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Posted to commits@phoenix.apache.org by ja...@apache.org on 2017/10/04 07:09:50 UTC
svn commit: r1811040 [2/2] - in /phoenix: phoenix-docs/src/docsrc/help/
phoenix-docs/src/tools/org/h2/build/doc/ site/publish/ site/publish/images/
site/publish/language/ site/source/src/site/ site/source/src/site/markdown/
site/source/src/site/resourc...
Added: phoenix/site/publish/tablesample.html
URL: http://svn.apache.org/viewvc/phoenix/site/publish/tablesample.html?rev=1811040&view=auto
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--- phoenix/site/publish/tablesample.html (added)
+++ phoenix/site/publish/tablesample.html Wed Oct 4 07:09:48 2017
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+ <h1>Table Sampliing</h1>
+</div>
+<p>To support table sampling, similar to the Postgresql and T-SQL syntax, a TABLESAMPLE clause has been encorporated into the table aliases ref as of Phoenix 4.12. The general syntax is described <a href="language/index.html#aliased_table_ref">here</a>. This feature limits the number of rows returned from a table to a PERCENT of rows. See <a class="externalLink" href="https://issues.apache.org/jira/browse/PHOENIX-153">PHOENIX-153</a> for implementation details.</p>
+<p>This feature is implmented with a Bernoulli trial, a consistent-hashing based tablesampler, to achieve a Bernoulli sampling on a given population (rows). When executed, given a sampling rate, it leverages Phoenixâs stats as well HBaseâs region distribution to perform a table sampling.</p>
+<p>As part of the <a href="update_statistics.html">statistics collection</a>, a guide post (a row referrence) is created for rows at equidistant bytes from each other. When sampling is required, a Bernoulli trial process is applied repeatedly on each guide post on each region with a probability that porportional to the sampling rate. A included guide post will result in all the rows in between this and next guide post being included into sample population.</p>
+<div class="section">
+ <div class="section">
+ <div class="section">
+ <h4 id="Performance">Performance</h4>
+ <p>Sampling on a table with a sampling rate = 100% will cost same amount of computational resource as a query without sampling. This resouces consumption quicky fall off when a smaller sampling rate is choosing. In general, the amortized complexity for sampling process is <tt>O(k + mn)</tt> , with n being the number of the regions of the sampled hbase table, m being the number of guide posts, and k being the size of sampled population.</p>
+ </div>
+ <div class="section">
+ <h4 id="Repeatable">Repeatable</h4>
+ <p>Repeatable means it guarantees that the repeated sampling on the same table will resulted in the same sampled result.</p>
+ <p>Repeatable feature is made possible by a consistent-hashing process applied on the binary representation of the start rowkey of each guideposts of each region when sampling process is in action. During hashing, a FNV1 implementation with lazy mod method is adopted by default. see <a class="externalLink" href="http://www.isthe.com/chongo/tech/comp/fnv/">FNV1</a>.</p>
+ </div>
+ </div>
+</div>
+<div class="section">
+ <h2 id="Examples">Examples</h2>
+ <p>For example, to tablesample a table , you would execute the following command. Please note, the sampling rate is a numeric value between 0 and 100, inclusive.</p>
+ <div class="source">
+ <pre>select * from PERSON TABLESAMPLE(12.08);
+</pre>
+ </div>
