code: func (p *Pool) PutChunk(fields []*types.FieldType, chk *Chunk) {
  	for i, f := range fields {
  		switch elemLen := getFixedLen(f); elemLen {
  		case varElemLen:
  			p.varLenColPool.Put(chk.columns[i])
  		case 4:
  			p.fixLenColPool4.Put(chk.columns[i])
  		case 8:
  			p.fixLenColPool8.Put(chk.columns[i])
  		case 16:
  			p.fixLenColPool16.Put(chk.columns[i])
  		case 40:
  			p.fixLenColPool40.Put(chk.columns[i])
  		}
  	}
  	chk.columns = nil // release the column references.
  }
docs: // PutChunk puts a Chunk back to the Pool.