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| <h1 class="entry-title">Predicate Improvements in Kudu 0.8</h1> |
| <p class="meta">Posted 19 Apr 2016 by Dan Burkert</p> |
| </header> |
| <div class="entry-content"> |
| <p>The recently released Kudu version 0.8 ships with a host of new improvements to |
| scan predicates. Performance and usability have been improved, especially for |
| tables taking advantage of <a href="http://kudu.apache.org/docs/schema_design.html#data-distribution">advanced partitioning |
| options</a>.</p> |
| |
| <!--more--> |
| |
| <h2 id="scan-optimizations-in-the-server-and-c-client">Scan Optimizations in the Server and C++ Client</h2> |
| |
| <p>The server and C++ client have gotten more sophisticated in how they handle and |
| optimize scan constraints. Constraints specified in the predicates and lower |
| and upper bound primary keys are better unified, resulting in more predicates |
| being pushed into primary key bounds, which can turn full table scans with |
| predicates into much more efficient bounded scans.</p> |
| |
| <p>Additionally, the server and C++ client now recognize more opportunities to |
| prune entire tablets during scans. For example, for the following schema and |
| query Kudu will now be able to skip scanning 15 out of the 16 tablets in the |
| table:</p> |
| |
| <div class="highlight"><pre><code class="language-sql" data-lang="sql"><span class="c1">-- create a table with 16 tablets</span> |
| <span class="k">CREATE</span> <span class="k">TABLE</span> <span class="n">users</span> <span class="p">(</span><span class="n">id</span> <span class="n">INT64</span><span class="p">,</span> <span class="n">name</span> <span class="n">STRING</span><span class="p">,</span> <span class="n">address</span> <span class="n">STRING</span><span class="p">)</span> |
| <span class="n">DISTRIBUTE</span> <span class="k">BY</span> <span class="n">HASH</span> <span class="p">(</span><span class="n">id</span><span class="p">)</span> <span class="k">INTO</span> <span class="mi">16</span> <span class="n">BUCKETS</span><span class="p">;</span> |
| |
| <span class="c1">-- scan over a single tablet</span> |
| <span class="k">SELECT</span> <span class="n">id</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="n">address</span> <span class="k">FROM</span> <span class="n">users</span> |
| <span class="k">WHERE</span> <span class="n">id</span> <span class="o">=</span> <span class="mi">876932</span><span class="p">;</span></code></pre></div> |
| |
| <p>For a deeper look at the newly implemented scan and partition pruning |
| optimizations, see the associated <a href="https://github.com/apache/incubator-kudu/blob/master/docs/design-docs/scan-optimization-partition-pruning.md">design |
| document</a>. |
| These optimizations will eventually be incorporated into the Java client as |
| well, but until that time they are still used on the server side for scans |
| initiated by Java clients. If you would like to help with this effort, let us |
| know on the <a href="https://issues.apache.org/jira/browse/KUDU-1065">JIRA issue</a>.</p> |
| |
| <h2 id="redesigned-predicate-api-in-the-java-client">Redesigned Predicate API in the Java Client</h2> |
| |
| <p>The Java client has a new way to express scan predicates: the |
| <a href="http://kudu.apache.org/apidocs/org/kududb/client/KuduPredicate.html"><code>KuduPredicate</code></a>. |
| The API matches the corresponding C++ API more closely, and adds support for |
| specifying exclusive, as well as inclusive, range predicates. The existing |
| <a href="http://kudu.apache.org/apidocs/org/kududb/client/ColumnRangePredicate.html"><code>ColumnRangePredicate</code></a> |
| API has been deprecated, and will be removed soon. Example of transitioning from |
| the old to new API:</p> |
| |
| <div class="highlight"><pre><code class="language-java" data-lang="java"><span class="n">ColumnSchema</span> <span class="n">myIntColumnSchema</span> <span class="o">=</span> <span class="o">...;</span> |
| <span class="n">KuduScanner</span><span class="o">.</span><span class="na">KuduScannerBuilder</span> <span class="n">scannerBuilder</span> <span class="o">=</span> <span class="o">...;</span> |
| |
| <span class="c1">// Old predicate API</span> |
| <span class="n">ColumnRangePredicate</span> <span class="n">predicate</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">ColumnRangePredicate</span><span class="o">(</span><span class="n">myIntColumnSchema</span><span class="o">);</span> |
| <span class="n">predicate</span><span class="o">.</span><span class="na">setLowerBound</span><span class="o">(</span><span class="mi">20</span><span class="o">);</span> |
| <span class="n">scannerBuilder</span><span class="o">.</span><span class="na">addColumnRangePredicate</span><span class="o">(</span><span class="n">predicate</span><span class="o">);</span> |
| |
| <span class="c1">// New predicate API</span> |
| <span class="n">scannerBuilder</span><span class="o">.</span><span class="na">newPredicate</span><span class="o">(</span> |
| <span class="n">KuduPredicate</span><span class="o">.</span><span class="na">newComparisonPredicate</span><span class="o">(</span><span class="n">myIntColumnSchema</span><span class="o">,</span> <span class="n">ComparisonOp</span><span class="o">.</span><span class="na">GREATER_EQUAL</span><span class="o">,</span> <span class="mi">20</span><span class="o">));</span></code></pre></div> |
| |
| <h2 id="under-the-covers-changes">Under the Covers Changes</h2> |
| |
| <p>The scan optimizations in the server and C++ client, and the new <code>KuduPredicate</code> |
| API in the Java client are made possible by an overhaul of how predicates are |
| handled internally. A new protobuf message type, |
| <a href="https://github.com/apache/incubator-kudu/blob/master/src/kudu/common/common.proto#L273"><code>ColumnPredicatePB</code></a> |
| has been introduced, and will allow more column predicate types to be introduced |
| in the future. If you are interested in contributing to Kudu but don’t know |
| where to start, consider adding a new predicate type; for example the <code>IS NULL</code>, |
| <code>IS NOT NULL</code>, <code>IN</code>, and <code>LIKE</code> predicates types are currently not implemented.</p> |
| |
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| <h3>Recent posts</h3> |
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