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=pod
=head1 NAME
MongoDB::Indexing - Indexing collections
=head1 CREATING AN INDEX
Unique and non-unique indexes can be created on collections using 
C<MongoDB::Collection::ensure_index>.
For example, to create a non-unique index on C<x>:
    $collection->ensure_index({'x' => 1})
To create a unique index on C<y>:
    $collection->ensure_index({"y" => 1}, {"unique" => 1});
Multi-key indexes can be created to speed up queries like "sort by name, then by
age."  Index direction (1 or -1) is only important for multi-key indexes and
should be the sort order.  So, for example, if we want a fast sort by name 
ascending and age descending, we'd write:
    my $idx = Tie::IxHash->new(name => 1, age => -1);
    $collection->ensure_index($idx);
Keep in mind that you should use L<Tie::IxHash> for multi-key indexes to 
guarantee the keys will be saved in the correct order.
=head2 Options
The second parameter to C<MongoDB::Collection::ensure_index> specifies index
options.  Available options are:
=over
=item C<< unique =E<gt> boolean >>
By default, indexes are not unique. To create a unique index, pass 
C<< unique =E<gt> true >>.  C<true> can be L<boolean::true> or any other true value.
=item C<< drop_dups => boolean >>
If a unique index is being created on an existing set of data that has duplicate
values, creating the index will fail.  To force the index creation by deleting 
duplicate values, use this option.  Again, any value that evaluates to true will
work.
=item C<safe =E<gt> boolean>
If the update fails and safe is set, this function will return 0.  You should 
check C<MongoDB::Database::last_error> to find out why the update failed.
=item C<< background =E<gt> boolean >>
Create the index as a background operation.
=item C<< name =E<gt> string >>
Give the index a non-default name.  This can be useful if the index contains so
many keys that you get an "index name too long" assertion, or if you just prefer
a more human-readable name.
=back
=head2 See Also
MongoDB documentation on indexing: 
L<http://dochub.mongodb.org/core/indexes>.
=head1 GEOSPATIAL INDEXES
Starting in version 1.3.3 of MongoDB, you can create geospatial indexes.  These
are useful for querying for "N documents nearest this point" or "documents 
within this shape."
To create an index for geospatial queries, use "2d" instead of 1 or -1.  
For example, this would create an index on the "location" field:
    $coll->ensure_index({"location" => "2d"});
Then, you can query for documents using C<$near>:
    my $cursor = $coll->query({"location" => {'$near' => [44, -70]}})->limit(10);
This finds the 10 nearest documents (automatically sorted by distance ascending) 
to latitude -70, longitude 44.
Documents must have some sort of pair in the "location" field, although the 
database is pretty flexible as to what it will accept:
    # valid geospatial locations
    $coll->insert({"location" => [44, -70]}); 
    $coll->insert({"location" => {"x" => 44, "y" => -70}}); 
    $coll->insert({"location" => {"foo" => 44, "bar" => -70}}); 
You can save values in C<(x,y)> or C<(y,x)> order, but you must be consistent.
By default, the geospatial index assumes that points will lie between -180 and 
180, for longitude and latitude queries.
=head2 Options
=over
=item C<< min =E<gt> int >>
By default, the geospatial index assumes that points will lie between -180 and 
180, for longitude and latitude queries.  If you need an alternative minimum
value, you can use this option.  This value is exclusive: if you specify
C<< min =E<gt> 0 >>, you cannot save a point with a 0 value coordinate.
=item C<< max =E<gt> int >>
Alternative maximum value, exclusive.
=back
=head2 See Also
MongoDB documentation on geospatial indexes:
L<http://dochub.mongodb.org/core/geo>.