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<h1>Cassandra adapter</h1>
<!--
-->
<p>For instructions on downloading and building Calcite, start with the
<a href="/docs/tutorial.html">tutorial</a>.</p>
<p>Once you’ve managed to compile the project, you can return here to
start querying Cassandra with Calcite. First, we need a
<a href="/docs/model.html">model definition</a>.
The model gives Calcite the necessary parameters to create an instance
of the Cassandra adapter. Note that while models can contain
definitions of
<a href="/docs/model.html#materialization">materializations</a>,
the adapter will attempt to automatically populate any materialized views
<a href="https://www.datastax.com/dev/blog/new-in-cassandra-3-0-materialized-views">defined in Cassandra</a>.</p>
<p>A basic example of a model file is given below:</p>
<figure class="highlight"><pre><code class="language-json" data-lang="json"><span class="p">{</span><span class="w">
</span><span class="err">version</span><span class="p">:</span><span class="w"> </span><span class="err">'</span><span class="mf">1.0</span><span class="err">'</span><span class="p">,</span><span class="w">
</span><span class="err">defaultSchema</span><span class="p">:</span><span class="w"> </span><span class="err">'twissandra'</span><span class="p">,</span><span class="w">
</span><span class="err">schemas</span><span class="p">:</span><span class="w"> </span><span class="p">[</span><span class="w">
</span><span class="p">{</span><span class="w">
</span><span class="err">name</span><span class="p">:</span><span class="w"> </span><span class="err">'twissandra'</span><span class="p">,</span><span class="w">
</span><span class="err">type</span><span class="p">:</span><span class="w"> </span><span class="err">'custom'</span><span class="p">,</span><span class="w">
</span><span class="err">factory</span><span class="p">:</span><span class="w"> </span><span class="err">'org.apache.calcite.adapter.cassandra.CassandraSchemaFactory'</span><span class="p">,</span><span class="w">
</span><span class="err">operand</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w">
</span><span class="err">host</span><span class="p">:</span><span class="w"> </span><span class="err">'localhost'</span><span class="p">,</span><span class="w">
</span><span class="err">keyspace</span><span class="p">:</span><span class="w"> </span><span class="err">'twissandra'</span><span class="w">
</span><span class="p">}</span><span class="w">
</span><span class="p">}</span><span class="w">
</span><span class="p">]</span><span class="w">
</span><span class="p">}</span></code></pre></figure>
<p>Note that you can also specify <code class="highlighter-rouge">username</code> and <code class="highlighter-rouge">password</code> keys along with
the <code class="highlighter-rouge">host</code> and <code class="highlighter-rouge">keyspace</code> if your server requires authentication.
Assuming this file is stored as <code class="highlighter-rouge">model.json</code>, you can connect to
Cassandra via <a href="https://github.com/julianhyde/sqlline"><code class="highlighter-rouge">sqlline</code></a> as
follows:</p>
<figure class="highlight"><pre><code class="language-bash" data-lang="bash"><span class="nv">$ </span>./sqlline
sqlline&gt; <span class="o">!</span>connect jdbc:calcite:model<span class="o">=</span>model.json admin admin</code></pre></figure>
<p><code class="highlighter-rouge">sqlline</code> will now accept SQL queries which access your CQL tables.
However, you’re not restricted to issuing queries supported by
<a href="https://cassandra.apache.org/doc/cql3/CQL-2.2.html">CQL</a>.
Calcite allows you to perform complex operations such as aggregations
or joins. The adapter will attempt to compile the query into the most
efficient CQL possible by exploiting filtering and sorting directly in
Cassandra where possible.</p>
<p>For example, in the example dataset there is a CQL table named <code class="highlighter-rouge">timeline</code>
with <code class="highlighter-rouge">username</code> as the partition key and <code class="highlighter-rouge">time</code> as the clustering key.</p>
<p>We can issue a simple query to fetch the most recent tweet ID of the
user by writing standard SQL:</p>
<figure class="highlight"><pre><code class="language-sql" data-lang="sql"><span class="n">sqlline</span><span class="o">&gt;</span> <span class="k">SELECT</span> <span class="nv">"tweet_id"</span>
<span class="k">FROM</span> <span class="nv">"timeline"</span>
<span class="k">WHERE</span> <span class="nv">"username"</span> <span class="o">=</span> <span class="s1">'JmuhsAaMdw'</span>
<span class="k">ORDER</span> <span class="k">BY</span> <span class="nv">"time"</span> <span class="k">DESC</span> <span class="k">LIMIT</span> <span class="mi">1</span><span class="p">;</span>
<span class="o">+</span><span class="c1">--------------------------------------+</span>
<span class="o">|</span> <span class="n">tweet_id</span> <span class="o">|</span>
<span class="o">+</span><span class="c1">--------------------------------------+</span>
<span class="o">|</span> <span class="n">f3d3d4dc</span><span class="o">-</span><span class="n">d05b</span><span class="o">-</span><span class="mi">11</span><span class="n">e5</span><span class="o">-</span><span class="n">b58b</span><span class="o">-</span><span class="mi">90</span><span class="n">e2ba530b12</span> <span class="o">|</span>
<span class="o">+</span><span class="c1">--------------------------------------+</span></code></pre></figure>
<p>While executing this query, the Cassandra adapter is able to recognize
that <code class="highlighter-rouge">username</code> is the partition key and can be filtered by Cassandra.
It also recognizes the clustering key <code class="highlighter-rouge">time</code> and pushes the ordering to
Cassandra as well.</p>
<p>The final CQL query given to Cassandra is below:</p>
<figure class="highlight"><pre><code class="language-sql" data-lang="sql"><span class="k">SELECT</span> <span class="n">username</span><span class="p">,</span> <span class="n">time</span><span class="p">,</span> <span class="n">tweet_id</span>
<span class="k">FROM</span> <span class="nv">"timeline"</span>
<span class="k">WHERE</span> <span class="n">username</span> <span class="o">=</span> <span class="s1">'JmuhsAaMdw'</span>
<span class="k">ORDER</span> <span class="k">BY</span> <span class="n">time</span> <span class="k">DESC</span> <span class="n">ALLOW</span> <span class="n">FILTERING</span><span class="p">;</span></code></pre></figure>
<p>There is still significant work to do in improving the flexibility and
performance of the adapter, but if you’re looking for a quick way to
gain additional insights into data stored in Cassandra, Calcite should
prove useful.</p>
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