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From George Walker <>
Subject emit(key,doc) vs. emit(key,null) and the wiki
Date Mon, 01 Apr 2013 15:06:38 GMT
In the CouchDB wiki (, it says:

In a view, why should I not emit(key,doc) ?

The key point here is that by emitting ,doc you are duplicating the document which is already
present in the database (a .couch file), and including it in the results of the view (a .view
file, with similar structure). This is the same as having a SQL Index that includes the original
table, instead of using a foreign key.

The same effect can be acheived by using emit(key,null) and ?include_docs=true with the view
request. This approach has the benefit of not duplicating the document data in the view index,
which reduces the disk space consumed by the view. On the other hand, the file access pattern
is slightly more expensive for CouchDB. It is usually a premature optimization to include
the document in the view. As always, if you think you may need to emit the document it's always
best to test.

In my own research, that seems to not be the case. I have a CouchDB 1.2 instance, and when
I have a view that uses emit(key, doc) on my 0.8 GB dataset, I see a 1.4 GB .view file. When
I create the exact same view, but instead emit(key, null) or emit(key, {"_id":doc._id}) it
creates a .view file that is also 1.4 GB. I should also add that the indexing time to create
these views is non-trivial on my current machine. Taking around 10-15 minutes. Examining the
.view files with a text editor seems to indicate that they are all storing the full documents
in the view index, which does not follow what it says in the wiki.

It seems like the wiki should be updated? Or a bug? As I see no difference between using emit(key,
doc) and emit(key, null).

George W.

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