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hbase-5252. book.xml, added section in Data Model about joins 

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1234674 13f79535-47bb-0310-9956-ffa450edef68
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Doug Meil 2012-01-23 02:27:54 +00:00
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commit 8f90bbef96
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src/docbkx/book.xml
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@ -522,6 +522,19 @@ htable.put(put);
</section>
</section>
</section>
<section xml:id="joins"><title>Joins</title>
<para>Whether HBase supports joins is a common question on the dist-list, and there is a simple answer: it doesn't,
at not least in the way that RDBMS' support them (e.g., with equi-joins or outer-joins in SQL). As has been illustrated
in this chapter, the read data model operations in HBase are Get and Scan.
</para>
<para>However, that doesn't mean that equivalent join functionality can't be supported in your application, but
you have to do it yourself. The two primary strategies are either denormalizing the data upon writing to HBase,
or to have lookup tables and do the join between HBase tables in your application or MapReduce code (and as RDBMS'
demonstrate, there are several strategies for this depending on the size of the tables, e.g., nested loops vs.
hash-joins). So which is the best approach? It depends on what you are trying to do, and as such there isn't a single
answer that works for every use case.
</para>
</section>
</chapter> <!-- data model -->
<chapter xml:id="schema">
@ -756,6 +769,10 @@ System.out.println("md5 digest as string length: " + sbDigest.length); // ret
</para>
</section>
</section>
<section xml:id="schema.joins"><title>Joins</title>
<para>If you have multiple tables, don't forget to factor in the potential for <xref linkend="joins"/> into the schema design.
</para>
</section>
<section xml:id="ttl">
<title>Time To Live (TTL)</title>
<para>ColumnFamilies can set a TTL length in seconds, and HBase will automatically delete rows once the expiration time is reached.