+ <p>Some more examples:</p>
+ <div class="source">
+ <pre>select * from PERSON TABLESAMPLE (12.08) where ADDRESS = 'CA' OR name>'aaa';
+select count( * ) from PERSON TABLESAMPLE (12.08) LIMIT 2
+select count( * ) from (select NAME from PERSON TABLESAMPLE (49) limit 20)
+select * from (select /+NO_INDEX/ * from PERSON tablesample (10) where Name > 'tina10') where ADDRESS = 'CA'
+select * from PERSON1, PERSON2 tablesample (70) where PERSON1.Name = PERSON2.NAME
+select /+NO_INDEX/ count( * ) from PERSON tableSample (19), US_POPULATION tableSample (28) where PERSON.Name > US_POPULATION.STATE
+upsert into personbig(ID, ADDRESS) select id, address from personbig tablesample (1);
+</pre>
+ </div>
+ <p>To use it in aggregation:</p>
+ <div class="source">
+ <pre>select count( * ) from PERSON TABLESAMPLE (49) LIMIT 2
+select count( * ) from (select NAME from PERSON TABLESAMPLE (49) limit 20)
+</pre>
+ </div>
+ <p>To explain the query:</p>
+ <div class="source">
+ <pre>explain select count( * ) from PERSON TABLESAMPLE (49) LIMIT 2)
+</pre>
+ </div>
+</div>
+<div class="section">
+ <h2 id="Tuning">Tuning</h2>
+ <ul>
+ <li> <p>Due to sampling process, the TABLESAMPLE clause should be used with some caution. For example, a join of two tables is likely to return a match for each row in both tables; however, when tablesampling process is applied on either one or both of the two tables, the joining may not match the expectation when without sampling.</p></li>
+ <li> <p>The stats needs to be collected in order to achieve the best sampling accuracy. To turn on statistics collection, refer to <a href="update_statistics.html">Statistics Collection</a>.</p></li>
+ </ul>
+ <p></p>
+ <div>
+ <pre> ALTER TABLE my_table SET GUIDE_POSTS_WIDTH = 10000000 </pre>
+ </div> * A denser guide post setting improves the sampling accuracy, but reduces the performance at the same time. A comparison is experimented as fig below:
+ <p><img src="images/sampling_accuracy_perf.jpg" alt="table samping performance" /></p>
+</div>
+ </div>
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Modified: phoenix/site/publish/team.html
URL: http://svn.apache.org/viewvc/phoenix/site/publish/team.html?rev=1811040&r1=1811039&r2=1811040&view=diff
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--- phoenix/site/publish/team.html (original)
+++ phoenix/site/publish/team.html Wed Oct 4 07:09:48 2017
@@ -1,7 +1,7 @@
<!DOCTYPE html>
<!--
- Generated by Apache Maven Doxia at 2017-09-24
+ Generated by Apache Maven Doxia at 2017-10-04
Rendered using Reflow Maven Skin 1.1.0 (http://andriusvelykis.github.io/reflow-maven-skin)
-->
<html xml:lang="en" lang="en">
@@ -115,6 +115,7 @@
<li ><a href="paged.html" title="Paged Queries">Paged Queries</a></li>
<li ><a href="salted.html" title="Salted Tables">Salted Tables</a></li>
<li ><a href="skip_scan.html" title="Skip Scan">Skip Scan</a></li>
+ <li ><a href="tablesample.html" title="Table Sampling">Table Sampling</a></li>
<li class="divider"/>
<li ><a href="views.html" title="Views">Views</a></li>
<li ><a href="multi-tenancy.html" title="Multi tenancy">Multi tenancy</a></li>
@@ -528,6 +529,9 @@
<a href="skip_scan.html" title="Skip Scan">Skip Scan</a>
</li>
<li >
+ <a href="tablesample.html" title="Table Sampling">Table Sampling</a>
+ </li>
+ <li >
<a href="http:divider" title=""></a>
</li>
<li >
Modified: phoenix/site/publish/tracing.html
URL: http://svn.apache.org/viewvc/phoenix/site/publish/tracing.html?rev=1811040&r1=1811039&r2=1811040&view=diff
