delete_metadata()

function delete_metadata( $meta_type, $object_id, $meta_key, $meta_value = '', $delete_all = false ) {
	global $gcdb;

	if ( ! $meta_type || ! $meta_key || ! is_numeric( $object_id ) && ! $delete_all ) {
		return false;
	}

	$object_id = absint( $object_id );
	if ( ! $object_id && ! $delete_all ) {
		return false;
	}

	$table = _get_meta_table( $meta_type );
	if ( ! $table ) {
		return false;
	}

	$type_column = sanitize_key( $meta_type . '_id' );
	$id_column   = ( 'user' === $meta_type ) ? 'umeta_id' : 'meta_id';

	// expected_slashed ($meta_key)
	$meta_key   = gc_unslash( $meta_key );
	$meta_value = gc_unslash( $meta_value );

	/**
	 * Short-circuits deleting metadata of a specific type.
	 *
	 * The dynamic portion of the hook, `$meta_type`, refers to the meta object type
	 * (post, comment, term, user, or any other type with an associated meta table).
	 * 响应ing a non-null value will effectively short-circuit the function.
	 *
	 * @since 3.1.0
	 *
	 * @param null|bool $delete     Whether to allow metadata deletion of the given type.
	 * @param int       $object_id  ID of the object metadata is for.
	 * @param string    $meta_key   Metadata key.
	 * @param mixed     $meta_value Metadata value. Must be serializable if non-scalar.
	 * @param bool      $delete_all Whether to delete the matching metadata entries
	 *                              for all objects, ignoring the specified $object_id.
	 *                              Default false.
	 */
	$check = apply_filters( "delete_{$meta_type}_metadata", null, $object_id, $meta_key, $meta_value, $delete_all );
	if ( null !== $check ) {
		return (bool) $check;
	}

	$_meta_value = $meta_value;
	$meta_value  = maybe_serialize( $meta_value );

	$query = $gcdb->prepare( "SELECT $id_column FROM $table WHERE meta_key = %s", $meta_key );

	if ( ! $delete_all ) {
		$query .= $gcdb->prepare( " AND $type_column = %d", $object_id );
	}

	if ( '' !== $meta_value && null !== $meta_value && false !== $meta_value ) {
		$query .= $gcdb->prepare( ' AND meta_value = %s', $meta_value );
	}

	$meta_ids = $gcdb->get_col( $query );
	if ( ! count( $meta_ids ) ) {
		return false;
	}

	if ( $delete_all ) {
		if ( '' !== $meta_value && null !== $meta_value && false !== $meta_value ) {
			$object_ids = $gcdb->get_col( $gcdb->prepare( "SELECT $type_column FROM $table WHERE meta_key = %s AND meta_value = %s", $meta_key, $meta_value ) );
		} else {
			$object_ids = $gcdb->get_col( $gcdb->prepare( "SELECT $type_column FROM $table WHERE meta_key = %s", $meta_key ) );
		}
	}

	/**
	 * Fires immediately before deleting metadata of a specific type.
	 *
	 * The dynamic portion of the hook, `$meta_type`, refers to the meta object type
	 * (post, comment, term, user, or any other type with an associated meta table).
	 *
	 * @since 3.1.0
	 *
	 * @param string[] $meta_ids    An array of metadata entry IDs to delete.
	 * @param int      $object_id   ID of the object metadata is for.
	 * @param string   $meta_key    Metadata key.
	 * @param mixed    $_meta_value Metadata value. Serialized if non-scalar.
	 */
	do_action( "delete_{$meta_type}_meta", $meta_ids, $object_id, $meta_key, $_meta_value );

	// Old-style action.
	if ( 'post' === $meta_type ) {
		/**
		 * Fires immediately before deleting metadata for a post.
		 *
		 * @since 2.9.0
		 *
		 * @param string[] $meta_ids An array of metadata entry IDs to delete.
		 */
		do_action( 'delete_postmeta', $meta_ids );
	}

	$query = "DELETE FROM $table WHERE $id_column IN( " . implode( ',', $meta_ids ) . ' )';

	$count = $gcdb->query( $query );

	if ( ! $count ) {
		return false;
	}

	if ( $delete_all ) {
		foreach ( (array) $object_ids as $o_id ) {
			gc_cache_delete( $o_id, $meta_type . '_meta' );
		}
	} else {
		gc_cache_delete( $object_id, $meta_type . '_meta' );
	}

	/**
	 * Fires immediately after deleting metadata of a specific type.
	 *
	 * The dynamic portion of the hook, `$meta_type`, refers to the meta object type
	 * (post, comment, term, user, or any other type with an associated meta table).
	 *
	 * @since 2.9.0
	 *
	 * @param string[] $meta_ids    An array of metadata entry IDs to delete.
	 * @param int      $object_id   ID of the object metadata is for.
	 * @param string   $meta_key    Metadata key.
	 * @param mixed    $_meta_value Metadata value. Serialized if non-scalar.
	 */
	do_action( "deleted_{$meta_type}_meta", $meta_ids, $object_id, $meta_key, $_meta_value );

	// Old-style action.
	if ( 'post' === $meta_type ) {
		/**
		 * Fires immediately after deleting metadata for a post.
		 *
		 * @since 2.9.0
		 *
		 * @param string[] $meta_ids An array of metadata entry IDs to delete.
		 */
		do_action( 'deleted_postmeta', $meta_ids );
	}

	return true;
}

		/*
		 * Remove all post meta data for Custom Post Type (CPT)
		 * at the time of uninstall of plugin that created this CPT.
		 * So that plugin do not leave behind any orphan post meta data
		 * related to its CPT. 
		 * 
		 * You may place this code in uninstall.php file in your plugin root directory.
		 */
		$meta_type  = 'post';           // since we are deleting data for CPT 
		$object_id  = 0;                // no need to put id of object since we are deleting all
		$meta_key   = 'my_meta_key';    // Your target meta_key added using update_post_meta()
		$meta_value = '';               // No need to check for value since we are deleting all
		$delete_all = true;             // This is important to have TRUE to delete all post meta

		// This will delete all post meta data having the specified key 
		delete_metadata( $meta_type, $object_id, $meta_key, $meta_value, $delete_all );