Register custom query args parameter for WP_Query()

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I have written a plugin which can store a table of technical specifications for each post (product) in an own database table.
For the search function of the site I want to add a custom parameter to the array passed to WP_Query().
I have no clue where to register this custom parameter so that it is handled by the plugin as soon as WP_Query() executes the search.

Is there a hook that I can use in the plugin to restrict the posts found by WP_Query() to a certain set matching the specs given?
Or do I eventually have to build the entire query using SQL?

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See the following example: next to “standard”-parameters I have a custom parameter _spec its value I want to be parsed in some way by the plugin. 

<?php

new WP_Query(
    array(

        's' => 'some keyword', //keyword criterion
        'category' => 'some category', //taxonomy criterion

        '_spec' => 'year_min=1980;year_max=2010', //custom criterion to be handled by the plugin

    )
);

?>
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  1. After further research I figured out a solution. Given the following WP_Query:

    <?php

new WP_Query(
    array(

        's' => 'some keyword', //keyword criterion
        'category' => 'some category', //taxonomy criterion

        '_spec' => 'some value', //custom criterion to be handled by the plugin

    )
);

?>

    You can handle custom parameters using pre_get_posts in combination with posts_where:

    <?php

add_action( 'pre_get_posts', 'my_pre_get_posts' ); //hook into the query before it is executed

$custom_where_string = ''; //used to save the generated where string between filter functions

function my_pre_get_posts( $query )
{
    global $custom_where_string;

    //if the custom parameter is used
    if(isset($query->query_vars['_spec'])){

        //here you can parse the contents of $query->query_vars['_spec'] to modify the query
        //even the first WHERE starts with AND, because WP adds a "WHERE 1=1" in front of every WHERE section
        $custom_where_string = 'AND ...';

        //only if the custom parameter is used, hook into the generation of the query
        add_filter('posts_where', 'my_posts_where'));
    }
}

function my_posts_where( $where )
{
    global $custom_where_string;

    //append our custom where expression(s)
    $where .= $custom_where_string;

    //clean up to avoid unexpected things on other queries
    remove_filter('posts_where', 'my_posts_where'));
    $custom_where_string = '';

    return $where;
}


?>

    PS: Take a look at http://codex.wordpress.org/Class_Reference/WP_Query#Filters for manipulating other parts of the SQL-query (e.g. JOIN)