==============================================================================
--- phoenix/site/publish/tracing.html (original)
+++ phoenix/site/publish/tracing.html Wed Oct 4 07:09:48 2017
@@ -1,7 +1,7 @@
<!DOCTYPE html>
<!--
- Generated by Apache Maven Doxia at 2017-09-15
+ Generated by Apache Maven Doxia at 2017-10-04
Rendered using Reflow Maven Skin 1.1.0 (http://andriusvelykis.github.io/reflow-maven-skin)
-->
<html xml:lang="en" lang="en">
@@ -115,6 +115,7 @@
<li ><a href="paged.html" title="Paged Queries">Paged Queries</a></li>
<li ><a href="salted.html" title="Salted Tables">Salted Tables</a></li>
<li ><a href="skip_scan.html" title="Skip Scan">Skip Scan</a></li>
+ <li ><a href="tablesample.html" title="Table Sampling">Table Sampling</a></li>
<li class="divider"/>
<li ><a href="views.html" title="Views">Views</a></li>
<li ><a href="multi-tenancy.html" title="Multi tenancy">Multi tenancy</a></li>
@@ -523,6 +524,9 @@ Connection conn = DriverManager.getConne
<a href="skip_scan.html" title="Skip Scan">Skip Scan</a>
</li>
<li >
+ <a href="tablesample.html" title="Table Sampling">Table Sampling</a>
+ </li>
+ <li >
<a href="http:divider" title=""></a>
</li>
<li >
Modified: phoenix/site/publish/transactions.html
URL: http://svn.apache.org/viewvc/phoenix/site/publish/transactions.html?rev=1811040&r1=1811039&r2=1811040&view=diff
==============================================================================
--- phoenix/site/publish/transactions.html (original)
+++ phoenix/site/publish/transactions.html Wed Oct 4 07:09:48 2017
@@ -1,7 +1,7 @@
<!DOCTYPE html>
<!--
- Generated by Apache Maven Doxia at 2017-09-15
+ Generated by Apache Maven Doxia at 2017-10-04
Rendered using Reflow Maven Skin 1.1.0 (http://andriusvelykis.github.io/reflow-maven-skin)
-->
<html xml:lang="en" lang="en">
@@ -115,6 +115,7 @@
<li ><a href="paged.html" title="Paged Queries">Paged Queries</a></li>
<li ><a href="salted.html" title="Salted Tables">Salted Tables</a></li>
<li ><a href="skip_scan.html" title="Skip Scan">Skip Scan</a></li>
+ <li ><a href="tablesample.html" title="Table Sampling">Table Sampling</a></li>
<li class="divider"/>
<li ><a href="views.html" title="Views">Views</a></li>
<li ><a href="multi-tenancy.html" title="Multi tenancy">Multi tenancy</a></li>
@@ -420,6 +421,9 @@ DELETE FROM my_other_table WHERE k=2;
<a href="skip_scan.html" title="Skip Scan">Skip Scan</a>
</li>
<li >
+ <a href="tablesample.html" title="Table Sampling">Table Sampling</a>
+ </li>
+ <li >
<a href="http:divider" title=""></a>
</li>
<li >
Modified: phoenix/site/publish/tuning.html
URL: http://svn.apache.org/viewvc/phoenix/site/publish/tuning.html?rev=1811040&r1=1811039&r2=1811040&view=diff
==============================================================================
--- phoenix/site/publish/tuning.html (original)
+++ phoenix/site/publish/tuning.html Wed Oct 4 07:09:48 2017
@@ -1,7 +1,7 @@
<!DOCTYPE html>
<!--
- Generated by Apache Maven Doxia at 2017-09-15
+ Generated by Apache Maven Doxia at 2017-10-04
Rendered using Reflow Maven Skin 1.1.0 (http://andriusvelykis.github.io/reflow-maven-skin)
-->
<html xml:lang="en" lang="en">
@@ -115,6 +115,7 @@
<li ><a href="paged.html" title="Paged Queries">Paged Queries</a></li>
<li ><a href="salted.html" title="Salted Tables">Salted Tables</a></li>
<li ><a href="skip_scan.html" title="Skip Scan">Skip Scan</a></li>
+ <li ><a href="tablesample.html" title="Table Sampling">Table Sampling</a></li>
<li class="divider"/>
