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<title>MADlib: Principal Component Analysis</title>
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<div class="header">
  <div class="headertitle">
<div class="title">Principal Component Analysis<div class="ingroups"><a class="el" href="group__grp__pca.html">Dimensionality Reduction</a></div></div>  </div>
</div><!--header-->
<div class="contents">
<div class="toc"><b>Contents</b> </p>
<ul>
<li class="level1">
<a href="#train">Training Function</a> </li>
<li class="level1">
<a href="#examples">Examples</a> </li>
<li class="level1">
<a href="#notes">Notes</a> </li>
<li class="level1">
<a href="#background_pca">Technical Background</a> </li>
<li class="level1">
<a href="#literature">Literature</a> </li>
<li class="level1">
<a href="#related">Related Topics</a> </li>
</ul>
</div><p>Principal component analysis (PCA) is a mathematical procedure that uses an orthogonal transformation to convert a set of observations of possibly correlated variables into a set of values of linearly uncorrelated variables called principal components. This transformation is defined in such a way that the first principal component has the largest possible variance (i.e., accounts for as much of the variability in the data as possible), and each succeeding component in turn has the highest variance possible under the constraint that it be orthogonal to (i.e., uncorrelated with) the preceding components.</p>
<p>See the <a class="el" href="group__grp__pca__train.html#background_pca">Technical Background</a> for an introduction to principal component analysis and the implementation notes.</p>
<p><a class="anchor" id="train"></a></p>
<dl class="section user"><dt>Training Function</dt><dd>The training functions have the following formats: <pre class="syntax">
pca_train( source_table,
           out_table,
           row_id,
           k,
           grouping_cols,
           lanczos_iter,
           use_correlation,
           result_summary_table
         )
</pre> and <pre class="syntax">
pca_sparse_train( source_table,
                  out_table,
                  row_id,
                  col_id,
                  val_id,
                  row_dim,
                  col_dim,
                  k,
                  grouping_cols,
                  lanczos_iter,
                  use_correlation,
                  result_summary_table
                )
</pre></dd></dl>
<p><b>Arguments</b> </p>
<dl class="arglist">
<dt>source_table </dt>
<dd><p class="startdd">TEXT. Name of the input table containing the data for PCA training. The input data matrix should have \( N \) rows and \( M \) columns, where \( N \) is the number of data points, and \( M \) is the number of features for each data point.</p>
<p>A dense input table is expected to be in the one of the two standard MADlib dense matrix formats, and a sparse input table should be in the standard MADlib sparse matrix format.</p>
<p>The two standard MADlib dense matrix formats are </p>
<pre>{TABLE|VIEW} <em>source_table</em> (
    <em>row_id</em> INTEGER,
    <em>row_vec</em> FLOAT8[],
)</pre><p> and </p>
<pre>{TABLE|VIEW} <em>source_table</em> (
    <em>row_id</em> INTEGER,
    <em>col1</em> FLOAT8,
    <em>col2</em> FLOAT8,
    ...
)</pre><p>Note that the column name <em>row_id</em> is taken as an input parameter, and should contain a list of row indices (starting at 0) for the input matrix.</p>
<p>The input table for sparse PCA is expected to be in the form:</p>
<pre>{TABLE|VIEW} <em>source_table</em> (
    ...
    <em>row_id</em> INTEGER,
    <em>col_id</em> INTEGER,
    <em>val_id</em> FLOAT8,
    ...
