code: func (c *Constant) Equal(b Expression, ctx context.Context) bool {
  	y, ok := b.(*Constant)
  	if !ok {
  		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.