<li ><a href="views.html" title="Views">Views</a></li>
<li ><a href="multi-tenancy.html" title="Multi tenancy">Multi tenancy</a></li>
@@ -594,6 +595,9 @@
<a href="skip_scan.html" title="Skip Scan">Skip Scan</a>
</li>
<li >
+ <a href="tablesample.html" title="Table Sampling">Table Sampling</a>
+ </li>
+ <li >
<a href="http:divider" title=""></a>
</li>
<li >
Modified: phoenix/site/publish/tuning_guide.html
URL: http://svn.apache.org/viewvc/phoenix/site/publish/tuning_guide.html?rev=1811040&r1=1811039&r2=1811040&view=diff
==============================================================================
--- phoenix/site/publish/tuning_guide.html (original)
+++ phoenix/site/publish/tuning_guide.html Wed Oct 4 07:09:48 2017
@@ -1,7 +1,7 @@
<!DOCTYPE html>
<!--
- Generated by Apache Maven Doxia at 2017-09-15
+ Generated by Apache Maven Doxia at 2017-10-04
Rendered using Reflow Maven Skin 1.1.0 (http://andriusvelykis.github.io/reflow-maven-skin)
-->
<html xml:lang="en" lang="en">
@@ -115,6 +115,7 @@
<li ><a href="paged.html" title="Paged Queries">Paged Queries</a></li>
<li ><a href="salted.html" title="Salted Tables">Salted Tables</a></li>
<li ><a href="skip_scan.html" title="Skip Scan">Skip Scan</a></li>
+ <li ><a href="tablesample.html" title="Table Sampling">Table Sampling</a></li>
<li class="divider"/>
<li ><a href="views.html" title="Views">Views</a></li>
<li ><a href="multi-tenancy.html" title="Multi tenancy">Multi tenancy</a></li>
@@ -592,6 +593,9 @@
<a href="skip_scan.html" title="Skip Scan">Skip Scan</a>
</li>
<li >
+ <a href="tablesample.html" title="Table Sampling">Table Sampling</a>
+ </li>
+ <li >
<a href="http:divider" title=""></a>
</li>
<li >
Modified: phoenix/site/publish/udf.html
URL: http://svn.apache.org/viewvc/phoenix/site/publish/udf.html?rev=1811040&r1=1811039&r2=1811040&view=diff
==============================================================================
--- phoenix/site/publish/udf.html (original)
+++ phoenix/site/publish/udf.html Wed Oct 4 07:09:48 2017
@@ -1,7 +1,7 @@
<!DOCTYPE html>
<!--
- Generated by Apache Maven Doxia at 2017-09-15
+ Generated by Apache Maven Doxia at 2017-10-04
Rendered using Reflow Maven Skin 1.1.0 (http://andriusvelykis.github.io/reflow-maven-skin)
-->
<html xml:lang="en" lang="en">
@@ -115,6 +115,7 @@
<li ><a href="paged.html" title="Paged Queries">Paged Queries</a></li>
<li ><a href="salted.html" title="Salted Tables">Salted Tables</a></li>
<li ><a href="skip_scan.html" title="Skip Scan">Skip Scan</a></li>
+ <li ><a href="tablesample.html" title="Table Sampling">Table Sampling</a></li>
<li class="divider"/>
<li ><a href="views.html" title="Views">Views</a></li>
<li ><a href="multi-tenancy.html" title="Multi tenancy">Multi tenancy</a></li>
@@ -473,6 +474,9 @@ Connection conn = DriverManager.getConne
<a href="skip_scan.html" title="Skip Scan">Skip Scan</a>
</li>
<li >
+ <a href="tablesample.html" title="Table Sampling">Table Sampling</a>
+ </li>
+ <li >
<a href="http:divider" title=""></a>
</li>
<li >
Modified: phoenix/site/publish/update_statistics.html
URL: http://svn.apache.org/viewvc/phoenix/site/publish/update_statistics.html?rev=1811040&r1=1811039&r2=1811040&view=diff
==============================================================================
--- phoenix/site/publish/update_statistics.html (original)
+++ phoenix/site/publish/update_statistics.html Wed Oct 4 07:09:48 2017
@@ -1,7 +1,7 @@
<!DOCTYPE html>
<!--
- Generated by Apache Maven Doxia at 2017-09-15
+ Generated by Apache Maven Doxia at 2017-10-04
