Get all IDs from $wp_query

Try it with the fields parameter:
https://codex.www.ads-software.com/Function_Reference/WP_Query#Return_Fields_Parameter
Example:

<?php
// preserve the original query
global $query_string;
$the_query = new WP_Query( $query_string . '&fields=ids' );
if(isset($the_query->posts) && !empty($the_query->posts)){
	foreach((array) $the_query->posts as $id) {
		echo $id;
	}
}
?>

If you need the ids from a specific query you can also use get_posts:

<?php
// get 10 recent post IDs from category 1
$the_query = get_posts( 'cat=1&posts_per_page=10&fields=ids' );
if($the_query){
	foreach($the_query as $id) {
		echo $id;
	}
}
?>

keesiemeijer, thanks for the quick reply!
This is exactly what I needed.
But now I have another question – what are possible values of “fields” option?

In docs it is:
‘ids’ – Return an array of post IDs.
‘id=>parent’ – Return an associative array [ parent => ID, … ].
any other value or empty (default): return an array of post objects

I don’t understand the second and third options – which keys/values are supported and why are they reversed – id=>parent then parent => ID?
And “any other value or empty” – value of what?

The second option will return all parent (page) ID’s:

$args = array('fields' => 'id=>post_status');
$the_query = get_posts( $args );
// $the_query returns all parent post IDs

All other values will return full post objects. Only ‘ids’ and ‘id=>parent’ is supported for now. the output for the parent post IDs is an array like:

[posts] => Array
        (
            [0] => stdClass Object
                (
                    [ID] => 25
                    [post_parent] => 21
                )

            [1] => stdClass Object
                (
                    [ID] => 32
                    [post_parent] => 0
                )
        )

I understood. Thank you very much!

You’re welcome. I’m glad you’ve got it resolved ??