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From "Chris Anderson (JIRA)" <j...@apache.org>
Subject [jira] Created: (COUCHDB-495) Make views twice as fast
Date Thu, 03 Sep 2009 22:34:57 GMT
Make views twice as fast
------------------------

                 Key: COUCHDB-495
                 URL: https://issues.apache.org/jira/browse/COUCHDB-495
             Project: CouchDB
          Issue Type: Improvement
          Components: JavaScript View Server
            Reporter: Chris Anderson
             Fix For: 0.11


Devs,

Damien's identified view collation as the most significant bottleneck for the view generation.
We've done some testing, and some preliminary patches, and the upshot seems to be that even
removing ICU from the collator is not a significant boost. What does speed things up greatly
is using raw Erlang term comparison. Eg, instead of using couch_view:less_json, using fun(A,B)
A < B end. provides a roughly 2x speedup.

However, the patch is challenging for a few reasons: Making the collation strategy switchable
at all is tough. It's actually quite easy to get an alternate less function into the btree
writer (all you've got to do is set it in couch_view_group:init_group). The hard part is propagating
the same less function to the PassedEndFun. There's a secondary problem that when you use
raw term comparison, a lot of terms turn out to come before nil, and after {}, which we use
as artificial first and last terms in the less_json function. So just switching to raw collation
alone will leave you with a view with unreachable rows.

I tried two different approaches to the problem last night, and both of them led to (instructive)
dead ends. I'll attach them for illustration purposes.

The next line of attack we think should be tried is this:

First - remove _all_docs_by_seq, as it is just adding complexity to the problem, and has been
deprecated by _changes anyway. Along the same lines, _all_docs should no longer use couch_httpd_view:make_view_fold_fun
as it has completely different collation needs than make_view_fold_fun. We'll end up duplicating
a little code in the _all_docs implementation, but it should be worth it because it will make
the other work much simpler.

Once those changes have laid the groundwork, the next step is to change make_view_fold_fun
and couch_view:fold, so that rather than make_view_fold_fun being responsible for detecting
when we've passed the endkey. That means make_passed_end_fun and all references to PassedEnd
and PassedEnd fun will be stripped from couch_httpd_view and moved to couch_btree.

couch_view:fold (and the underlying btree) will need to accept not just a start, but also
an endkey. This will make it much easier to use the less fun that is stored on View#view.btree#btree.less
to determine PassedEnd funs. This will move some complexity to the btree code from the view
code, but will keep the concerns more aligned. This also means that the btree will need to
accept not only an endkey for folds, but also an inclusive_end parameter.

Once we have all these refactorings done, it will be easy to make the less fun for an index
configurable, as both the index writer and the index reader will look for it in the same place
(on the #btree record).

My aim is to start a discussion and get someone excited to work on this patch. Think of all
the fast-views glory you'll get! Please ask questions and otherwise force me to clarify the
above discussion.


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