Rendered using Reflow Maven Skin 1.1.0 (http://andriusvelykis.github.io/reflow-maven-skin)
-->
<html xml:lang="en" lang="en">
@@ -115,6 +115,7 @@
<li ><a href="paged.html" title="Paged Queries">Paged Queries</a></li>
<li ><a href="salted.html" title="Salted Tables">Salted Tables</a></li>
<li ><a href="skip_scan.html" title="Skip Scan">Skip Scan</a></li>
+ <li ><a href="tablesample.html" title="Table Sampling">Table Sampling</a></li>
<li class="divider"/>
<li ><a href="views.html" title="Views">Views</a></li>
<li ><a href="multi-tenancy.html" title="Multi tenancy">Multi tenancy</a></li>
@@ -394,6 +395,9 @@
<a href="skip_scan.html" title="Skip Scan">Skip Scan</a>
</li>
<li >
+ <a href="tablesample.html" title="Table Sampling">Table Sampling</a>
+ </li>
+ <li >
<a href="http:divider" title=""></a>
</li>
<li >
Modified: phoenix/site/publish/upgrading.html
URL: http://svn.apache.org/viewvc/phoenix/site/publish/upgrading.html?rev=1811040&r1=1811039&r2=1811040&view=diff
==============================================================================
--- phoenix/site/publish/upgrading.html (original)
+++ phoenix/site/publish/upgrading.html Wed Oct 4 07:09:48 2017
@@ -1,7 +1,7 @@
<!DOCTYPE html>
<!--
- Generated by Apache Maven Doxia at 2017-09-15
+ Generated by Apache Maven Doxia at 2017-10-04
Rendered using Reflow Maven Skin 1.1.0 (http://andriusvelykis.github.io/reflow-maven-skin)
-->
<html xml:lang="en" lang="en">
@@ -115,6 +115,7 @@
<li ><a href="paged.html" title="Paged Queries">Paged Queries</a></li>
<li ><a href="salted.html" title="Salted Tables">Salted Tables</a></li>
<li ><a href="skip_scan.html" title="Skip Scan">Skip Scan</a></li>
+ <li ><a href="tablesample.html" title="Table Sampling">Table Sampling</a></li>
<li class="divider"/>
<li ><a href="views.html" title="Views">Views</a></li>
<li ><a href="multi-tenancy.html" title="Multi tenancy">Multi tenancy</a></li>
@@ -355,6 +356,9 @@
<a href="skip_scan.html" title="Skip Scan">Skip Scan</a>
</li>
<li >
+ <a href="tablesample.html" title="Table Sampling">Table Sampling</a>
+ </li>
+ <li >
<a href="http:divider" title=""></a>
</li>
<li >
Modified: phoenix/site/publish/views.html
URL: http://svn.apache.org/viewvc/phoenix/site/publish/views.html?rev=1811040&r1=1811039&r2=1811040&view=diff
==============================================================================
--- phoenix/site/publish/views.html (original)
+++ phoenix/site/publish/views.html Wed Oct 4 07:09:48 2017
@@ -1,7 +1,7 @@
<!DOCTYPE html>
<!--
- Generated by Apache Maven Doxia at 2017-09-15
+ Generated by Apache Maven Doxia at 2017-10-04
Rendered using Reflow Maven Skin 1.1.0 (http://andriusvelykis.github.io/reflow-maven-skin)
-->
<html xml:lang="en" lang="en">
@@ -115,6 +115,7 @@
<li ><a href="paged.html" title="Paged Queries">Paged Queries</a></li>
<li ><a href="salted.html" title="Salted Tables">Salted Tables</a></li>
<li ><a href="skip_scan.html" title="Skip Scan">Skip Scan</a></li>
+ <li ><a href="tablesample.html" title="Table Sampling">Table Sampling</a></li>
<li class="divider"/>
<li class="active"><a href="" title="Views">Views</a></li>
<li ><a href="multi-tenancy.html" title="Multi tenancy">Multi tenancy</a></li>
@@ -384,6 +385,9 @@ VALUES('John Doe', CURRENT_DATE(), NEXT
<a href="skip_scan.html" title="Skip Scan">Skip Scan</a>
</li>
<li >
+ <a href="tablesample.html" title="Table Sampling">Table Sampling</a>
+ </li>
+ <li >
<a href="http:divider" title=""></a>
</li>
<li class="active">
