On Mon, Jun 15, 2009 at 02:28:17PM +0800, hhsuper wrote:
> Hi, guys, I need some help, i want to know when we should implement the
> rereduce in reduce function?
Answer: always.
Once you get above a handful of documents you will see re-reduces taking
place. This is because reduce is done in multiple steps:
doc \
doc -> reduce -
doc / \
doc \ \
doc -> reduce ---rereduce --> answer
doc / /
doc \ /
doc -> reduce -
doc /
Also: each btree node contains a bunch of documents, and the btree node also
contains a cached reduce value for that set of docs. This makes it much
cheaper for couchdb to calculate the reduce value across a range of keys; it
can use already-reduced values where available, and combine them using
rereduce.
There's more info linked from the Wiki.
Regards,
Brian.
