code: func (c *Cluster) SplitIndex(mvccStore MVCCStore, tableID, indexID int64, count int) {
  	indexStart := tablecodec.EncodeTableIndexPrefix(tableID, indexID)
  	indexEnd := indexStart.PrefixNext()
  	c.splitRange(mvccStore, NewMvccKey(indexStart), NewMvccKey(indexEnd), count)
  }
docs: // SplitIndex evenly splits the data in index into count regions.
  // Only works for single store.