add error message when more than 100 groups are found

performanceDb/src/main/java/org/lucares/performance/db/PerformanceDb.java

@@ -183,6 +183,14 @@ public class PerformanceDb implements AutoCloseable {
 
         final Grouping grouping = Grouping.groupBy(pdbFiles, groupBy);
 
+        if (grouping.getGroups().size() > 100) {
+            // we know groupBy is not empty, because we have groupings
+            final String example = " For example '" + query.getQuery() + " and " + groupBy.get(0) + "=someValue'";
+
+            throw new RuntimeException("too many groups - try adding restrictions for one of these fields "
+                    + String.join(",", groupBy) + "." + example + " Or you can remove a field from 'Group By'.");
+        }
+
         final Result result = toResult(grouping);
         METRICS_LOGGER.debug("query execution took: " + (System.nanoTime() - start) / 1_000_000.0 + "ms: " + query
                 + " (" + groupBy + "): files found: " + pdbFiles.size());