Disallow the empty string as an index key type

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    • Minor Change
    • ALL
    • v4.2
    • Hide 
      db.repro.createIndex({x:""})

      Outputs:

      > db.repro.createIndex({x:""})
{
	"createdCollectionAutomatically" : true,
	"numIndexesBefore" : 1,
	"numIndexesAfter" : 2,
	"ok" : 1
}

      A few additional lines from 4.1.10:

      > db.version()
4.1.10
> db.repro.createIndex({x:""})
{
	"createdCollectionAutomatically" : true,
	"numIndexesBefore" : 1,
	"numIndexesAfter" : 2,
	"ok" : 1
}
> db.repro.createIndex({x:1})
{
	"createdCollectionAutomatically" : false,
	"numIndexesBefore" : 2,
	"numIndexesAfter" : 3,
	"ok" : 1
}
> db.repro.getIndices()
[
	{
		"v" : 2,
		"key" : {
			"_id" : 1
		},
		"name" : "_id_",
		"ns" : "repo.repro"
	},
	{
		"v" : 2,
		"key" : {
			"x" : ""
		},
		"name" : "x_",
		"ns" : "repo.repro"
	},
	{
		"v" : 2,
		"key" : {
			"x" : 1
		},
		"name" : "x_1",
		"ns" : "repo.repro"
	}
]
      Show
      db.repro.createIndex({x:""}) Outputs: > db.repro.createIndex({x:""}) { "createdCollectionAutomatically" : true, "numIndexesBefore" : 1, "numIndexesAfter" : 2, "ok" : 1 } A few additional lines from 4.1.10 : > db.version() 4.1.10 > db.repro.createIndex({x:""}) { "createdCollectionAutomatically" : true, "numIndexesBefore" : 1, "numIndexesAfter" : 2, "ok" : 1 } > db.repro.createIndex({x:1}) { "createdCollectionAutomatically" : false, "numIndexesBefore" : 2, "numIndexesAfter" : 3, "ok" : 1 } > db.repro.getIndices() [ { "v" : 2, "key" : { "_id" : 1 }, "name" : "_id_", "ns" : "repo.repro" }, { "v" : 2, "key" : { "x" : "" }, "name" : "x_", "ns" : "repo.repro" }, { "v" : 2, "key" : { "x" : 1 }, "name" : "x_1", "ns" : "repo.repro" } ]
    • Query 2019-06-17
    • None
    • 3
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      At present it is possible to generate an index with an empty string as the key type (eg a definition of {x:""}).  The optimizer seems to be able to use this index when the key is a predicate in the query:

       

      > db.repro.find({x:123}).explain()
{
	"queryPlanner" : {
		"plannerVersion" : 1,
		"namespace" : "repro.repro",
		"indexFilterSet" : false,
		"parsedQuery" : {
			"x" : {
				"$eq" : 123
			}
		},
		"queryHash" : "716F281A",
		"planCacheKey" : "0FA0E5FD",
		"winningPlan" : {
			"stage" : "FETCH",
			"inputStage" : {
				"stage" : "IXSCAN",
				"keyPattern" : {
					"x" : ""
				},
				"indexName" : "x_",
				"isMultiKey" : false,
				"multiKeyPaths" : {
					"x" : [ ]
				},
				"isUnique" : false,
				"isSparse" : false,
				"isPartial" : false,
				"indexVersion" : 2,
				"direction" : "forward",
				"indexBounds" : {
					"x" : [
						"[123.0, 123.0]"
					]
				}
			}
		},
		"rejectedPlans" : [ ]
	},
	"serverInfo" : {
		"host" : "Chriss-MacBook-Pro.local",
		"port" : 27017,
		"version" : "4.1.10",
		"gitVersion" : "8cdc51e7810f7fd8898a4c60b935e389f04659ee"
	},
	"ok" : 1
}

      However it is unable to use the index to sort:

      > db.repro.find({x:123}).sort({x:1}).explain()
{
	"queryPlanner" : {
		"plannerVersion" : 1,
		"namespace" : "repro.repro",
		"indexFilterSet" : false,
		"parsedQuery" : {
			"x" : {
				"$eq" : 123
			}
		},
		"queryHash" : "87B348CE",
		"planCacheKey" : "F46EA305",
		"winningPlan" : {
			"stage" : "FETCH",
			"inputStage" : {
				"stage" : "SORT",
				"sortPattern" : {
					"x" : 1
				},
				"inputStage" : {
					"stage" : "SORT_KEY_GENERATOR",
					"inputStage" : {
						"stage" : "IXSCAN",
						"keyPattern" : {
							"x" : ""
						},
						"indexName" : "x_",
						"isMultiKey" : false,
						"multiKeyPaths" : {
							"x" : [ ]
						},
						"isUnique" : false,
						"isSparse" : false,
						"isPartial" : false,
						"indexVersion" : 2,
						"direction" : "forward",
						"indexBounds" : {
							"x" : [
								"[123.0, 123.0]"
							]
						}
					}
				}
			}
		},
		"rejectedPlans" : [ ]
	},
	"serverInfo" : {
		"host" : "Chriss-MacBook-Pro.local",
		"port" : 27017,
		"version" : "4.1.10",
		"gitVersion" : "8cdc51e7810f7fd8898a4c60b935e389f04659ee"
	},
	"ok" : 1
}
> db.repro.find().sort({x:1}).explain()
{
	"queryPlanner" : {
		"plannerVersion" : 1,
		"namespace" : "repro.repro",
		"indexFilterSet" : false,
		"parsedQuery" : {
			
		},
		"queryHash" : "B7791BAD",
		"planCacheKey" : "B7791BAD",
		"winningPlan" : {
			"stage" : "SORT",
			"sortPattern" : {
				"x" : 1
			},
			"inputStage" : {
				"stage" : "SORT_KEY_GENERATOR",
				"inputStage" : {
					"stage" : "COLLSCAN",
					"direction" : "forward"
				}
			}
		},
		"rejectedPlans" : [ ]
	},
	"serverInfo" : {
		"host" : "Chriss-MacBook-Pro.local",
		"port" : 27017,
		"version" : "4.1.10",
		"gitVersion" : "8cdc51e7810f7fd8898a4c60b935e389f04659ee"
	},
	"ok" : 1
}

      This type of index should fail validation and not be created.

            Assignee:
            Xin Hao Zhang (Inactive)
            Reporter:
            Chris Harris
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              Created:
              Updated:
              Resolved: