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Type:
Bug
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Resolution: Unresolved
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Priority:
Major - P3
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None
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Affects Version/s: None
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Component/s: Diagnostics, Querying
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Query Execution
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ALL
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When run against a sharded collection with multiple shards, it looks like the explain implementation will simply forward the count command to each shard, which would be incorrect and is not what the regular, non-explain implementation does. More specifically, if there is a skip specified, that should not be forwarded to the shards.
This can be observed by running the test jstests/core/explain_count.js against a cluster with multiple shards - although without modification the test will fail earlier because it doesn't correctly add the 'nSkipped' totals from each shard. Once that issue is resolved, this assertion fails, because each shard will apply the skip of 3 in the explained version, resulting in the following explain output, implying there were only 4 in the resulting count:
[js_test:explain_count] 2018-01-04T23:56:43.232+0000 {
[js_test:explain_count] 2018-01-04T23:56:43.232+0000 "queryPlanner" : {
[js_test:explain_count] 2018-01-04T23:56:43.232+0000 "mongosPlannerVersion" : 1,
[js_test:explain_count] 2018-01-04T23:56:43.232+0000 "winningPlan" : {
[js_test:explain_count] 2018-01-04T23:56:43.232+0000 "stage" : "SHARD_MERGE",
[js_test:explain_count] 2018-01-04T23:56:43.232+0000 "shards" : [
[js_test:explain_count] 2018-01-04T23:56:43.232+0000 {
[js_test:explain_count] 2018-01-04T23:56:43.233+0000 "shardName" : "shard0000",
[js_test:explain_count] 2018-01-04T23:56:43.233+0000 "connectionString" : "localhost:20503",
[js_test:explain_count] 2018-01-04T23:56:43.233+0000 "serverInfo" : {
[js_test:explain_count] 2018-01-04T23:56:43.233+0000 "host" : "ip-10-218-215-76",
[js_test:explain_count] 2018-01-04T23:56:43.233+0000 "port" : 20503,
[js_test:explain_count] 2018-01-04T23:56:43.233+0000 "version" : "3.7.0-405-g534b1d0c5a-patch-5a4eb2bfe3c33103b7004eb0",
[js_test:explain_count] 2018-01-04T23:56:43.233+0000 "gitVersion" : "534b1d0c5abc8f62d7e3961081f54fd965480ddc"
[js_test:explain_count] 2018-01-04T23:56:43.234+0000 },
[js_test:explain_count] 2018-01-04T23:56:43.234+0000 "plannerVersion" : 1,
[js_test:explain_count] 2018-01-04T23:56:43.234+0000 "namespace" : "test.jstests_explain_count",
[js_test:explain_count] 2018-01-04T23:56:43.234+0000 "indexFilterSet" : false,
[js_test:explain_count] 2018-01-04T23:56:43.234+0000 "winningPlan" : {
[js_test:explain_count] 2018-01-04T23:56:43.234+0000 "stage" : "COUNT"
[js_test:explain_count] 2018-01-04T23:56:43.234+0000 },
[js_test:explain_count] 2018-01-04T23:56:43.234+0000 "rejectedPlans" : [ ]
[js_test:explain_count] 2018-01-04T23:56:43.235+0000 },
[js_test:explain_count] 2018-01-04T23:56:43.235+0000 {
[js_test:explain_count] 2018-01-04T23:56:43.235+0000 "shardName" : "shard0001",
[js_test:explain_count] 2018-01-04T23:56:43.235+0000 "connectionString" : "localhost:20504",
[js_test:explain_count] 2018-01-04T23:56:43.235+0000 "serverInfo" : {
[js_test:explain_count] 2018-01-04T23:56:43.235+0000 "host" : "ip-10-218-215-76",
[js_test:explain_count] 2018-01-04T23:56:43.235+0000 "port" : 20504,
[js_test:explain_count] 2018-01-04T23:56:43.235+0000 "version" : "3.7.0-405-g534b1d0c5a-patch-5a4eb2bfe3c33103b7004eb0",
[js_test:explain_count] 2018-01-04T23:56:43.236+0000 "gitVersion" : "534b1d0c5abc8f62d7e3961081f54fd965480ddc"
[js_test:explain_count] 2018-01-04T23:56:43.236+0000 },
[js_test:explain_count] 2018-01-04T23:56:43.236+0000 "plannerVersion" : 1,
[js_test:explain_count] 2018-01-04T23:56:43.236+0000 "namespace" : "test.jstests_explain_count",
[js_test:explain_count] 2018-01-04T23:56:43.236+0000 "indexFilterSet" : false,
[js_test:explain_count] 2018-01-04T23:56:43.237+0000 "winningPlan" : {
[js_test:explain_count] 2018-01-04T23:56:43.237+0000 "stage" : "COUNT"
[js_test:explain_count] 2018-01-04T23:56:43.237+0000 },
[js_test:explain_count] 2018-01-04T23:56:43.237+0000 "rejectedPlans" : [ ]
[js_test:explain_count] 2018-01-04T23:56:43.237+0000 }
[js_test:explain_count] 2018-01-04T23:56:43.238+0000 ]
[js_test:explain_count] 2018-01-04T23:56:43.238+0000 }
[js_test:explain_count] 2018-01-04T23:56:43.238+0000 },
[js_test:explain_count] 2018-01-04T23:56:43.238+0000 "executionStats" : {
[js_test:explain_count] 2018-01-04T23:56:43.238+0000 "nReturned" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.238+0000 "executionTimeMillis" : 2,
