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From "Filipe Manana (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (COUCHDB-1076) _all_docs performance degrades as doc_del_count increases
Date Mon, 23 May 2011 11:38:47 GMT

    [ https://issues.apache.org/jira/browse/COUCHDB-1076?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13037900#comment-13037900
] 

Filipe Manana commented on COUCHDB-1076:
----------------------------------------

Looks good Randall, only 2 minor comments:

1) Do you have to add the skip function to couch_db_updater and export it if it's only used
in couch_httpd_db.erl?

2) The skip function doesn't account for db reduction values created by 1.1.x and older releases,
which are 2 tuple terms {NotDeleted, Deleted}; So either another clause should be added or
simply do a  "Skip >= element(1, Reduction)"

Do you think there's a sane way do add some etap tests (to 020-btree-basics.t)? At least for
the trivial cases it's easy to test (all branches skipped, or none of the branches is skipped)

> _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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