When 2 or more design documents have exactly the same map and reduce functions (but different IDs of course), they get the same signature (see Section D.7, “Index filesystem structure and meaning”). This means that both point to the same index files, and it's exactly this feature that allows publishing development design documents into production, which consists of creating a copy of the development design document (ID matches _design/dev_foobar) with an ID not containing the dev_ prefix and then deleting the original development document, which ensure the index files are preserved after deleting the development design document. It's also possible to have multiple "production" aliases for the same production design document. The view engine itself has no notion of development and production design documents, this is a notion only at the UI and cluster layers, which exploits the design document signatures/aliases feature.

The following example shows this property.

We create 2 identical design documents, only their IDs differ:

shell> curl -H 'Content-Type: application/json' \
    -X PUT 'http://localhost:9500/default/_design/ddoc1' \
    -d '{ "views": {"view1": {"map": "function(doc, meta) { emit(doc.level, meta.id); }"}}}'
{"ok":true,"id":"_design/ddoc1"}

shell> curl -H 'Content-Type: application/json' \
    -X PUT 'http://localhost:9500/default/_design/ddoc2' 
    -d '{ "views": {"view1": {"map": "function(doc, meta) { emit(doc.level, meta.id); }"}}}'
{"ok":true,"id":"_design/ddoc2"}

Next we query view1 from _design/ddoc1 with stale=false, and get:

shell> curl -s 'http://localhost:9500/default/_design/ddoc1/_view/view1?limit=10&stale=false'
{"total_rows":1000000,"rows":[
{"id":"0000025","key":1,"value":"0000025"},
{"id":"0000136","key":1,"value":"0000136"},
{"id":"0000158","key":1,"value":"0000158"},
{"id":"0000205","key":1,"value":"0000205"},
{"id":"0000208","key":1,"value":"0000208"},
{"id":"0000404","key":1,"value":"0000404"},
{"id":"0000464","key":1,"value":"0000464"},
{"id":"0000496","key":1,"value":"0000496"},
{"id":"0000604","key":1,"value":"0000604"},
{"id":"0000626","key":1,"value":"0000626"}
]
}

If immediately after you query view1 from _design/ddoc2 with stale=ok, you'll get exactly the same results, because both design documents are aliases, they share the same signature:

shell> curl -s 'http://localhost:9500/default/_design/ddoc2/_view/view1?limit=10&stale=ok'
{"total_rows":1000000,"rows":[
{"id":"0000025","key":1,"value":"0000025"},
{"id":"0000136","key":1,"value":"0000136"},
{"id":"0000158","key":1,"value":"0000158"},
{"id":"0000205","key":1,"value":"0000205"},
{"id":"0000208","key":1,"value":"0000208"},
{"id":"0000404","key":1,"value":"0000404"},
{"id":"0000464","key":1,"value":"0000464"},
{"id":"0000496","key":1,"value":"0000496"},
{"id":"0000604","key":1,"value":"0000604"},
{"id":"0000626","key":1,"value":"0000626"}
]
}

If you look into the data directory, there's only one main index file and one replica index file:

shell> tree couch/0/\@indexes
couch/0/@indexes
 ??? default
     ???
main_1909e1541626269ef88c7107f5123feb.view.1
     ???
replica_1909e1541626269ef88c7107f5123feb.view.1
     ???
tmp_1909e1541626269ef88c7107f5123feb_main

 2 directories, 2 files

Also, while the indexer is running, if you query_active_tasksfor a node, you'll see one single indexer task, which lists both design documents in the design_documents array field:

shell> curl -s http://localhost:9500/_active_tasks | json_xs
[
   {
      "waiting" : 0,
      "started_on" : 1345662986,
      "pid" :   "<0.234.0>",
      "type" :   "couch_main_index_barrier",
      "running" : 1,
      "limit" : 4,
      "updated_on" : 1345663590
   },
   {
      "waiting" : 0,
      "started_on" : 1345662986,
      "pid" : "<0.235.0>",
      "type" :   "couch_replica_index_barrier",
      "running" : 0,
      "limit" : 2,
      "updated_on" : 1345662986
   },
   {
      "indexer_type" : "main",
      "started_on" : 1345663590,
      "progress" : 75,
      "initial_build" : true,
      "updated_on" : 1345663634,
      "total_changes" : 250000,
      "design_documents" : [
         "_design/ddoc1",
         "_design/ddoc2"
      ],
      "pid" :   "<0.6567.0>",
      "changes_done" : 189635,
      "signature" : "1909e1541626269ef88c7107f5123feb",
      "type" :   "indexer",
      "set" :   "default"
   }
]