)</pre><p>The <em>row_id</em> and <em>col_id</em> columns specify which entries in the matrix are nonzero, and the <em>val_id</em> column defines the values of the nonzero entries. </p>
<p class="enddd"></p>
</dd>
<dt>out_table </dt>
<dd><p class="startdd">TEXT. The name of the table that will contain the output. The output is divided into three tables.</p>
<p>The primary output table (<em>out_table</em>) encodes the principal components with the <em>k</em> highest eigenvalues. The table has the following columns: </p>
<table  class="output">
<tr>
<th>row_id </th><td>Eigenvalue rank in descending order of the eigenvalue size.  </td></tr>
<tr>
<th>principal_components </th><td>Vectors containing elements of the principal components.  </td></tr>
<tr>
<th>eigen_values </th><td>The eigenvalues associated with each principal component.  </td></tr>
</table>
<p>The table <em>out_table</em>_means contains the column means. This table has just one column: </p>
<table  class="output">
<tr>
<th>column_means </th><td>A vector containing the column means for the input matrix.  </td></tr>
</table>
<p>The optional table <em>result_summary_table</em> contains information about the performance of the PCA. The contents of this table are described under the <em>result_summary_table</em> argument. </p>
<p class="enddd"></p>
</dd>
<dt>row_id </dt>
<dd><p class="startdd">TEXT. Column name containing the row IDs in the input source table.</p>
<p class="enddd"></p>
</dd>
<dt>col_id </dt>
<dd><p class="startdd">TEXT. Name of 'col_id' column in sparse matrix representation (sparse matrices only). </p>
<p class="enddd"></p>
</dd>
<dt>val_id </dt>
<dd><p class="startdd">TEXT. Name of 'val_id' column in sparse matrix representation (sparse matrices only). </p>
<p class="enddd"></p>
</dd>
<dt>row_dim </dt>
<dd><p class="startdd">INTEGER. The number of rows in the sparse matrix (sparse matrices only). </p>
<p class="enddd"></p>
</dd>
<dt>col_dim </dt>
<dd><p class="startdd">INTEGER. The number of columns in the sparse matrix (sparse matrices only). </p>
<p class="enddd"></p>
</dd>
<dt>k </dt>
<dd><p class="startdd">INTEGER. The number of principal components to calculate from the input data. </p>
<p class="enddd"></p>
</dd>
<dt>grouping_cols (optional) </dt>
<dd><p class="startdd">TEXT, default: NULL. Currently <em>grouping_cols</em> is present as a placeholder for forward compatibility. The parameter is planned to be implemented as a comma-separated list of column names, with the source data grouped using the combination of all the columns. An independent PCA model will be computed for each combination of the grouping columns.</p>
<p class="enddd"></p>
</dd>
<dt>lanczos_iter (optional) </dt>
<dd><p class="startdd">INTEGER, default: minimum of {k+40, smallest matrix dimension}. The number of Lanczos iterations for the SVD calculation. The Lanczos iteration number roughly corresponds to the accuracy of the SVD calculation, and a higher iteration number corresponds to greater accuracy but longer computation time. The number of iterations must be at least as large as the value of <em>k</em>, but no larger than the smallest dimension of the matrix. If the iteration number is given as zero, then the default number of iterations is used.</p>
<p class="enddd"></p>
</dd>
<dt>use_correlation (optional) </dt>
<dd><p class="startdd">BOOLEAN, default FALSE. Whether to use the correlation matrix for calculating the principal components instead of the covariance matrix. Currently <em>use_correlation</em> is a placeholder for forward compatibility, and this value must be set to false.</p>
<p class="enddd"></p>
</dd>
<dt>result_summary_table (optional) </dt>
<dd><p class="startdd">TEXT, default NULL. Name of the optional summary table. When NULL, no summary table is generated.</p>
<p class="enddd">This sumary table has the following columns: </p>
<table  class="output">
<tr>
<th>rows_used </th><td>INTEGER. Number of data points in the input.  </td></tr>
<tr>
<th>exec_time (ms) </th><td>FLOAT8. Number of milliseconds for the PCA calculation to run.  </td></tr>
<tr>
<th>iter </th><td>INTEGER. Number of iterations used in the SVD calculation.  </td></tr>
<tr>
<th>recon_error </th><td>FLOAT8. The absolute error in the SVD approximation.  </td></tr>
<tr>
<th>relative_recon_error </th><td>FLOAT8. The relative error in the SVD approximation.  </td></tr>
<tr>
<th>use_correlation </th><td>BOOLEAN. Indicates if the correlation matrix was used.  </td></tr>
</table>
</dd>
</dl>
<p><a class="anchor" id="examples"></a></p>
<dl class="section user"><dt>Examples</dt><dd><ol type="1">
<li>View online help for the PCA training function. <pre class="example">
SELECT madlib.pca_train();
</pre></li>
<li>Create the sample data. <pre class="example">
DROP TABLE IF EXISTS mat;
CREATE TABLE mat (
    row_id integer,
    row_vec double precision[]
);
COPY mat (row_id, row_vec) FROM stdin;
0   {1,2,3}
1   {2,1,2}
2   {3,2,1}
\.
