Fix/course average query

changelog/course-averages-add-caching

@@ -0,0 +1,4 @@
+Significance: patch
+Type: fixed
+
+Add caching to expensive query

includes/reports/overview/services/class-sensei-reports-overview-service-courses.php

@@ -94,6 +94,14 @@ public function get_courses_average_grade( array $course_ids ) {
 		if ( empty( $course_ids ) ) {
 			return 0;
 		}
+		// Sets a cache key with a determinate string length for `set_transient`.
+		$cache_key      = 'get_courses_average_grade_' . md5( implode( '', $course_ids ) );
+		$average_cached = get_transient( $cache_key );
+
+		if ( false !== $average_cached ) {
+			return $average_cached;
+		}
+
 		global $wpdb;
 		/**
 		 * The subquery calculates the average grade per course, and the outer query then calculates the
@@ -125,7 +133,10 @@ public function get_courses_average_grade( array $course_ids ) {
 		) averages_by_course'
 		);
 
-		return floatval( $result->courses_average );
+		$course_result = floatval( $result->courses_average );
+		set_transient( $cache_key, $course_result, HOUR_IN_SECONDS );
+
+		return $course_result;
 	}
 
 	/**