Filtered Gridview Paging Problem?

protected void Page_Load(object sender, EventArgs e)
{

}
protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
{
    if (ViewState["flag"] !=null)
    {
        if ((bool)ViewState["flag"] == true)
        {
            SqlDataSource1.FilterExpression = "Item_name like '%trr4%'";
        }
    }
}
protected void Button1_Click(object sender, EventArgs e)
{
    ViewState.Add("flag", true);

    SqlDataSource1.FilterExpression = "Item_name like '%trr4%'";
    GridView1.DataSourceID = "";
    GridView1.DataSourceID = "SqlDataSource1";
    //GridView1.DataSource = SqlDataSource1;
    GridView1.DataBind();
}

<div>
    <asp:GridView ID="GridView1" runat="server" AllowPaging="True"
        AutoGenerateColumns="False" DataKeyNames="item_code"
        DataSourceID="SqlDataSource1" onpageindexchanging="GridView1_PageIndexChanging"
        PageSize="3">
        <Columns>
            <asp:BoundField DataField="item_code" HeaderText="item_code"
                InsertVisible="False" ReadOnly="True" SortExpression="item_code" />
            <asp:BoundField DataField="Item_name" HeaderText="Item_name"
                SortExpression="Item_name" />
            <asp:BoundField DataField="brand" HeaderText="brand" SortExpression="brand" />
            <asp:BoundField DataField="size" HeaderText="size" SortExpression="size" />
            <asp:BoundField DataField="section" HeaderText="section"
                SortExpression="section" />
            <asp:BoundField DataField="price" HeaderText="price" SortExpression="price" />
            <asp:BoundField DataField="Material" HeaderText="Material"
                SortExpression="Material" />
            <asp:BoundField DataField="Qty" HeaderText="Qty" SortExpression="Qty" />
            <asp:BoundField DataField="tax" HeaderText="tax" SortExpression="tax" />
            <asp:BoundField DataField="tt" HeaderText="tt" SortExpression="tt" />
        </Columns>
    </asp:GridView>
    <asp:SqlDataSource ID="SqlDataSource1" runat="server"
        ConnectionString="<%$ ConnectionStrings:testConnectionString %>"
        SelectCommand="SELECT * FROM [Items]"></asp:SqlDataSource>
    <br />
    <asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Button" />
</div>