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From "Nathan Vander Wilt (JIRA)" <j...@apache.org>
Subject [jira] Created: (COUCHDB-1076) _all_docs performance degrades as doc_del_count increases
Date Fri, 25 Feb 2011 19:18:22 GMT
_all_docs performance degrades as doc_del_count increases
---------------------------------------------------------

                 Key: COUCHDB-1076
                 URL: https://issues.apache.org/jira/browse/COUCHDB-1076
             Project: CouchDB
          Issue Type: Bug
            Reporter: Nathan Vander Wilt


The time required to query _all_docs?limit=1 can be proportional to the doc_del_count of the
database depending on where the deleted docs are in the sort order. As kocolosk explained
on IRC:

The tree that serves as the source for _all_docs contains a small record for each document,
deleted or not ... if you have a large number of deleted docs and their IDs are interspersed
with the non-deleted ones i can imagine that it would cause additional seeks when streaming
the _all_docs response

In my use case (https://github.com/natevw/RQMS) and in other cases (e.g. a rolling log or
any long-lived database), the deleted docs may all be at the beginning of the _all_docs view,
making query performance end up like using "?limit=N&skip=doc_del_count".

To improve the performance in cases where large blocks of documents have been deleted, kocolosk
notes:

[10:30am] kocolosk: the inner nodes in the btree currently report the doc_count and doc_del_count
[10:31am] kocolosk: we might be able to rewrite the function that walks the btree so that
it checks if the doc_count underneath an inner node is zero
[10:31am] kocolosk: and then it can skip that part of the tree entirely
[10:31am] kocolosk: instead of descending all the way to the leaf nodes and skipping deleted
documents one by one
[10:31am] n[ate]vw: yeah, that'd have the benefit of being a code-only change
[10:31am] kocolosk: right
[10:32am] kocolosk: i think it should work rather nicely, though it probably requires some
interesting spelunking into the depths of couch_btree
[10:33am] kocolosk: davisp: whaddya think?  would what i'm proposing be possible? (checking
for doc_count=0 in the inner node reduction and then skipping ahead when serving _all_docs)
...
[10:39am] davisp: n[ate]vw: Its definitely doable, its just a question of how to do it so
that I don't bleed from the eyes when trying to read through the implementation
...
[10:40am] rnewson: davisp: right. we definitely want to preserve the high readability of the
current btree code. /s
[10:40am] davisp: kocolosk: The only thing I can think of is passing a function to the view
iteration that gets called to evaluate whether it should decend to a child node based on the
key/reduction pair
[10:41am] kocolosk: davisp: what you're proposing would also allow for (more) efficient implementation
of skip

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