type: "y", i_id: i, price: i * 50});
db.items.insert({item: "b", i_type: "x", i_id: i, price: i * 50});
db.items.insert({item: "b", i_type: "y", i_id: i, price: i * 50});
}
db.items.createIndex({item: 1, i_type: 1, i_id: 1}, {unique: true})
db.items.createIndex({item: 1, i_type: 1, price: 1})
sh.enableSharding("test")
sh.shardCollection("test.items", {item: 1, i_type: 1})
我跑了这个：
db.items.find(
{item: "a", i_type: {$in: ["x", "y"]}},
{_id: 0, item: 1, i_type: 1}
).sort({price: 1}).limit(10).explain("executionStats")
获取以下executionStats：
"nReturned" : NumberInt(10),
"executionTimeMillis" : NumberInt(22),
"totalKeysExamined" : NumberInt(2000),
"totalDocsExamined" : NumberInt(2000),
"executionStages" : {
"stage" : "SINGLE_SHARD",
"nReturned" : NumberInt(10),
"executionTimeMillis" : NumberInt(22),
"totalKeysExamined" : NumberInt(2000),
"totalDocsExamined" : NumberInt(2000),
"totalChildMillis" : NumberLong(21),
"shards" : [
{
"shardName" : "rs1",
"executionSuccess" : true,
"executionStages" : {
"stage" : "PROJECTION",
"nReturned" : NumberInt(10),
"executionTimeMillisEstimate" : NumberInt(0),
"works" : NumberInt(2013),
"advanced" : NumberInt(10),
"needTime" : NumberInt(2002),
"needFetch" : NumberInt(0),
"saveState" : NumberInt(47),
"restoreState" : NumberInt(47),
"isEOF" : NumberInt(1),
"invalidates" : NumberInt(0),
"transformBy" : {
"_id" : NumberInt(0),
"item" : NumberInt(1),
"i_type" : NumberInt(1)
},
"inputStage" : {
"stage" : "SORT",
"nReturned" : NumberInt(10),
"executionTimeMillisEstimate" : NumberInt(0),
"works" : NumberInt(2013),
"advanced" : NumberInt(10),
"needTime" : NumberInt(2001),
"needFetch" : NumberInt(0),
"saveState" : NumberInt(47),
"restoreState" : NumberInt(47),
"isEOF" : NumberInt(1),
"invalidates" : NumberInt(0),
"sortPattern" : {
"price" : NumberInt(1)
},
"memUsage" : NumberInt(850),
"memLimit" : NumberInt(33554432),
"limitAmount" : NumberInt(10),
"inputStage" : {
"stage" : "FETCH",
"nReturned" : NumberInt(2000),
"executionTimeMillisEstimate" : NumberInt(0),
"works" : NumberInt(2001),
"advanced" : NumberInt(2000),
"needTime" : NumberInt(0),
"needFetch" : NumberInt(0),
"saveState" : NumberInt(47),
"restoreState" : NumberInt(47),
"isEOF" : NumberInt(1),
"invalidates" : NumberInt(0),
"docsExamined" : NumberInt(2000),
"alreadyHasObj" : NumberInt(0),
"inputStage" : {
"stage" : "SHARDING_FILTER",
"nReturned" : NumberInt(2000),
"executionTimeMillisEstimate" : NumberInt(0),
"works" : NumberInt(2001),
"advanced" : NumberInt(2000),
"needTime" : NumberInt(0),
"needFetch" : NumberInt(0),
"saveState" : NumberInt(47),
"restoreState" : NumberInt(47),
"isEOF" : NumberInt(1),
"invalidates" : NumberInt(0),
"chunkSkips" : NumberInt(0),
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : NumberInt(2000),
"executionTimeMillisEstimate" : NumberInt(0),
"works" : NumberInt(2001),
"advanced" : NumberInt(2000),
"needTime" : NumberInt(0),
"needFetch" : NumberInt(0),
"saveState" : NumberInt(47),
"restoreState" : NumberInt(47),
"isEOF" : NumberInt(1),
"invalidates" : NumberInt(0),
"keyPattern" : {
"item" : NumberInt(1),
"i_type" : NumberInt(1),
"price" : NumberInt(1)
},
"indexName" : "item_1_i_type_1_price_1",
"isMultiKey" : false,
"direction" : "forward",
"indexBounds" : {
"item" : [
"[\"a\", \"a\"]"
],
"i_type" : [
"[\"x\", \"x\"]",
"[\"y\", \"y\"]"
],
"price" : [
"[MinKey, MaxKey]"
]
},
"keysExamined" : NumberInt(2000),
"dupsTested" : NumberInt(0),
"dupsDropped" : NumberInt(0),
"seenInvalidated" : NumberInt(0),
"matchTested" : NumberInt(0)
}
}
}
}
}
}
]
}
我很难理解为什么需要在分片上检查包含{item: "a", i_type: "x"}和{item: "a", i_type: "y"}的所有密钥。该解释表明它正在使用适当的索引。
如果我在文档所在的副本集上运行相同的查询，则totalKeysExamined为10.如果我删除了mongos上单个i_type的排序或查询，totalKeysExamined是10（或11取决于自动平衡）。所有这些查询计划都有executionStage LIMIT，我在上面发布的解释中明显遗漏了executionStage。
根据自动平衡器决定执行操作的方式，层次结构中的第一个SHARD_MERGE_SORT可能是SINGLE_SHARD而不是{{1}}，但即使查询需要从两者获取信息碎片我认为只需要检查20个密钥（每个碎片10个）。
这是我遇到的mongos的限制，我需要改进选择的分片键和索引，还是完全不同的东西？