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DBD::SQLite::Cookbook(User Contributed Perl DocumentatDBD::SQLite::Cookbook(3)

NAME
       DBD::SQLite::Cookbook - The DBD::SQLite Cookbook

DESCRIPTION
       This is the DBD::SQLite cookbook.

       It is intended to provide a place to keep a variety of functions and
       formals for use in callback APIs in DBD::SQLite.

AGGREGATE FUNCTIONS
   Variance
       This is a simple aggregate function which returns a variance. It is
       adapted from an example implementation in pysqlite.

	 package variance;

	 sub new { bless [], shift; }

	 sub step {
	     my ( $self, $value ) = @_;

	     push @$self, $value;
	 }

	 sub finalize {
	     my $self = $_[0];

	     my $n = @$self;

	     # Variance is NULL unless there is more than one row
	     return undef unless $n || $n == 1;

	     my $mu = 0;
	     foreach my $v ( @$self ) {
		 $mu += $v;
	     }
	     $mu /= $n;

	     my $sigma = 0;
	     foreach my $v ( @$self ) {
		 $sigma += ($v - $mu)**2;
	     }
	     $sigma = $sigma / ($n - 1);

	     return $sigma;
	 }

	 # NOTE: If you use an older DBI (< 1.608),
	 # use $dbh->func(..., "create_aggregate") instead.
	 $dbh->sqlite_create_aggregate( "variance", 1, 'variance' );

       The function can then be used as:

	 SELECT group_name, variance(score)
	 FROM results
	 GROUP BY group_name;

   Variance (Memory Efficient)
       A more efficient variance function, optimized for memory usage at the
       expense of precision:

	 package variance2;

	 sub new { bless {sum => 0, count=>0, hash=> {} }, shift; }

	 sub step {
	     my ( $self, $value ) = @_;
	     my $hash = $self->{hash};

	     # by truncating and hashing, we can comsume many more data points
	     $value = int($value); # change depending on need for precision
				   # use sprintf for arbitrary fp precision
	     if (exists $hash->{$value}) {
		 $hash->{$value}++;
	     } else {
		 $hash->{$value} = 1;
	     }
	     $self->{sum} += $value;
	     $self->{count}++;
	 }

	 sub finalize {
	     my $self = $_[0];

	     # Variance is NULL unless there is more than one row
	     return undef unless $self->{count} > 1;

	     # calculate avg
	     my $mu = $self->{sum} / $self->{count};

	     my $sigma = 0;
	     while (my ($h, $v) = each %{$self->{hash}}) {
		 $sigma += (($h - $mu)**2) * $v;
	     }
	     $sigma = $sigma / ($self->{count} - 1);

	     return $sigma;
	 }

       The function can then be used as:

	 SELECT group_name, variance2(score)
	 FROM results
	 GROUP BY group_name;

   Variance (Highly Scalable)
       A third variable implementation, designed for arbitrarily large data
       sets:

	 package variance3;

	 sub new { bless {mu=>0, count=>0, S=>0}, shift; }

	 sub step {
	     my ( $self, $value ) = @_;
	     $self->{count}++;
	     my $delta = $value - $self->{mu};
	     $self->{mu} += $delta/$self->{count};
	     $self->{S} += $delta*($value - $self->{mu});
	 }

	 sub finalize {
	     my $self = $_[0];
	     return $self->{S} / ($self->{count} - 1);
	 }

       The function can then be used as:

	 SELECT group_name, variance3(score)
	 FROM results
	 GROUP BY group_name;

FTS3 fulltext indexing
   Sparing database disk space
       As explained in <http://www.sqlite.org/fts3.html#section_6>, each FTS3
       table "t" is stored internally within three regular tables "t_content",
       "t_segments" and "t_segdir".  The last two tables contain the fulltext
       index.  The first table "t_content" stores the complete documents being
       indexed ... but if copies of the same documents are already stored
       somewhere else, or can be computed from external resources (for example
       as HTML or MsWord files in the filesystem), then this is quite a waste
       of space. SQLite itself only needs the "t_content" table for
       implementing the "offsets()" and "snippet()" functions, which are not
       always usable anyway (in particular when using utf8 characters greater
       than 255).

       So an alternative strategy is to use SQLite only for the fulltext index
       and metadata, and to keep the full documents outside of SQLite : to do
       so, after each insert or update in the FTS3 table, do an update in the
       "t_content" table, setting the content column(s) to NULL. Of course
       your application will need an algorithm for finding the external
       resource corresponding to any docid stored within SQLite. Furthermore,
       SQLite "offsets()" and "snippet()" functions cannot be used, so if such
       functionality is needed, it has to be directly programmed within the
       Perl application.  In short, this strategy is really a hack, because
       FTS3 was not originally programmed with that behaviour in mind; however
       it is workable and has a strong impact on the size of the database
       file.

SUPPORT
       Bugs should be reported via the CPAN bug tracker at

       http://rt.cpan.org/NoAuth/ReportBug.html?Queue=DBD-SQLite
       <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=DBD-SQLite>

TO DO
       * Add more and varied cookbook recipes, until we have enough to turn
       them into a separate CPAN distribution.

       * Create a series of tests scripts that validate the cookbook recipies.

AUTHOR
       Adam Kennedy <adamk@cpan.org>

       Laurent Dami <dami@cpan.org>

COPYRIGHT
       Copyright 2009 - 2012 Adam Kennedy.

       This program is free software; you can redistribute it and/or modify it
       under the same terms as Perl itself.

       The full text of the license can be found in the LICENSE file included
       with this module.

perl v5.16.2			  2012-06-09	      DBD::SQLite::Cookbook(3)
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