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| <div class="title">CountMin (Cormode-Muthukrishnan)</div> </div> |
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| <dl class="user"><dt><b>About:</b></dt><dd>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.</dd></dl> |
| <dl class="user"><dt><b>Usage:</b></dt><dd></dd></dl> |
| <ul> |
| <li>Get a sketch of a selected column specified by <em>col_name</em>. <pre>SELECT <a class="el" href="sketch_8sql__in.html#a323b932332f2db302469f14dc5fae369">cmsketch</a>(<em>col_name</em>) FROM table_name;</pre></li> |
| </ul> |
| <ul> |
| <li>Get the number of rows where <em>col_name = p</em>, computed from the sketch obtained from <code>cmsketch</code>. <pre>SELECT <a class="el" href="sketch_8sql__in.html#a3498d2c778d1289154f61d34e84c609e">cmsketch_count</a>(<em>cmsketch</em>,<em>p</em>) FROM table_name;</pre></li> |
| </ul> |
| <ul> |
| <li>Get the number of rows where <em>col_name</em> is between <em>m</em> and <em>n</em> inclusive. <pre>SELECT <a class="el" href="sketch_8sql__in.html#aeff9e36cfb3338c4e405d4ac77d3968c">cmsketch_rangecount</a>(<em>cmsketch</em>,<em>m</em>,<em>n</em>) FROM table_name;</pre></li> |
| </ul> |
| <ul> |
| <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>SELECT <a class="el" href="sketch_8sql__in.html#a2f2ab2fe3244515f5f73d49690e73b39">cmsketch_centile</a>(<em>cmsketch</em>,<em>k</em>,<em>count</em>) FROM table_name;</pre></li> |
| </ul> |
| <ul> |
| <li>Get the median of <em>col_name</em> 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>SELECT <a class="el" href="sketch_8sql__in.html#a0ef6428fa7ba4b7f7b1f633e6f8003ea">cmsketch_median</a>(<em>cmsketch</em>,<em>count</em>) FROM table_name;</pre></li> |
| </ul> |
| <ul> |
| <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>SELECT <a class="el" href="sketch_8sql__in.html#a8482f62849adf40a2c7df78c23ea33a4">cmsketch_width_histogram</a>(<em>cmsketch</em>,<em>min</em>,<em>max</em>,<em>n</em>) FROM table_name;</pre></li> |
| </ul> |
| <ul> |
| <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>SELECT <a class="el" href="sketch_8sql__in.html#a9e6d30f20b724b96249cc4a0f67a279e">cmsketch_depth_histogram</a>(<em>cmsketch</em>,<em>n</em>) FROM table_name;</pre></li> |
| </ul> |
| <dl class="user"><dt><b>Examples:</b></dt><dd></dd></dl> |
| <ol type="1"> |
| <li>Generate some data <div class="fragment"><pre class="fragment"> |
| sql> CREATE TABLE data(class INT, a1 INT); |
| sql> INSERT INTO data SELECT 1,1 FROM generate_series(1,10000); |
| sql> INSERT INTO data SELECT 1,2 FROM generate_series(1,15000); |
| sql> INSERT INTO data SELECT 1,3 FROM generate_series(1,10000); |
| sql> INSERT INTO data SELECT 2,5 FROM generate_series(1,1000); |
| sql> INSERT INTO data SELECT 2,6 FROM generate_series(1,1000); |
| </pre></div></li> |
| <li>Count number of rows where a1 = 2 in each class <div class="fragment"><pre class="fragment"> |
| sql> SELECT class,cmsketch_count(cmsketch(a1),2) FROM data GROUP BY data.class; |
| class | cmsketch_count |
| -------+---------------- |
| 2 | 0 |
| 1 | 15000 |
| (2 rows) |
| </pre></div></li> |
| <li>Count number of rows where a1 is between 3 and 6 <div class="fragment"><pre class="fragment"> |
| sql> SELECT class,cmsketch_rangecount(cmsketch(a1),3,6) FROM data GROUP BY data.class; |
| class | cmsketch_rangecount |
| -------+--------------------- |
| 2 | 2000 |
| 1 | 10000 |
| (2 rows) |
| </pre></div></li> |
| <li>Compute the 90th percentile of all of a1 <div class="fragment"><pre class="fragment"> |
| sql> SELECT cmsketch_centile(cmsketch(a1),90,count(*)) FROM data; |
| cmsketch_centile |
| ------------------ |
| 3 |
| (1 row) |
| </pre></div></li> |
| <li>Produce an equi-width histogram with 2 bins between 0 and 10 <div class="fragment"><pre class="fragment"> |
| sql> SELECT cmsketch_width_histogram(cmsketch(a1),0,10,2) FROM data; |
| cmsketch_width_histogram |
| ------------------------------------ |
| [[0L, 4L, 35000], [5L, 10L, 2000]] |
| (1 row) |
| </pre></div></li> |
| <li>Produce an equi-depth histogram of a1 with 2 bins of approximately equal depth <div class="fragment"><pre class="fragment"> |
| sql> SELECT cmsketch_depth_histogram(cmsketch(a1),2) FROM data; |
| cmsketch_depth_histogram |
| ----------------------------------------------------------------------- |
| [[-9223372036854775807L, 1, 10000], [2, 9223372036854775807L, 27000]] |
| (1 row) |
| </pre></div></li> |
| </ol> |
| <dl class="user"><dt><b>Literature:</b></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> |
| <dl class="see"><dt><b>See also:</b></dt><dd>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. <br/> |
| <br/> |
| Module <a class="el" href="group__grp__quantile.html">Quantile</a> for a different implementation of <a class="el" href="quantile_8sql__in.html#a6955e44659a0817803ab7a11a9a7e8a5" title="Computes quantile.">quantile</a> function. </dd></dl> |
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