Get all posts without tags

Question

I'm working with a tag plugin. Is there a WordPress way to get_posts or query all posts that have no tags?

EDIT

I had already queried the WP Codex, search Stackexchange and Google for a related question at the time of the question.

I found several results for helping finding tags, but not the NOT IN operator in a tax_query. I didn't have any code to share yet as I didn't have the information I needed to build the query $args.

Answer 1

A WP_Query where 'tax_query' has all tag terms and operator 'NOT IN':

$tags = get_terms('post_tag', array('fields'=>'ids') );
$args = array(
  'post_type' => 'post',
  'posts_per_page' => -1,
  'tax_query' => array(
    array(
      'taxonomy' => 'post_tag',
      'field' => 'id',
      'terms' => $tags,
      'operator' => 'NOT IN'
    )
  )
);
$untagged = new WP_Query( $args );

Answer 2

Not exactly answers the question, but might be a better solution than the above when dealing with bigger databases

// Get all posts IDs - Query took 0.0030 seconds
$all_posts_ids_query = $wpdb->prepare( "SELECT $wpdb->posts.ID FROM $wpdb->posts WHERE $wpdb->posts.post_type = '%s' GROUP BY $wpdb->posts.ID", 'your-post-type' );

$all_posts_ids = $wpdb->get_col( $all_posts_ids_query );


// Get all posts IDs that has tags - Query took 0.0300 seconds
$all_posts_ids_with_tags_query = $wpdb->prepare( "SELECT $wpdb->posts.ID FROM $wpdb->posts
    INNER JOIN $wpdb->term_relationships ON ($wpdb->posts.ID = $wpdb->term_relationships.object_id)
    INNER JOIN $wpdb->term_taxonomy ON ($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id)
    WHERE 1=1 AND ( $wpdb->term_taxonomy.taxonomy = '%s' )
    AND $wpdb->posts.post_type = '%s' GROUP BY $wpdb->posts.ID", 'your-taxonomy-name', 'your-post-type' );

$all_posts_ids_with_tags = $wpdb->get_col( $all_posts_ids_with_tags_query );


// Diff IDs arrays to get posts without tags
$all_posts_ids_without_tags = array_diff( $all_posts_ids, $all_posts_ids_with_tags );

DB size this was tested on (quite small but may give some representation), posts: ~3.000, terms: ~1.000, term relationships: ~6.000

Answer 3

<?php
  $args = array(
    'post_type' => 'post',
    'posts_per_page' => -1,
    'tax_query' => array(
        array(
            'taxonomy' => 'post_tag',
            'field' => 'id',
            'terms' => 'null',
            'operator' => 'NOT IN'
    )));
  $untagged = new WP_Query( $args );?>