In MongoDB, if I were to store an array (say ["red", "blue"]) in a field "color", does it index "red" and "blue" so I could query for "red", for example, or does in make {"red", "blue"} a composite index?
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2 Answers
When it comes to indexing arrays, MongoDB indexes each value of the array so you can query for individual items, such as "red." For example:
> db.col1.save({'colors': ['red','blue']})
> db.col1.ensureIndex({'colors':1})
> db.col1.find({'colors': 'red'})
{ "_id" : ObjectId("4ccc78f97cf9bdc2a2e54ee9"), "colors" : [ "red", "blue" ] }
> db.col1.find({'colors': 'blue'})
{ "_id" : ObjectId("4ccc78f97cf9bdc2a2e54ee9"), "colors" : [ "red", "blue" ] }
For more information, check out MongoDB's documentation on Multikeys: http://www.mongodb.org/display/DOCS/Multikeys
4 Comments
Gaurav Abbi
is it possible to search for a partial array like for ("colors" : [ "red", "blue" , "green"]), can i have a search for ("colors" : [ "red", "blue" ]) or I have to do them individually?
Mnebuerquo
@GauravAbbi - I think that works, but mongo will only use the index to find the first array key. After that it's scanning that set of documents for those matching the rest of the keys.
eran otzap
What considerations would be taken to use multikey indexes as apposed to a compound index. for instance an index describing a main color and a secondary color ?
Fred Mériot
@GauravAbbi : I think you should do a "colors" : {$in:["red", "blue"]}.
You can simply test index usage by appending "explain" to your query:
> db.col1.save({'colors': ['red','blue']})
# without index
> db.col1.find({'colors': 'red'}).explain()
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "protrain.col1",
"indexFilterSet" : false,
"parsedQuery" : {
"colors" : {
"$eq" : "red"
}
},
"winningPlan" : {
"stage" : "COLLSCAN", <--- simple column scan
"filter" : {
"colors" : {
"$eq" : "red"
}
},
"direction" : "forward"
},
"rejectedPlans" : [ ]
},
"serverInfo" : {
"host" : "bee34f15fe28",
"port" : 27017,
"version" : "3.4.4",
"gitVersion" : "888390515874a9debd1b6c5d36559ca86b44babd"
},
"ok" : 1
}
# query with index
> db.col1.createIndex( { "colors":1 } )
> db.col1.find({'colors': 'red'}).explain()
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "protrain.col1",
"indexFilterSet" : false,
"parsedQuery" : {
"colors" : {
"$eq" : "red"
}
},
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN", <!---- INDEX HAS BEEN USED
"keyPattern" : {
"colors" : 1
},
"indexName" : "colors_1",
"isMultiKey" : true,
"multiKeyPaths" : {
"colors" : [
"colors"
]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"colors" : [
"[\"red\", \"red\"]"
]
}
}
},
"rejectedPlans" : [ ]
},
"serverInfo" : {
"host" : "bee34f15fe28",
"port" : 27017,
"version" : "3.4.4",
"gitVersion" : "888390515874a9debd1b6c5d36559ca86b44babd"
},
"ok" : 1
}
For structures where you have structured indexes, you could use the array position to index fields inside arrays:
{
'_id': 'BB167E2D61909E848EBC96C7B33251AC',
'hist': {
'map': {
'10': 1
}
},
'wayPoints': [{
'bhf_name': 'Zinsgutstr.(Berlin)',
'ext_no': 900180542,
'lat': 52.435158,
'lon': 13.559086,
'puic': 86,
'time': {
'dateTime': '2018-01-10T09: 38: 00',
'offset': {
'totalSeconds': 3600
}
},
'train_name': 'Bus162'
},
{
'bhf_name': 'SAdlershof(Berlin)',
'ext_no': 900193002,
'lat': 52.435104,
'lon': 13.54055,
'puic': 86,
'time': {
'dateTime': '2018-01-10T09: 44: 00',
'offset': {
'totalSeconds': 3600
}
},
'train_name': 'Bus162'
}]
}
db.col.createIndex( { "wayPoints.0.ext_no":1 } )
db.col.createIndex( { "wayPoints.0.train_name":1 } )
db.col.createIndex( { "wayPoints.1.ext_no":1 } )
db.col.createIndex( { "wayPoints.1.train_name":1 } )
> db.col.find(
... {
... "wayPoints.ext_no": 900180542
... }
... ,
... {
... "wayPoints.ext_no":1,
... "wayPoints.train_name":1,
... "wayPoints.time":1
... }
... ).explain()
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "db.col",
"indexFilterSet" : false,
"parsedQuery" : {
"wayPoints.ext_no" : {
"$eq" : 900180542
}
},
"winningPlan" : {
"stage" : "PROJECTION",
"transformBy" : {
"wayPoints.ext_no" : 1,
"wayPoints.train_name" : 1,
"wayPoints.time" : 1
},
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"wayPoints.ext_no" : 1
},
"indexName" : "wayPoints.ext_no_1",
"isMultiKey" : true,
"multiKeyPaths" : {
"wayPoints.ext_no" : [
"wayPoints"
]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"wayPoints.ext_no" : [
"[900180542.0, 900180542.0]"
]
}
}
}
},
"rejectedPlans" : [ ]
},
"serverInfo" : {
"host" : "bee34f15fe28",
"port" : 27017,
"version" : "3.4.4",
"gitVersion" : "888390515874a9debd1b6c5d36559ca86b44babd"
},
"ok" : 1
}
