Description
On a mongodb setup a long running revision garbage collection operation has been witnessed. The query was running for hours. Doing a planCacheSetFilter, which hinted mongodb to use a specific index, together with killing the running command resolved the situation.
The problem was that mongodb generated a query plan which scored high (2.0003) but included an index scan through the _id_ index (and the collection contained millions of documents). It also generated other, better, plans, but they all "only" had the same high score, so it seemed legitimate that mongodb would choose this one.
The reason why this, problematic, query plan resulted in a high score seems to be that it does indeed find 101 documents after entering the first "or" - but during query execution it would also enter the other "or" parts where it has chosen to do a _id_ index scan.
The query involved was:
{
"_sdType" : {
"$in" : [
50,
60,
70
]
},
"$or" : [
{
"_sdType" : 50
},
{
"_sdType" : 60
},
{
"_sdType" : 70,
"$or" : [
{
"_id" : /.*-1\/0/
},
{
"_id" : /[^-]*/,
"_path" : /.*-1\/0/
}
],
"_sdMaxRevTime" : {
"$lt" : NumberLong(1551843365)
}
},
{
"_sdType" : 70,
"$or" : [
{
"_id" : /.*-2\/0/
},
{
"_id" : /[^-]*/,
"_path" : /.*-2\/0/
}
],
"_sdMaxRevTime" : {
"$lt" : NumberLong(1550757370)
}
}
],
"_sdMaxRevTime" : {
"$lt" : NumberLong(1551756965)
}
}
The problematic plan was:
{
"details" : {
"solution" : "(index-tagged expression tree: tree=Node\n---Node\n------Node\n---------Node\n------------Node\n---------------Leaf _id_, pos: 0, can combine? 1\n---------------Leaf \n------------Leaf _id_, pos: 0, can combine? 1\n---------Leaf \n---------Leaf \n------Node\n---------Node\n------------Node\n---------------Leaf _id_, pos: 0, can combine? 1\n---------------Leaf \n------------Leaf _id_, pos: 0, can combine? 1\n---------Leaf \n---------Leaf \n------Leaf _sdType_1__sdMaxRevTime_1, pos: 0, can combine? 1\n------Leaf _sdType_1__sdMaxRevTime_1, pos: 0, can combine? 1\n---Leaf Move to 2: _sdType_1__sdMaxRevTime_1, pos: 1, can combine? 1. Move to 3: _sdType_1__sdMaxRevTime_1, pos: 1, can combine? 1. \n---Leaf Move to 2: _sdType_1__sdMaxRevTime_1, pos: 0, can combine? 1. Move to 3: _sdType_1__sdMaxRevTime_1, pos: 0, can combine? 1. \n)"
},
"reason" : {
"score" : 2.0003,
"stats" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"_sdMaxRevTime" : {
"$lt" : NumberLong(1551410180)
}
},
{
"_sdType" : {
"$in" : [
50,
60,
70
]
}
}
]
},
"nReturned" : 101,
"executionTimeMillisEstimate" : 0,
"works" : 101,
"advanced" : 101,
"needTime" : 0,
"needYield" : 0,
"saveState" : 3,
"restoreState" : 3,
"isEOF" : 0,
"invalidates" : 0,
"docsExamined" : 101,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "OR",
"nReturned" : 101,
"executionTimeMillisEstimate" : 0,
"works" : 101,
"advanced" : 101,
"needTime" : 0,
"needYield" : 0,
"saveState" : 3,
"restoreState" : 3,
"isEOF" : 0,
"invalidates" : 0,
"dupsTested" : 101,
"dupsDropped" : 0,
"recordIdsForgotten" : 0,
"inputStages" : [
{
"stage" : "IXSCAN",
"nReturned" : 101,
"executionTimeMillisEstimate" : 0,
"works" : 101,
"advanced" : 101,