Modified: phoenix/site/publish/who_is_using.html
URL: http://svn.apache.org/viewvc/phoenix/site/publish/who_is_using.html?rev=1811040&r1=1811039&r2=1811040&view=diff
==============================================================================
--- phoenix/site/publish/who_is_using.html (original)
+++ phoenix/site/publish/who_is_using.html Wed Oct 4 07:09:48 2017
@@ -1,7 +1,7 @@
<!DOCTYPE html>
<!--
- Generated by Apache Maven Doxia at 2017-09-15
+ Generated by Apache Maven Doxia at 2017-10-04
Rendered using Reflow Maven Skin 1.1.0 (http://andriusvelykis.github.io/reflow-maven-skin)
-->
<html xml:lang="en" lang="en">
@@ -115,6 +115,7 @@
<li ><a href="paged.html" title="Paged Queries">Paged Queries</a></li>
<li ><a href="salted.html" title="Salted Tables">Salted Tables</a></li>
<li ><a href="skip_scan.html" title="Skip Scan">Skip Scan</a></li>
+ <li ><a href="tablesample.html" title="Table Sampling">Table Sampling</a></li>
<li class="divider"/>
<li ><a href="views.html" title="Views">Views</a></li>
<li ><a href="multi-tenancy.html" title="Multi tenancy">Multi tenancy</a></li>
@@ -437,6 +438,9 @@
<a href="skip_scan.html" title="Skip Scan">Skip Scan</a>
</li>
<li >
+ <a href="tablesample.html" title="Table Sampling">Table Sampling</a>
+ </li>
+ <li >
<a href="http:divider" title=""></a>
</li>
<li >
Modified: phoenix/site/source/src/site/markdown/recent.md
URL: http://svn.apache.org/viewvc/phoenix/site/source/src/site/markdown/recent.md?rev=1811040&r1=1811039&r2=1811040&view=diff
==============================================================================
--- phoenix/site/source/src/site/markdown/recent.md (original)
+++ phoenix/site/source/src/site/markdown/recent.md Wed Oct 4 07:09:48 2017
@@ -2,6 +2,7 @@
As items are implemented from our road map, they are moved here to track the progress we've made:
+1. **[Table Sampling](tablesample.html)**. Support the <code>TABLESAMPLE</code> clause by implementing a filter that uses the guideposts established by stats gathering to only return a percentage of the rows. **Available in our 4.12 release**
1. **[Reduce on disk storage](https://phoenix.apache.org/columnencoding.html)**. Reduce on disk storage to improve performance by a) packing all values into a single cell per column family and b) provide an indirection between the column name and the column qualifier. **Available in our 4.10 release**
1. **[Atomic update](https://phoenix.apache.org/atomic_upsert.html)**. Atomic update is now possible in the UPSERT VALUES statement in support of counters and other use cases. **Available in our 4.9 release**
6. **[DEFAULT declaration](https://phoenix.apache.org/language/index.html#column_def)**. When defining a column it is now possible to provide a DEFAULT declaration for the initial value. **Available in our 4.9 release**
Modified: phoenix/site/source/src/site/markdown/roadmap.md
URL: http://svn.apache.org/viewvc/phoenix/site/source/src/site/markdown/roadmap.md?rev=1811040&r1=1811039&r2=1811040&view=diff
==============================================================================
--- phoenix/site/source/src/site/markdown/roadmap.md (original)
+++ phoenix/site/source/src/site/markdown/roadmap.md Wed Oct 4 07:09:48 2017
@@ -14,7 +14,6 @@ Our roadmap is driven by our user commun
7. **[STRUCT type](https://issues.apache.org/jira/browse/PHOENIX-477)**. Allow declarations of multiple fields of different data types that would be packed into a single cell to reduce the per column storage overhead.
