Index WooCommerce product variation SKUs with the parent product

gistfile1.php

<?php

// index WooCommerce product_variation SKUs with the parent post
function my_searchwp_index_woocommerce_variation_skus( $extra_meta, $post_being_indexed ) {

	// we only care about WooCommerce Products
	if ( 'product' !== get_post_type( $post_being_indexed ) ) {
		return $extra_meta;
	}

	// retrieve all the product variations
	$args = array(
		'post_type'       => 'product_variation',
		'posts_per_page'  => -1,
		'fields'          => 'ids',
		'post_parent'     => $post_being_indexed->ID,
	);
	$product_variations = get_posts( $args );

	if ( ! empty( $product_variations ) ) {

		// store all SKUs as a Custom Field with a key of 'my_product_variation_skus'
		$extra_meta['my_product_variation_skus'] = array();

		// loop through all product variations, grab and store the SKU
		foreach ( $product_variations as $product_variation ) {
			$extra_meta['my_product_variation_skus'][] = get_post_meta( absint( $product_variation ), '_sku', true );
		}
	}

	return $extra_meta;
}

add_filter( 'searchwp_extra_metadata', 'my_searchwp_index_woocommerce_variation_skus', 10, 2 );