code: protected void buildIndexBlocking()
      {
          logger.info(String.format("Submitting index build of %s for data in %s",
                  getIndexName(), StringUtils.join(baseCfs.getSSTables(), ", ")));
  
          try (Refs<SSTableReader> sstables = baseCfs.selectAndReference(ColumnFamilyStore.CANONICAL_SSTABLES).refs)
          {
              SecondaryIndexBuilder builder = new SecondaryIndexBuilder(baseCfs,
                                                                        Collections.singleton(getIndexName()),
                                                                        new ReducingKeyIterator(sstables));
              Future<?> future = CompactionManager.instance.submitIndexBuild(builder);
              FBUtilities.waitOnFuture(future);
              forceBlockingFlush();
              setIndexBuilt();
          }
          logger.info("Index build of {} complete", getIndexName());
      }
docs: Builds the index using the data in the underlying CFS
  Blocks till it's complete