Why do we need to use rereduce?

I have gone through the documentation available in website to understand the usage of rereduce functions, but I was not able to figure it out. Can somebody explain me the proper use of rereduce with exampled illustrations.

Thanks in advance,
SK

1 Answer

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Have you seen this post from Aaron? http://crate.im/posts/couchbase-views-reddit-data/

It covers rereduce too

I tried understanding what he explained, however, I could not understand in which situation REREDUCE will be true. I understand that rereduce accepts returned values of previous call of reduce function in value parameter something like

function('James', [ 19000, function('James', [ 13000,20000,5000 ]) ]) { ... }

but the battle I am fighting is when should I use the code something like below

if (rereduce) {...}

I have read the documentation at http://docs.couchbase.com/couchbase-manual-2.5/cb-admin/#handling-re-reduce that it is used to include new or updated or deleted documents to update views incrementally. What I understand is that if 'stale'=false then the index of view will get updated and include the (new/updated/deleted) documents as well and reduce will aggregate them based on the keys returned(emitted) from map function.

If it's true then how 'rereduce' is playing the role here for incremental update?

In short rereduce will be true on large databases. On a large datasets reducer will be called on batches. These batches will be broken up on B-tree boundaries, which may occur in arbitrary places.

"On a large datasets reducer will be called on batches" is new for me. I didn't find this info anywhere in couchbase documentation.