What is the _id_hashed index for in mongoDB? -


i sharded mongodb cluster hashed _id. checked index size, there lies _id_hashed index taking space:

   "indexsizes" : {            "_id_" : 14060169088,            "_id_hashed" : 9549780576     },

mongodb manual says index on sharded key created if shard collection. guess reason _id_hashed index out there.

my question : _id_hashed index if query document _id field? can delete it? takes space.

ps: seems mongodb use _id index when query, not _id_hashed index. execution plan query:

   "clusteredtype" : "parallelsort",     "shards" : {             "rs1/192.168.62.168:27017,192.168.62.181:27017" : [                     {                             "cursor" : "btreecursor _id_",                             "ismultikey" : false,                             "n" : 0,                             "nscannedobjects" : 0,                             "nscanned" : 1,                             "nscannedobjectsallplans" : 0,                             "nscannedallplans" : 1,                             "scanandorder" : false,                             "indexonly" : false,                             "nyields" : 0,                             "nchunkskips" : 0,                             "millis" : 0,                             "indexbounds" : {                                     "start" : {                                             "_id" : "spiderman_task_captainstatus_30491467_2387600"                                     },                                     "end" : {                                             "_id" : "spiderman_task_captainstatus_30491467_2387600"                                     }                             },                             "server" : "localhost:27017"                     }             ]     },     "cursor" : "btreecursor _id_",     "n" : 0,     "nchunkskips" : 0,     "nyields" : 0,     "nscanned" : 1,     "nscannedallplans" : 1,     "nscannedobjects" : 0,     "nscannedobjectsallplans" : 0,     "millisshardtotal" : 0,     "millisshardavg" : 0,     "numqueries" : 1,     "numshards" : 1,     "indexbounds" : {             "start" : {                     "_id" : "spiderman_task_captainstatus_30491467_2387600"             },             "end" : {                     "_id" : "spiderman_task_captainstatus_30491467_2387600"             }     },     "millis" : 574

mongodb uses range based sharding approach. if choose use hashed based sharding, must have hashed index on shard key , cannot drop since used determine shard use subsequent queries ( note there open ticket allow drop _id index once hashed indexes allowed unique server-8031 ).

as why query appears using _id index rather _id_hashed index - ran tests , think optimizer choosing _id index because unique , results in more efficient plan. can see similar behavior if shard on key has pre-existing unique index.


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