"needTime" : 0,
"needYield" : 0,
"saveState" : 3,
"restoreState" : 3,
"isEOF" : 0,
"invalidates" : 0,
"keyPattern" : {
"_sdType" : 1,
"_sdMaxRevTime" : 1
},
"indexName" : "_sdType_1__sdMaxRevTime_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"_sdType" : [ ],
"_sdMaxRevTime" : [ ]
},
"isUnique" : false,
"isSparse" : true,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"_sdType" : [
"[50, 50]"
],
"_sdMaxRevTime" : [
"[-inf.0, 1551410180)"
]
},
"keysExamined" : 101,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
},
{
"stage" : "IXSCAN",
"nReturned" : 0,
"executionTimeMillisEstimate" : 0,
"works" : 0,
"advanced" : 0,
"needTime" : 0,
"needYield" : 0,
"saveState" : 3,
"restoreState" : 3,
"isEOF" : 0,
"invalidates" : 0,
"keyPattern" : {
"_sdType" : 1,
"_sdMaxRevTime" : 1
},
"indexName" : "_sdType_1__sdMaxRevTime_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"_sdType" : [ ],
"_sdMaxRevTime" : [ ]
},
"isUnique" : false,
"isSparse" : true,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"_sdType" : [
"[60, 60]"
],
"_sdMaxRevTime" : [
"[-inf.0, 1551410180)"
]
},
"keysExamined" : 0,
"seeks" : 0,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
},
{
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"_sdType" : {
"$eq" : 70
}
},
{
"_sdMaxRevTime" : {
"$lt" : NumberLong(1551496575)
}
}
]
},
"nReturned" : 0,
"executionTimeMillisEstimate" : 0,
"works" : 0,
"advanced" : 0,
"needTime" : 0,
"needYield" : 0,
"saveState" : 3,
"restoreState" : 3,
"isEOF" : 0,
"invalidates" : 0,
"docsExamined" : 0,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "OR",
"nReturned" : 0,
"executionTimeMillisEstimate" : 0,
"works" : 0,
"advanced" : 0,
"needTime" : 0,
"needYield" : 0,
"saveState" : 3,
"restoreState" : 3,
"isEOF" : 0,
"invalidates" : 0,
"dupsTested" : 0,
"dupsDropped" : 0,
"recordIdsForgotten" : 0,
"inputStages" : [
{
"stage" : "FETCH",
"filter" : {
"_path" : {
"$regex" : ".*-1/0"
}
},
"nReturned" : 0,
"executionTimeMillisEstimate" : 0,
"works" : 0,
"advanced" : 0,
"needTime" : 0,
"needYield" : 0,
"saveState" : 3,
"restoreState" : 3,
"isEOF" : 0,
"invalidates" : 0,
"docsExamined" : 0,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"filter" : {
"_id" : {
"$regex" : "[^-]*"
}
},
"nReturned" : 0,
"executionTimeMillisEstimate" : 0,
"works" : 0,
"advanced" : 0,
"needTime" : 0,
"needYield" : 0,
"saveState" : 3,
"restoreState" : 3,
"isEOF" : 0,
"invalidates" : 0,
"keyPattern" : {
"_id" : 1
},
"indexName" : "_id_",
"isMultiKey" : false,
"multiKeyPaths" : {
"_id" : [ ]
},
"isUnique" : true,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"_id" : [
"[\"\", {})",
"[/[^-]*/, /[^-]*/]"
]
},
"keysExamined" : 0,
"seeks" : 0,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
},
{
"stage" : "IXSCAN",
"filter" : {
"$or" : [
{
"_id" : {
"$regex" : ".*-1/0"
}
}
]
},
"nReturned" : 0,
"executionTimeMillisEstimate" : 0,
"works" : 0,
"advanced" : 0,
"needTime" : 0,
"needYield" : 0,
"saveState" : 3,
"restoreState" : 3,
"isEOF" : 0,
"invalidates" : 0,
"keyPattern" : {
"_id" : 1
},
"indexName" : "_id_",
"isMultiKey" : false,
"multiKeyPaths" : {
