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| <div class="title">FM (Flajolet-Martin)<div class="ingroups"><a class="el" href="group__grp__stats.html">Statistics</a> » <a class="el" href="group__grp__desc__stats.html">Descriptive Statistics</a> » <a class="el" href="group__grp__sketches.html">Cardinality Estimators</a></div></div> </div> |
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| <div class="toc"><b>Contents</b> <ul> |
| <li> |
| <a href="#syntax">Syntax</a> </li> |
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| <a href="#examples">Examples</a> </li> |
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| <a href="#literature">Literature</a> </li> |
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| </div><p><a class="el" href="sketch_8sql__in.html#ae27d5aaa5e4b426bcfe55e05a89c8e0b">fmsketch_dcount</a> can be run on a column of any type. It returns an approximation to the number of distinct values (a la <code>COUNT(DISTINCT x)</code>), but faster and approximate. Like any aggregate, it can be combined with a GROUP BY clause to do distinct counts per group.</p> |
| <p><a class="anchor" id="syntax"></a></p><dl class="section user"><dt>Syntax</dt><dd></dd></dl> |
| <p>Get the number of distinct values in a designated column. </p><pre class="syntax"> |
| fmsketch_dcount( col_name ) |
| </pre><dl class="section note"><dt>Note</dt><dd>This is a <a href="https://www.postgresql.org/docs/current/static/xaggr.html">User Defined Aggregate</a> which returns the results when used in a query. Use "CREATE TABLE AS ", with the UDA as subquery if the results are to be stored. This is unlike the usual MADlib stored procedure interface which places the results in a table instead of returning it.</dd></dl> |
| <p><a class="anchor" id="examples"></a></p><dl class="section user"><dt>Examples</dt><dd><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>Find the distinct number of values for each class. <pre class="example"> |
| SELECT class, fmsketch_dcount(a1) |
| FROM data |
| GROUP BY data.class; |
| </pre> Result: <pre class="result"> |
| class | fmsketch_dcount |
|  ------+----------------- |
| 2 | 2 |
| 1 | 3 |
| (2 rows) |
| </pre></li> |
| </ol> |
| </dd></dl> |
| <p><a class="anchor" id="literature"></a></p><dl class="section user"><dt>Literature</dt><dd>[1] P. Flajolet and N.G. Martin. Probabilistic counting algorithms for data base applications, Journal of Computer and System Sciences 31(2), pp 182-209, 1985. <a href="http://algo.inria.fr/flajolet/Publications/FlMa85.pdf">http://algo.inria.fr/flajolet/Publications/FlMa85.pdf</a></dd></dl> |
| <p><a class="anchor" id="related"></a></p><dl class="section user"><dt>Related Topics</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 function. </dd></dl> |
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