</pre></li>
<li>Run the PCA function: <pre class="example">
DROP TABLE result_table;
SELECT pca_train( 'mat',
                  'result_table',
                  'row_id',
                  3
                );
</pre></li>
<li>View the PCA results: <pre class="example">
SELECT * FROM result_table;
</pre> Result <pre class="result">
 row_id |                     principal_components                     |     eigen_values
--------+--------------------------------------------------------------+----------------------
      0 | {0.707106781186547,0.408248290459781,-0.577350269192513}     |                    2
      2 | {-0.707106781186547,0.408248290459781,-0.577350269192512}    | 1.26294130828989e-08
      1 | {2.08166817117217e-17,-0.816496580931809,-0.577350269183852} |    0.816496580927726
</pre></li>
</ol>
</dd></dl>
<p><a class="anchor" id="notes"></a></p>
<dl class="section user"><dt>Notes</dt><dd><ul>
<li>Table names can be optionally schema qualified (current_schemas() would be searched if a schema name is not provided) and all table and column names should follow case-sensitivity and quoting rules per the database. (For instance, 'mytable' and 'MyTable' both resolve to the same entity, i.e. 'mytable'. If mixed-case or multi-byte characters are desired for entity names then the string should be double-quoted; in this case the input would be '"MyTable"').</li>
<li>Because of the centering step in PCA (see <a class="el" href="group__grp__pca__train.html#background_pca">Technical Background</a>), sparse matrices almost always become dense during the training process. Thus, this implementation automatically densifies sparse matrix input, and there should be no expected performance improvement in using sparse matrix input over dense matrix input.</li>
</ul>
</dd></dl>
<p><a class="anchor" id="background_pca"></a></p>
<dl class="section user"><dt>Technical Background</dt><dd></dd></dl>
<p>The PCA implemented here uses an SVD decomposition implementation to recover the principal components (as opposed to the directly computing the eigenvectors of the covariance matrix). Let \( \boldsymbol X \) be the data matrix, and let \( \hat{x} \) be a vector of the column averages of \( \boldsymbol{X}\). PCA computes the matrix \( \hat{\boldsymbol X} \) as </p>
<p class="formulaDsp">
\[ \hat{\boldsymbol X} = {\boldsymbol X} - \vec{e} \hat{x}^T \]
</p>
<p> where \( \vec{e} \) is the vector of all ones.</p>
<p>PCA then computes the SVD matrix factorization </p>
<p class="formulaDsp">
\[ \hat{\boldsymbol X} = {\boldsymbol U}{\boldsymbol \Sigma}{\boldsymbol V}^T \]
</p>
<p> where \( {\boldsymbol \Sigma} \) is a diagonal matrix. The eigenvalues are recovered as the entries of \( {\boldsymbol \Sigma}/(\sqrt{N-1}) \), and the principal components are the rows of \( {\boldsymbol V} \).</p>
<p>It is important to note that the PCA implementation assumes that the user will use only the principal components that have non-zero eigenvalues. The SVD calculation is done with the Lanczos method, with does not guarantee correctness for singular vectors with zero-valued eigenvalues. Consequently, principal components with zero-valued eigenvalues are not guaranteed to be correct. Generally, this will not be problem unless the user wants to use the principal components for the entire eigenspectrum.</p>
<p><a class="anchor" id="literature"></a></p>
<dl class="section user"><dt>Literature</dt><dd></dd></dl>
<p>[1] Principal Component Analysis. <a href="http://en.wikipedia.org/wiki/Principal_component_analysis">http://en.wikipedia.org/wiki/Principal_component_analysis</a></p>
<p>[2] Shlens, Jonathon (2009), A Tutorial on Principal Component Analysis</p>
<p><a class="anchor" id="related"></a></p>
<dl class="section user"><dt>Related Topics</dt><dd></dd></dl>
<p>File <a class="el" href="pca_8sql__in.html" title="Principal Component Analysis. ">pca.sql_in</a> documenting the SQL functions</p>
<p><a class="el" href="group__grp__pca__project.html">Principal Component Projection</a> </p>
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