Due to auto-generation of queries or user error, it is possible to issue a query to the server which cannot possibly return any results. One such query is
db.foo.find({$expr:{$eq:["a","b"]}});
as described by related ticket SERVER-33642. Since the constant string "a" is never equal to the constant string "b", this query can never match any documents. In fact, the server optimizes this expression to be a constant of false. However, it still runs a collection scan, filtering out each of the documents that do not match one by one:
> db.foo.drop();
> db.foo.insert([{}, {}, {}]);
> db.foo.find({$expr:{$eq:["a","b"]}}).explain("executionStats");
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.foo",
"indexFilterSet" : false,
"parsedQuery" : {
"$expr" : {
"$const" : false
}
},
"winningPlan" : {
"stage" : "COLLSCAN",
"filter" : {
"$expr" : {
"$const" : false
}
},
"direction" : "forward"
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 0,
"executionTimeMillis" : 0,
"totalKeysExamined" : 0,
"totalDocsExamined" : 3,
"executionStages" : {
"stage" : "COLLSCAN",
"filter" : {
"$expr" : {
"$const" : false
}
},
"nReturned" : 0,
"executionTimeMillisEstimate" : 0,
"works" : 5,
"advanced" : 0,
"needTime" : 4,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"invalidates" : 0,
"direction" : "forward",
"docsExamined" : 3
}
},
"serverInfo" : {
"host" : "storchbox",
"port" : 27017,
"version" : "0.0.0",
"gitVersion" : "unknown"
},
"ok" : 1
}
Instead, the planner could notice that the plan is guaranteed to return no documents, and replace the entire execution tree with an EOF plan. EOF plans simply immediately return end-of-stream, without doing any wasteful query execution work.
- is related to
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SERVER-33642 Alert client when parsing a query expression that will never be true
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- Closed
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- Closed
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- Closed
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