Using WP Query to search within ALL keys in meta query

As @Howdy_McGee suggested and I made a quick test. Removing key do the trick so you can just remove the key and add compare LIKE if you do not want exact match.

Example:-

$posts = new WP_Query(array(
    'meta_query' => array(
        array(
            'value' => 'meta_value'
        )
    )
));

This will produce the SQL like 

WHERE 1=1  AND ( CAST(wp_postmeta.meta_value AS CHAR) = 'meta_value' )

That is what we want!

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