In sum;

In function.php of your theme:

function my_theme_join_mytable_to_WPQuery($join) {
    global $wpdb;
    $join .= " LEFT JOIN {$wpdb->prefix}mytable ON $wpdb->posts.ID = {$wpdb->prefix}mytable.postid ";
    return $join;
}
add_filter('posts_join', 'my_theme_join_mytable_to_WPQuery');

Jigsaw puzzle symbolizing joined tables © Ajv123ajv | Dreamstime Stock Photos

add_filter(‘posts_join’, ‘my_theme_join_mytable_to_WPQuery’);

What this does, it tells wordpress to call your custom function my_theme_join_mytable_to_WPQuery  when building the posts fetching query. so every time a user views your site and sees the list of posts or search posts. the function will be called before querying the database.  The filter hook passes the join statement sent by user.

Then add a function with the custom name you’ve chosen earlier: my_theme_join_mytable_to_WPQuery

function my_theme_join_mytable_to_WPQuery($join) {

}

The join variable is a string that contains the join statements added by other filters or wordpress.  you need concatenate to it. as:

$join .= " LEFT JOIN MYTABLE ON MYTABLE.id = wp_posts.post.id = MYTABLE.postid";

then you must return join back

return $join;

It’s better however to use wordpress table names and prefixes.  it’s possible the prefix of tables not wp_ or table names for wordpress change.  instead $wpdb->prefix to get the prefix  and $wpdb->posts to get wordpress post table (this includes the prefix).  I assume your custom table also has the same wordpress prefix as it should.

therefore your table becomes {$wpdb->prefix}mytable

You need to import the variable $wpdb from the global scope.  using global $wpdb.

Finally it would look like this:

function my_theme_join_mytable_to_WPQuery($join) {
    global $wpdb;
    $join .= " LEFT JOIN {$wpdb->prefix}mytable ON $wpdb->posts.ID = {$wpdb->prefix}mytable.postid ";
    return $join;
}
add_filter('posts_join', 'my_theme_join_mytable_to_WPQuery');