Source

File: wp-includes/class-wp-user-query.php

public function query() {
		global $wpdb;

		$qv =& $this->query_vars;

		/**
		 * Filters the users array before the query takes place.
		 *
		 * Return a non-null value to bypass WordPress's default user queries.
		 * Filtering functions that require pagination information are encouraged to set
		 * the `total_users` property of the WP_User_Query object, passed to the filter
		 * by reference. If WP_User_Query does not perform a database query, it will not
		 * have enough information to generate these values itself.
		 *
		 * @since 5.1.0
		 *
		 * @param array|null $results Return an array of user data to short-circuit WP's user query
		 *                            or null to allow WP to run its normal queries.
		 * @param WP_User_Query $this The WP_User_Query instance (passed by reference).
		 */
		$this->results = apply_filters_ref_array( 'users_pre_query', array( null, &$this ) );

		if ( null === $this->results ) {
			$this->request = "SELECT $this->query_fields $this->query_from $this->query_where $this->query_orderby $this->query_limit";

			if ( is_array( $qv['fields'] ) || 'all' === $qv['fields'] ) {
				$this->results = $wpdb->get_results( $this->request );
			} else {
				$this->results = $wpdb->get_col( $this->request );
			}

			if ( isset( $qv['count_total'] ) && $qv['count_total'] ) {
				/**
				 * Filters SELECT FOUND_ROWS() query for the current WP_User_Query instance.
				 *
				 * @since 3.2.0
				 * @since 5.1.0 Added the `$this` parameter.
				 *
				 * @global wpdb $wpdb WordPress database abstraction object.
				 *
				 * @param string $sql         The SELECT FOUND_ROWS() query for the current WP_User_Query.
				 * @param WP_User_Query $this The current WP_User_Query instance.
				 */
				$found_users_query = apply_filters( 'found_users_query', 'SELECT FOUND_ROWS()', $this );

				$this->total_users = (int) $wpdb->get_var( $found_users_query );
			}
		}

		if ( ! $this->results ) {
			return;
		}

		if ( 'all_with_meta' === $qv['fields'] ) {
			cache_users( $this->results );

			$r = array();
			foreach ( $this->results as $userid ) {
				$r[ $userid ] = new WP_User( $userid, '', $qv['blog_id'] );
			}

			$this->results = $r;
		} elseif ( 'all' === $qv['fields'] ) {
			foreach ( $this->results as $key => $user ) {
				$this->results[ $key ] = new WP_User( $user, '', $qv['blog_id'] );
			}
		}
	}