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| <div id="projectname">MADlib |
|  <span id="projectnumber">0.6</span> <span style="font-size:10pt; font-style:italic"><a href="../latest/./group__grp__summary.html"> A newer version is available</a></span> |
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| <dl class="user"><dt><b>About:</b></dt><dd></dd></dl> |
| <p>'summary' is a generic function used to produce summary statistics of any data table. The function invokes particular 'methods' from the MADlib library to provide an overview of the data.</p> |
| <dl class="user"><dt><b>Usage:</b></dt><dd>The summary function can be invoked in the following way: <div class="fragment"><pre class="fragment"> |
| SELECT MADlib.summary |
| ( |
| source_table TEXT, -- Source table name (Required) |
| output_table TEXT, -- Output table name (Required) |
| target_cols TEXT, -- Comma separated columns for which summary is desired |
| -- (Default: NULL - produces result for all columns) |
| grouping_cols TEXT, -- Comma separated columns on which to group results |
| -- (Default: NULL - provides summary on complete table) |
| get_distinct BOOLEAN, -- Are distinct values required? |
| -- (Default: True) |
| get_quartiles BOOLEAN, -- Are quartiles required? |
| -- (Default: True) |
| ntile_array FLOAT8[], -- Array of quantile values to compute |
| -- (Default: NULL - Quantile array not included) |
| how_many_mfv INTEGER, -- How many most-frequent-values to compute? |
| -- (Default: 10) |
| get_estimates BOOLEAN -- Should we produce exact or estimated values? |
| ) -- (Default: True) |
| </pre></div></dd></dl> |
| <p>Note:</p> |
| <ul> |
| <li>Currently, estimated values are only implemented for the distinct values computation.</li> |
| <li>The '<em>get_estimates</em>' parameter controls computation for two statistics<ul> |
| <li>If '<em>get_estimates</em>' is True then the distinct value computation is estimated. Further, the most frequent values computation is computed using a "quick and dirty" method that does parallel aggregation in GPDB at the expense of missing some of the most frequent values.</li> |
| <li>If '<em>get_estimates</em>' is False then the distinct values are computed in a slow but exact method. The most frequent values are computed using a faithful implementation that preserves the approximation guarantees of the Cormode/Muthukrishnan method (more information in <a class="el" href="group__grp__mfvsketch.html">MFV (Most Frequent Values)</a>)</li> |
| </ul> |
| </li> |
| </ul> |
| <p>The output of the function is a composite type containing: ouput_table TEXT, -- Name of the output table row_count INT4, -- Number of rows in the output table duration FLOAT8 -- Time taken (in seconds) to compute the summary</p> |
| <p>The summary stastics are stored in the 'output_table' relation provided in the arguments. The relation 'output_table' can contain the following table (presence of some columns depends on the argument values) </p> |
| <div class="fragment"><pre class="fragment"> |
| - group_by_column : Group-by column names (NULL if none provided) |
| - group_by_value : Values of the Group-by column (NULL if no grouping) |
| - target_column : Targeted column values for which summary is requested |
| - column_number : Physical column number for the target column, as described in pg_attribute |
| - data_type : Data type of target column. Standard GPDB descriptors will be displayed |
| - row_count : Number of rows for the target column |
| - distinct_values : Number of distinct values in the target column |
| - missing_values : Number of missing values in the target column |
| - blank_values : Number of blank values (blanks are defined by the regular expression '^\w*$') |
| - fraction_missing : Percentage of total rows that are missing. Will be expressed as a decimal (e.g. 0.3) |
| - fraction_blank : Percentage of total rows that are blank. Will be expressed as a decimal (e.g. 0.3) |
| - mean : Mean value of target column (if target is numeric, else NULL) |
| - variance : Variance of target columns (if target is numeric, else NULL for strings) |
| - min : Min value of target column (for strings this is the length of the shortest string) |
| - max : Max value of target column (for strings this is the length of the longest string) |
| - first_quartile : First quartile (25th percentile, only for numeric columns) |
| - median : Median value of target column (if target is numeric, else NULL) |
| - third_quartile : Third quartile (25th percentile, only for numeric columns) |
| - quantile_array : Percentile values corresponding to ntile_array |
| - most_frequent_values : Most frequent values |
| - mfv_frequencies : Frequency of the most frequent values |
| </pre></div><p>The output can be obtained as </p> |
| <div class="fragment"><pre class="fragment"> |
| sql> SELECT * FROM 'output_table'; |
| </pre></div><p>The usage information can be obtained at any time directly from the function using </p> |
| <div class="fragment"><pre class="fragment"> |
| sql> SELECT summary('usage'); |
| </pre></div> </div> |
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