17. **[OLAP Extensions](https://issues.apache.org/jira/browse/PHOENIX-154)**. Support the `WINDOW`, `PARTITION OVER`, `RANK`, and other SQL-92 extensions.
16. **[Multi-version Row Queries](https://issues.apache.org/jira/browse/PHOENIX-590)**. Expose the time dimension of rows through a built-in function to allow aggregation and trending over multiple row versions.
-18. **[Table Sampling](https://issues.apache.org/jira/browse/PHOENIX-153)**. Support the <code>TABLESAMPLE</code> clause by implementing a filter that uses the guideposts established by stats gathering to only return n rows per region.
14. **Security Features**. A number of existing HBase security features in 0.94 could be leverage and new security features being added to 0.98 could be leveraged in the future.
* **[Support GRANT and REVOKE](https://issues.apache.org/jira/browse/PHOENIX-672)**. Support the standard GRANT and REVOKE SQL commands through an HBase AccessController.
* **[Surface support for encryption](https://issues.apache.org/jira/browse/PHOENIX-673)**. Surface specification of what should be encrypted now that HBase supports transparent encryption.
Added: phoenix/site/source/src/site/markdown/tablesample.md
URL: http://svn.apache.org/viewvc/phoenix/site/source/src/site/markdown/tablesample.md?rev=1811040&view=auto
==============================================================================
--- phoenix/site/source/src/site/markdown/tablesample.md (added)
+++ phoenix/site/source/src/site/markdown/tablesample.md Wed Oct 4 07:09:48 2017
@@ -0,0 +1,80 @@
+# Table Sampliing
+
+To support table sampling, similar to the Postgresql and T-SQL syntax, a TABLESAMPLE clause has been
+encorporated into the table aliases ref as of Phoenix 4.12. The general syntax is described
+[here](language/index.html#aliased_table_ref). This feature limits the number of rows returned from
+a table to a PERCENT of rows. See [PHOENIX-153](https://issues.apache.org/jira/browse/PHOENIX-153) for
+implementation details.
+
+This feature is implmented with a Bernoulli trial, a consistent-hashing based tablesampler, to achieve a
+Bernoulli sampling on a given population (rows). When executed, given a sampling rate, it leverages Phoenix's
+stats as well HBase's region distribution to perform a table sampling.
+
+As part of the [statistics collection](update_statistics.html), a guide post (a row referrence) is created for
+rows at equidistant bytes from each other. When sampling is required, a Bernoulli trial process is applied
+repeatedly on each guide post on each region with a probability that porportional to the sampling rate. A
+included guide post will result in all the rows in between this and next guide post being included into
+sample population.
+
+#### Performance
+Sampling on a table with a sampling rate = 100% will cost same amount of computational resource as a query without sampling.
+This resouces consumption quicky fall off when a smaller sampling rate is choosing. In general,
+the amortized complexity for sampling process is `O(k + mn)` , with n being the number of the regions of the sampled hbase table,
+m being the number of guide posts, and k being the size of sampled population.
