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| <title>MADlib: CountMin (Cormode-Muthukrishnan)</title> |
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| <span id="projectnumber">1.10.0</span> |
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| <div id="projectbrief">User Documentation for MADlib</div> |
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| <div class="title">CountMin (Cormode-Muthukrishnan)<div class="ingroups"><a class="el" href="group__grp__early__stage.html">Early Stage Development</a> » <a class="el" href="group__grp__sketches.html">Cardinality Estimators</a></div></div> </div> |
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| <div class="contents"> |
| <div class="toc"><b>Contents</b> <ul> |
| <li> |
| <a href="#syntax">Syntax</a> </li> |
| <li> |
| <a href="#examples">Examples</a> </li> |
| <li> |
| <a href="#literature">Literature</a> </li> |
| <li> |
| <a href="#related">Related Topics</a> </li> |
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| </div><dl class="section warning"><dt>Warning</dt><dd><em> This MADlib method is still in early stage development. There may be some issues that will be addressed in a future version. Interface and implementation is subject to change. </em></dd></dl> |
| <p>This module implements Cormode-Muthukrishnan <em>CountMin</em> sketches on integer values, implemented as a user-defined aggregate. It also provides scalar functions over the sketches to produce approximate counts, order statistics, and histograms.</p> |
| <p><a class="anchor" id="syntax"></a></p><dl class="section user"><dt>Syntax</dt><dd><ul> |
| <li>Get a sketch of a selected column specified by <em>col_name</em>. <pre class="syntax"> |
| cmsketch( col_name ) |
| </pre></li> |
| <li>Get the number of rows where <em>col_name = p</em>, computed from the sketch obtained from <code>cmsketch</code>. <pre class="syntax"> |
| cmsketch_count( cmsketch, |
| p |
| ) |
| </pre></li> |
| <li>Get the number of rows where <em>col_name</em> is between <em>m</em> and <em>n</em> inclusive. <pre class="syntax"> |
| cmsketch_rangecount( cmsketch, |
| m, |
| n |
| ) |
| </pre></li> |
| <li>Get the <em>k</em>th percentile of <em>col_name</em> where <em>count</em> specifies number of rows. <em>k</em> should be an integer between 1 to 99. <pre class="syntax"> |
| cmsketch_centile( cmsketch, |
| k, |
| count |
| ) |
| </pre></li> |
| <li>Get the median of col_name where <em>count</em> specifies number of rows. This is equivalent to <code><a class="el" href="sketch_8sql__in.html#a2f2ab2fe3244515f5f73d49690e73b39">cmsketch_centile</a>(<em>cmsketch</em>,50,<em>count</em>)</code>. <pre class="syntax"> |
| cmsketch_median( cmsketch, |
| count |
| ) |
| </pre></li> |
| <li>Get an n-bucket histogram for values between min and max for the column where each bucket has approximately the same width. The output is a text string containing triples {lo, hi, count} representing the buckets; counts are approximate. <pre class="syntax"> |
| cmsketch_width_histogram( cmsketch, |
| min, |
| max, |
| n |
| ) |
| </pre></li> |
| <li>Get an n-bucket histogram for the column where each bucket has approximately the same count. The output is a text string containing triples {lo, hi, count} representing the buckets; counts are approximate. Note that an equi-depth histogram is equivalent to a spanning set of equi-spaced centiles. <pre class="syntax"> |
| cmsketch_depth_histogram( cmsketch, |
| n |
| ) |
| </pre></li> |
