How does MongoDB index arrays? - indexing

Question

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?

Answer 1

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

Answer 2

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
}