indexing - MongoDB: What index should I use? -

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i got highscore mongodb table contains documents such

{username:"bob",score:10,category:"mostlikes"} {username:"john",score:32,category:"mostlikes"} {username:"bob",score:2,category:"leastdeaths"}

the goal fetch top 100 (sorted) of specific category.

important: highscore categories ascending (lower better ex: leastdeaths) , others descending (bigger better ex: mostlikes). means depending on category, want either 100 biggest scores or 100 lowest scores.

there 2 main queries in application:

db.highscore.find({category:category}, {}).limit(100).sort({ score: 1 /*or -1*/ });

db.highscore.find({username:username});

what index recommend?

would keeping ascending category , descending categories in different tables result in better performance?

note: not want have 1 table per category.

i did test on local sample datasets , think best option create index on "category_1_score_1_username_1"

creating index on following fields gives covered query , documents returned index directly.

find below analysis 

> db.usr.find(); { "_id" : objectid("57bd20630744bd376277a795"), "username" : "bob", "score" : 10, "category" : "mostlikes" } { "_id" : objectid("57bd20630744bd376277a796"), "username" : "john", "score" : 32, "category" : "mostlikes" } { "_id" : objectid("57bd20630744bd376277a797"), "username" : "bob1", "score" : 2, "category" : "leastdeaths" } { "_id" : objectid("57bd20630744bd376277a798"), "username" : "john2", "score" : 132, "category" : "mostlikes" } { "_id" : objectid("57bd20630744bd376277a799"), "username" : "bob3", "score" : 20, "category" : "leastdeaths" } { "_id" : objectid("57bd20630744bd376277a79a"), "username" : "john4", "score" : 132, "category" : "mostlikes" } { "_id" : objectid("57bd20630744bd376277a79b"), "username" : "bob5", "score" : 22, "category" : "leastdeaths" } { "_id" : objectid("57bd20630744bd376277a79c"), "username" : "john6", "score" : 322, "category" : "mostlikes" } { "_id" : objectid("57bd20630744bd376277a79d"), "username" : "bob7", "score" : 232, "category" : "leastdeaths" } { "_id" : objectid("57bd20630744bd376277a79e"), "username" : "john8", "score" : 3112, "category" : "mostlikes" } { "_id" : objectid("57bd20630744bd376277a79f"), "username" : "bob4", "score" : 222, "category" : "leastdeaths" } { "_id" : objectid("57bd20630744bd376277a7a0"), "username" : "john22", "score" : 3210, "category" : "mostlikes" } { "_id" : objectid("57bd20630744bd376277a7a1"), "username" : "bob33", "score" : 2111, "category" : "leastdeaths" }

indexes:

> db.usr.getindexes();         {                 "v" : 1,                 "key" : {                         "category" : 1,                         "score" : 1,                         "username" : 1                 },                 "name" : "category_1_score_1_username_1",                 "ns" : "test.usr"         } ] >

now can modify query make return covered query.

    db.usr.find({"category":"mostlikes"},{"_id":0,"score":-1,"category":1,"username":1}).sort({"score":1}).explain("executionstats");  output of execution stats:  {         "queryplanner" : {                 "plannerversion" : 1,                 "namespace" : "test.usr",                 "indexfilterset" : false,                 "parsedquery" : {                         "category" : {                                 "$eq" : "mostlikes"                         }                 },                 "winningplan" : {                         "stage" : "projection",                         "transformby" : {                                 "_id" : 0,                                 "score" : -1,                                 "category" : 1,                                 "username" : 1                         },                         "inputstage" : {                                 "stage" : "ixscan",                                 "keypattern" : {                                         "category" : 1,                                         "score" : 1,                                         "username" : 1                                 },                                 "indexname" : "category_1_score_1_username_1",                                 "ismultikey" : false,                                 "isunique" : false,                                 "issparse" : false,                                 "ispartial" : false,                                 "indexversion" : 1,                                 "direction" : "forward",                                 "indexbounds" : {                                         "category" : [                                                 "[\"mostlikes\", \"mostlikes\"]"                                         ],                                         "score" : [                                                 "[minkey, maxkey]"                                         ],                                         "username" : [                                                 "[minkey, maxkey]"                                         ]                                 }                         }                 },                 "rejectedplans" : [ ]         },         "executionstats" : {                 "executionsuccess" : true,                 "nreturned" : 7,                 "executiontimemillis" : 0,                 "totalkeysexamined" : 7,                 "totaldocsexamined" : 0,                 "executionstages" : {                         "stage" : "projection",                         "nreturned" : 7,                         "executiontimemillisestimate" : 0,                         "works" : 8,                         "advanced" : 7,                         "needtime" : 0,                         "needyield" : 0,                         "savestate" : 0,                         "restorestate" : 0,                         "iseof" : 1,                         "invalidates" : 0,                         "transformby" : {                                 "_id" : 0,                                 "score" : -1,                                 "category" : 1,                                 "username" : 1                         },                         "inputstage" : {                                 "stage" : "ixscan",                                 "nreturned" : 7,                                 "executiontimemillisestimate" : 0,                                 "works" : 8,                                 "advanced" : 7,                                 "needtime" : 0,                                 "needyield" : 0,                                 "savestate" : 0,                                 "restorestate" : 0,                                 "iseof" : 1,                                 "invalidates" : 0,                                 "keypattern" : {                                         "category" : 1,                                         "score" : 1,                                         "username" : 1                                 },                                 "indexname" : "category_1_score_1_username_1",                                 "ismultikey" : false,                                 "isunique" : false,                                 "issparse" : false,                                 "ispartial" : false,                                 "indexversion" : 1,                                 "direction" : "forward",                                 "indexbounds" : {                                         "category" : [                                                 "[\"mostlikes\", \"mostlikes\"]"                                         ],                                         "score" : [                                                 "[minkey, maxkey]"                                         ],                                         "username" : [                                                 "[minkey, maxkey]"                                         ]                                 },                                 "keysexamined" : 7,                                 "dupstested" : 0,                                 "dupsdropped" : 0,                                 "seeninvalidated" : 0                         }                 }         },         "serverinfo" : {                 "host" : "l4156409",                 "port" : 27017,                 "version" : "3.2.5",                 "gitversion" : "34e65e5383f7ea1726332cb175b73077ec4a1b02"         },         "ok" : 1 } >

thus can see output no of documents scanned 0 while records fetched directly index. choosing index best bet first query.

for second query, should simple create index on username field , should solve second query you.

hth.


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