+
+
+#### Repeatable
+Repeatable means it guarantees that the repeated sampling on the same table will resulted in the same sampled result.
+
+Repeatable feature is made possible by a consistent-hashing process applied on the binary representation of the
+start rowkey of each guideposts of each region when sampling process is in action. During hashing,
+a FNV1 implementation with lazy mod method is adopted by default. see [FNV1](http://www.isthe.com/chongo/tech/comp/fnv/).
+
+
+## Examples
+
+For example, to tablesample a table , you would execute the following command.
+Please note, the sampling rate is a numeric value between 0 and 100, inclusive.
+
+ select * from PERSON TABLESAMPLE(12.08);
+
+Some more examples:
+
+ select * from PERSON TABLESAMPLE (12.08) where ADDRESS = 'CA' OR name>'aaa';
+ select count( * ) from PERSON TABLESAMPLE (12.08) LIMIT 2
+ select count( * ) from (select NAME from PERSON TABLESAMPLE (49) limit 20)
+ select * from (select /+NO_INDEX/ * from PERSON tablesample (10) where Name > 'tina10') where ADDRESS = 'CA'
+ select * from PERSON1, PERSON2 tablesample (70) where PERSON1.Name = PERSON2.NAME
+ select /+NO_INDEX/ count( * ) from PERSON tableSample (19), US_POPULATION tableSample (28) where PERSON.Name > US_POPULATION.STATE
+ upsert into personbig(ID, ADDRESS) select id, address from personbig tablesample (1);
+
+To use it in aggregation:
+
+ select count( * ) from PERSON TABLESAMPLE (49) LIMIT 2
+ select count( * ) from (select NAME from PERSON TABLESAMPLE (49) limit 20)
+
+To explain the query:
+
+ explain select count( * ) from PERSON TABLESAMPLE (49) LIMIT 2)
+
+
+
+## Tuning
+
+* Due to sampling process, the TABLESAMPLE clause should be used with some caution.
+For example, a join of two tables is likely to return a match for each row in both tables;
+however, when tablesampling process is applied on either one or both of the two tables, the joining
+may not match the expectation when without sampling.
+
+* The stats needs to be collected in order to achieve the best sampling accuracy. To turn on statistics collection, refer to [Statistics Collection](update_statistics.html).
+
+ <pre>
+ ALTER TABLE my_table SET GUIDE_POSTS_WIDTH = 10000000
+ </pre>
+* A denser guide post setting improves the sampling accuracy, but reduces the performance at the same time. A comparison
+is experimented as fig below:
+
+
+
+
+
Added: phoenix/site/source/src/site/resources/images/sampling_accuracy_perf.jpg
URL: http://svn.apache.org/viewvc/phoenix/site/source/src/site/resources/images/sampling_accuracy_perf.jpg?rev=1811040&view=auto
==============================================================================
Binary file - no diff available.
Propchange: phoenix/site/source/src/site/resources/images/sampling_accuracy_perf.jpg
------------------------------------------------------------------------------
svn:mime-type = application/octet-stream
Modified: phoenix/site/source/src/site/site.xml
URL: http://svn.apache.org/viewvc/phoenix/site/source/src/site/site.xml?rev=1811040&r1=1811039&r2=1811040&view=diff
==============================================================================
--- phoenix/site/source/src/site/site.xml (original)
+++ phoenix/site/source/src/site/site.xml Wed Oct 4 07:09:48 2017
@@ -111,6 +111,7 @@
<item href="paged.html" name="Paged Queries"/>
<item href="salted.html" name="Salted Tables"/>
<item href="skip_scan.html" name="Skip Scan"/>
+ <item href="tablesample.html" name="Table Sampling"/>
<item href="http:divider" name=""/>
<item href="views.html" name="Views"/>
<item href="multi-tenancy.html" name="Multi tenancy"/>