| </ul> |
| </dd></dl> |
| <p><a class="anchor" id="examples"></a></p><dl class="section user"><dt>Examples</dt><dd></dd></dl> |
| <ol type="1"> |
| <li>Generate some data. <pre class="example"> |
| CREATE TABLE data(class INT, a1 INT); |
| INSERT INTO data SELECT 1,1 FROM generate_series(1,10000); |
| INSERT INTO data SELECT 1,2 FROM generate_series(1,15000); |
| INSERT INTO data SELECT 1,3 FROM generate_series(1,10000); |
| INSERT INTO data SELECT 2,5 FROM generate_series(1,1000); |
| INSERT INTO data SELECT 2,6 FROM generate_series(1,1000); |
| </pre></li> |
| <li>Count number of rows where a1 = 2 in each class. <pre class="example"> |
| SELECT class, |
| cmsketch_count( |
| cmsketch( a1 ), |
| 2 |
| ) |
| FROM data GROUP BY data.class; |
| </pre> Result: <pre class="result"> |
| class | cmsketch_count |
|  ------+---------------- |
| 2 | 0 |
| 1 | 15000 |
| (2 rows) |
| </pre></li> |
| <li>Count number of rows where a1 is between 3 and 6. <pre class="example"> |
| SELECT class, |
| cmsketch_rangecount( |
| cmsketch(a1), |
| 3, |
| 6 |
| ) |
| FROM data GROUP BY data.class; |
| </pre> Result: <pre class="result"> |
| class | cmsketch_rangecount |
|  ------+--------------------- |
| 2 | 2000 |
| 1 | 10000 |
| (2 rows) |
| </pre></li> |
| <li>Compute the 90th percentile of all of a1. <pre class="example"> |
| SELECT cmsketch_centile( |
| cmsketch( a1 ), |
| 90, |
| count(*) |
| ) |
| FROM data; |
| </pre> Result: <pre class="result"> |
| cmsketch_centile |
|  ----------------- |
| 3 |
| (1 row) |
| </pre></li> |
| <li>Produce an equi-width histogram with 2 bins between 0 and 10. <pre class="example"> |
| SELECT cmsketch_width_histogram( |
| cmsketch( a1 ), |
| 0, |
| 10, |
| 2 |
| ) |
| FROM data; |
| </pre> Result: <pre class="result"> |
| cmsketch_width_histogram |
|  ----------------------------------- |
| [[0L, 4L, 35000], [5L, 10L, 2000]] |
| (1 row) |
| </pre></li> |
| <li>Produce an equi-depth histogram of a1 with 2 bins of approximately equal depth. <pre class="example"> |
| SELECT cmsketch_depth_histogram( |
| cmsketch( a1 ), |
| 2 |
| ) |
| FROM data; |
| </pre> Result: <pre class="result"> |
| cmsketch_depth_histogram |
|  ---------------------------------------------------------------------- |
| [[-9223372036854775807L, 1, 10000], [2, 9223372036854775807L, 27000]] |
| (1 row) |
| </pre></li> |
| </ol> |
| <p><a class="anchor" id="literature"></a></p><dl class="section user"><dt>Literature</dt><dd></dd></dl> |
| <p>[1] G. Cormode and S. Muthukrishnan. An improved data stream summary: The count-min sketch and its applications. LATIN 2004, J. Algorithm 55(1): 58-75 (2005) . <a href="http://dimacs.rutgers.edu/~graham/pubs/html/CormodeMuthukrishnan04CMLatin.html">http://dimacs.rutgers.edu/~graham/pubs/html/CormodeMuthukrishnan04CMLatin.html</a></p> |
| <p>[2] G. Cormode. Encyclopedia entry on 'Count-Min Sketch'. In L. Liu and M. T. Ozsu, editors, Encyclopedia of Database Systems, pages 511-516. Springer, 2009. <a href="http://dimacs.rutgers.edu/~graham/pubs/html/Cormode09b.html">http://dimacs.rutgers.edu/~graham/pubs/html/Cormode09b.html</a></p> |
| <p><a class="anchor" id="related"></a>File <a class="el" href="sketch_8sql__in.html" title="SQL functions for sketch-based approximations of descriptive statistics. ">sketch.sql_in</a> documenting the SQL functions.</p> |
| <p>Module grp_quantile for a different implementation of quantile function. </p> |
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