Reduce output must shrink more rapidly, what is this error about? - couchdb

Question

An user can post multiple comments in a thread, and I try to get list of threads (distinct) that an user has make comment to it, like :-
// comment table (relation table)
id, thread_id, user_id
select comment.thread_id, count(*)
from user
inner join comment on user.id=comment.user_id
where user.id = ?
group by comment.thread_id;
This is pretty easy in MySQL.
But to convert to couchdb :-
// map
function(doc)
{
emit(doc.user_id, doc.thread_id);
}
// reduce
function (key, thread_id)
{
return thread_id;
}
If I using the above map function, I will hit into an error like :-
"error": "reduce_overflow_error",
"reason": "Reduce output must shrink more rapidly: Current output: ...
I think I have applied the reduce function in wrong manner.
If using another way, like :-
// map
function (doc)
{
emit([doc.user_id, doc.thread_id], 1);
}
// reduce
function(keys, values)
{
return sum(values);
}
The group=true result is look exactly what mysql group-by does.
However, I'm unable to get ALL the list of thread by an user (given I only have the user_id during query time)
Third way, I can discard use of map reduce, and directly apply :-
emit(doc.user_id, doc.thread_id);
And do an PHP array like
foreach ( ... )
{
$threads[$thread_id] = TRUE;
}
array_keys($threads);
However, this is quite bloated and less efficient.
Second method look more accurate :-
key=[user_id, *] <-- it does not work, believe only work on exact match
key=[user_id, thread_id] <-- return one row
Is there a way to get all result without knowing the thread_id ?
(ps: I new to couchdb, and I might have describe the scenario in a bad manner)
Some reference I gotten via #jasonsmith :- http://guide.couchdb.org/draft/cookbook.html
As a rule of thumb, the reduce function should reduce to a single scalar value. That is, an integer; a string; or a small, fixed-size list or object that includes an aggregated value (or values) from the values argument. It should never just return values or similar. CouchDB will give you a warning if you try to use reduce “the wrong way”:

Answer 1

Follow closely to what this docs saying :-
http://wiki.apache.org/couchdb/View_Snippets#Generating_a_list_of_unique_values
// map
function(doc)
{
emit([doc.user_id, doc.thread_id], null);
}
// reduce
function (keys, values)
{
return null;
}
Query :-
?startkey=["$uid"]&endkey=["$uid",{}]&group=true
And the result now is accurate,
so the problem is lied on the reduce function and how the query being construct.