code: func (c *Constant) Equal(ctx sessionctx.Context, b Expression) bool {
  	y, ok := b.(*Constant)
  	if !ok {
  		return false
  	}
  	_, err1 := y.Eval(chunk.Row{})
  	_, err2 := c.Eval(chunk.Row{})
  	if err1 != nil || err2 != nil {
  		return false
  	}
  	con, err := c.Value.CompareDatum(ctx.GetSessionVars().StmtCtx, &y.Value)
  	if err != nil || con != 0 {
  		return false
  	}
  	return true
  }
docs: // Equal implements Expression interface.