"_id" : [ ]
},
"isUnique" : true,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"_id" : [
"[\"\", {})",
"[/.*-1/0/, /.*-1/0/]"
]
},
"keysExamined" : 0,
"seeks" : 0,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
]
}
},
{
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"_sdType" : {
"$eq" : 70
}
},
{
"_sdMaxRevTime" : {
"$lt" : NumberLong(1550757370)
}
}
]
},
"nReturned" : 0,
"executionTimeMillisEstimate" : 0,
"works" : 0,
"advanced" : 0,
"needTime" : 0,
"needYield" : 0,
"saveState" : 3,
"restoreState" : 3,
"isEOF" : 0,
"invalidates" : 0,
"docsExamined" : 0,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "OR",
"nReturned" : 0,
"executionTimeMillisEstimate" : 0,
"works" : 0,
"advanced" : 0,
"needTime" : 0,
"needYield" : 0,
"saveState" : 3,
"restoreState" : 3,
"isEOF" : 0,
"invalidates" : 0,
"dupsTested" : 0,
"dupsDropped" : 0,
"recordIdsForgotten" : 0,
"inputStages" : [
{
"stage" : "FETCH",
"filter" : {
"_path" : {
"$regex" : ".*-2/0"
}
},
"nReturned" : 0,
"executionTimeMillisEstimate" : 0,
"works" : 0,
"advanced" : 0,
"needTime" : 0,
"needYield" : 0,
"saveState" : 3,
"restoreState" : 3,
"isEOF" : 0,
"invalidates" : 0,
"docsExamined" : 0,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"filter" : {
"_id" : {
"$regex" : "[^-]*"
}
},
"nReturned" : 0,
"executionTimeMillisEstimate" : 0,
"works" : 0,
"advanced" : 0,
"needTime" : 0,
"needYield" : 0,
"saveState" : 3,
"restoreState" : 3,
"isEOF" : 0,
"invalidates" : 0,
"keyPattern" : {
"_id" : 1
},
"indexName" : "_id_",
"isMultiKey" : false,
"multiKeyPaths" : {
"_id" : [ ]
},
"isUnique" : true,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"_id" : [
"[\"\", {})",
"[/[^-]*/, /[^-]*/]"
]
},
"keysExamined" : 0,
"seeks" : 0,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
},
{
"stage" : "IXSCAN",
"filter" : {
"$or" : [
{
"_id" : {
"$regex" : ".*-2/0"
}
}
]
},
"nReturned" : 0,
"executionTimeMillisEstimate" : 0,
"works" : 0,
"advanced" : 0,
"needTime" : 0,
"needYield" : 0,
"saveState" : 3,
"restoreState" : 3,
"isEOF" : 0,
"invalidates" : 0,
"keyPattern" : {
"_id" : 1
},
"indexName" : "_id_",
"isMultiKey" : false,
"multiKeyPaths" : {
"_id" : [ ]
},
"isUnique" : true,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"_id" : [
"[\"\", {})",
"[/.*-2/0/, /.*-2/0/]"
]
},
"keysExamined" : 0,
"seeks" : 0,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
]
}
}
]
}
}
},
"feedback" : {
"nfeedback" : 18,
"scores" : [
{
"score" : 2.0003
},
{
"score" : 2.0003
},
{
"score" : 2.0003
},
{
"score" : 2.0003
},
{
"score" : 2.0003
},
{
"score" : 2.0003
},
{
"score" : 2.0003
},
{
"score" : 2.0003
},
{
"score" : 2.0003
},
{
"score" : 2.0003
},
{
"score" : 2.0003
},
{
"score" : 2.0003
},
{
"score" : 2.0003
},
{
"score" : 2.0003
},
{
"score" : 2.0003
},
{
"score" : 2.0003
},
{
"score" : 2.0003
},
{
"score" : 2.0003
}
]
},
"filterSet" : false
}
Note that this was primarily caused by SERVER-20616 and that SERVER-42090 and SERVER-24396 would also have been helpful preventing this.
Attachments
Attachments
Issue Links
- is related to
-
OAK-9880 Simplify rgc DEFAULT_NO_BRANCH query
-
- Closed
-