[js_test:explain_count] 2018-01-04T23:56:43.238+0000 "totalKeysExamined" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.238+0000 "totalDocsExamined" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.238+0000 "executionStages" : {
[js_test:explain_count] 2018-01-04T23:56:43.239+0000 "stage" : "SHARD_MERGE",
[js_test:explain_count] 2018-01-04T23:56:43.239+0000 "nReturned" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.239+0000 "executionTimeMillis" : 2,
[js_test:explain_count] 2018-01-04T23:56:43.239+0000 "totalKeysExamined" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.239+0000 "totalDocsExamined" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.239+0000 "totalChildMillis" : NumberLong(0),
[js_test:explain_count] 2018-01-04T23:56:43.239+0000 "shards" : [
[js_test:explain_count] 2018-01-04T23:56:43.239+0000 {
[js_test:explain_count] 2018-01-04T23:56:43.239+0000 "shardName" : "shard0000",
[js_test:explain_count] 2018-01-04T23:56:43.240+0000 "executionSuccess" : true,
[js_test:explain_count] 2018-01-04T23:56:43.240+0000 "executionStages" : {
[js_test:explain_count] 2018-01-04T23:56:43.240+0000 "stage" : "COUNT",
[js_test:explain_count] 2018-01-04T23:56:43.241+0000 "nReturned" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.241+0000 "executionTimeMillisEstimate" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.241+0000 "works" : 1,
[js_test:explain_count] 2018-01-04T23:56:43.241+0000 "advanced" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.241+0000 "needTime" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.241+0000 "needYield" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.241+0000 "saveState" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.241+0000 "restoreState" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.242+0000 "isEOF" : 1,
[js_test:explain_count] 2018-01-04T23:56:43.242+0000 "invalidates" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.242+0000 "nCounted" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.242+0000 "nSkipped" : 0
[js_test:explain_count] 2018-01-04T23:56:43.242+0000 }
[js_test:explain_count] 2018-01-04T23:56:43.243+0000 },
[js_test:explain_count] 2018-01-04T23:56:43.243+0000 {
[js_test:explain_count] 2018-01-04T23:56:43.243+0000 "shardName" : "shard0001",
[js_test:explain_count] 2018-01-04T23:56:43.243+0000 "executionSuccess" : true,
[js_test:explain_count] 2018-01-04T23:56:43.243+0000 "executionStages" : {
[js_test:explain_count] 2018-01-04T23:56:43.243+0000 "stage" : "COUNT",
[js_test:explain_count] 2018-01-04T23:56:43.244+0000 "nReturned" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.244+0000 "executionTimeMillisEstimate" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.244+0000 "works" : 1,
[js_test:explain_count] 2018-01-04T23:56:43.244+0000 "advanced" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.244+0000 "needTime" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.244+0000 "needYield" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.245+0000 "saveState" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.245+0000 "restoreState" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.245+0000 "isEOF" : 1,
[js_test:explain_count] 2018-01-04T23:56:43.245+0000 "invalidates" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.245+0000 "nCounted" : 0,
[js_test:explain_count] 2018-01-04T23:56:43.245+0000 "nSkipped" : 0
[js_test:explain_count] 2018-01-04T23:56:43.245+0000 }
[js_test:explain_count] 2018-01-04T23:56:43.245+0000 }
[js_test:explain_count] 2018-01-04T23:56:43.246+0000 ]
[js_test:explain_count] 2018-01-04T23:56:43.246+0000 }
[js_test:explain_count] 2018-01-04T23:56:43.246+0000 },
[js_test:explain_count] 2018-01-04T23:56:43.246+0000 "ok" : 1,
[js_test:explain_count] 2018-01-04T23:56:43.246+0000 "$clusterTime" : {
[js_test:explain_count] 2018-01-04T23:56:43.246+0000 "clusterTime" : Timestamp(1515110203, 12),
[js_test:explain_count] 2018-01-04T23:56:43.247+0000 "signature" : {
[js_test:explain_count] 2018-01-04T23:56:43.247+0000 "hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
[js_test:explain_count] 2018-01-04T23:56:43.247+0000 "keyId" : NumberLong(0)
[js_test:explain_count] 2018-01-04T23:56:43.247+0000 }
[js_test:explain_count] 2018-01-04T23:56:43.247+0000 },
[js_test:explain_count] 2018-01-04T23:56:43.247+0000 "operationTime" : Timestamp(1515110203, 12)
[js_test:explain_count] 2018-01-04T23:56:43.247+0000 }
- is related to
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- In Progress
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- Closed
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SERVER-82218 Unify explain and execution path in sharded_agg_helpers
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- Closed
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