code: func (c *SampleCollector) MergeSampleCollector(sc *stmtctx.StatementContext, rc *SampleCollector) {
  	c.NullCount += rc.NullCount
  	c.Count += rc.Count
  	c.TotalSize += rc.TotalSize
  	c.FMSketch.mergeFMSketch(rc.FMSketch)
  	if rc.CMSketch != nil {
  		err := c.CMSketch.MergeCMSketch(rc.CMSketch)
  		terror.Log(errors.Trace(err))
  	}
  	for _, item := range rc.Samples {
  		err := c.collect(sc, item.Value)
  		terror.Log(errors.Trace(err))
  	}
  }
docs: // MergeSampleCollector merges two sample collectors.