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From: Apache Wiki <wikidiffs@apache.org>
To: Apache Wiki <wikidiffs@apache.org>
Date: Tue, 06 Dec 2011 10:13:10 -0000
Message-ID: <20111206101310.79794.40278@eos.apache.org>
Subject: =?utf-8?q?=5BCouchdb_Wiki=5D_Update_of_=22FUQ=22_by_DaveCottlehuber?=
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Dear Wiki user,

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The "FUQ" page has been changed by DaveCottlehuber:
http://wiki.apache.org/couchdb/FUQ?action=3Ddiff&rev1=3D2&rev2=3D3

Comment:
reducing the number of FUQups

  =3D=3D Views =3D=3D
  =

   1. In a view, why should I not {{{emit(key,doc)}}} ?
+   The key point here is that by emitting {{{,doc}}} you are duplicating t=
he document which is already present in the database (a .couch file), and i=
ncluding it in the results of the view (a different .couch file, with simil=
ar structure). This is the same as having a SQL Index that includes the ori=
ginal table, instead of using a foreign key.
+ =

    The same effect can be acheived by using {{{emit(key,null)}}} and ?incl=
ude_docs=3Dtrue with the view request. This approach has the benefit of not=
 duplicating the document data in the view index, which reduces the disk sp=
ace consumed by the view. On the other hand, the file access pattern is sli=
ghtly more expensive for CouchDB. It is usually a premature optimization to=
 include the document in the view. As always, if you think you may need to =
emit the document it's always best to test.
+ =

   1. What happens if I don't ducktype the variables I am using in my view?
   1. Does it matter if my map function is complex, or takes a long time to=
 run?
  =

  =3D=3D Tools =3D=3D
  =

-  1. I decided to roll my own !CouchApp tool or !CouchDB client in <myfavo=
uritelanguage>. How cool is that?
+  1. I decided to roll my own !CouchApp tool or CouchDB client in <myfavou=
ritelanguage>. How cool is that?
+    Pretty cool! In fact its a great way to get familiar with the API. How=
ever - wrappers around the HTTP API are not necessarily of great use as Cou=
chDB already makes this very easy. Mapping CouchDB semantics onto your lang=
uage's native data structures is much more useful to people. Many languages=
 are already covered and we'd really like to see your ideas and enhancement=
s incorporated into the existing tools if possible, and helping to keep the=
m up to date. Ask on the mailing list about contributing!
  =

+ =3D=3D Log Files =3D=3D
+  1. Those Erlang messages in the log are pretty confusing. What gives?
+    Erlang kindly provides us with a stack trace. In many cases, this is e=
nough to identify the problem. For example, OS errors are reported as tagge=
d tuples {{{{error,enospc}}}} or {{{{error,enoacces}}}} which respectively =
is "You ran out of disk space", and "CouchDB doesn't have permission to acc=
ess that resource". Most of these errors are derived from C used to build t=
he Erlang VM and are documented in {{{errno.h}}} and related header files. =
[[http://www.ibm.com/developerworks/aix/library/au-errnovariable/|IBM]] pro=
vides a good introduction to these, and the relevant [[http://pubs.opengrou=
p.org/onlinepubs/9699919799/basedefs/errno.h.html|POSIX]] and [[http://www.=
gnu.org/s/hello/manual/libc/Error-Codes.html|GNU]] and [[http://msdn.micros=
oft.com/en-us/library/5814770t.aspx|Microsoft Windows]] standards will cove=
r most cases.
+=20