Source

File: wp-admin/includes/class-wp-posts-list-table.php

private function _display_rows_hierarchical( $pages, $pagenum = 1, $per_page = 20 ) {
		global $wpdb;

		$level = 0;

		if ( ! $pages ) {
			$pages = get_pages( array( 'sort_column' => 'menu_order' ) );

			if ( ! $pages ) {
				return;
			}
		}

		/*
		 * Arrange pages into two parts: top level pages and children_pages
		 * children_pages is two dimensional array, eg.
		 * children_pages[10][] contains all sub-pages whose parent is 10.
		 * It only takes O( N ) to arrange this and it takes O( 1 ) for subsequent lookup operations
		 * If searching, ignore hierarchy and treat everything as top level
		 */
		if ( empty( $_REQUEST['s'] ) ) {

			$top_level_pages = array();
			$children_pages  = array();

			foreach ( $pages as $page ) {

				// Catch and repair bad pages.
				if ( $page->post_parent == $page->ID ) {
					$page->post_parent = 0;
					$wpdb->update( $wpdb->posts, array( 'post_parent' => 0 ), array( 'ID' => $page->ID ) );
					clean_post_cache( $page );
				}

				if ( 0 == $page->post_parent ) {
					$top_level_pages[] = $page;
				} else {
					$children_pages[ $page->post_parent ][] = $page;
				}
			}

			$pages = &$top_level_pages;
		}

		$count      = 0;
		$start      = ( $pagenum - 1 ) * $per_page;
		$end        = $start + $per_page;
		$to_display = array();

		foreach ( $pages as $page ) {
			if ( $count >= $end ) {
				break;
			}

			if ( $count >= $start ) {
				$to_display[ $page->ID ] = $level;
			}

			$count++;

			if ( isset( $children_pages ) ) {
				$this->_page_rows( $children_pages, $count, $page->ID, $level + 1, $pagenum, $per_page, $to_display );
			}
		}

		// If it is the last pagenum and there are orphaned pages, display them with paging as well.
		if ( isset( $children_pages ) && $count < $end ) {
			foreach ( $children_pages as $orphans ) {
				foreach ( $orphans as $op ) {
					if ( $count >= $end ) {
						break;
					}

					if ( $count >= $start ) {
						$to_display[ $op->ID ] = 0;
					}

					$count++;
				}
			}
		}

		$ids = array_keys( $to_display );
		_prime_post_caches( $ids );

		if ( ! isset( $GLOBALS['post'] ) ) {
			$GLOBALS['post'] = reset( $ids );
		}

		foreach ( $to_display as $page_id => $level ) {
			echo "\t";
			$this->single_row( $page_id, $level );
		}
	}