Code:

unit ThreadS;

{$mode objfpc}{$H+}

interface

uses
  Classes, SysUtils, sqldb,mysql55conn,StdCtrls,Forms,Controls;

  type
    {TGetComboItems}
    TGetComboItems = class(TThread)
        private
          S: String;
          tCombo: TComboBox;
          StrList: TStringList;
          tField: String;
          tFieldID: String;
          tSQL: String;
          tIndex: Integer;
          tQ : TSQLQuery;
          tConn : TMySQL55Connection;
          tTrans : TSQLTransaction;
          procedure SetEnable;
          procedure SetDisable;
          procedure PropertyQuery;
        public
          IsFinish : Boolean;
          constructor Create(sSQL,sField,sFieldID: String;
                             ComboBox: TComboBox; sIndex: Integer);
          destructor Destroy; override;
        protected
          procedure Execute; override;
          procedure AddToCombo;
          procedure ShowStatus;
          procedure SetCursorWait;
          procedure SetCursorDefault;
    end;

implementation

uses glava1s;

{ TGetComboItem }

procedure TGetComboItems.SetEnable;
begin
     tCombo.Enabled:=true;
end;

procedure TGetComboItems.SetDisable;
begin
     tCombo.Enabled:=false;
end;

procedure TGetComboItems.PropertyQuery;
begin
  // kreiranje konekcije
     tConn := TMySQL55Connection.Create(nil);
     tConn.Hostname := 'localhost';
     tConn.DatabaseName := 'baza';
     tConn.UserName := 'root';
     tConn.Password := 'root';
     tConn.CharSet:='latin1';
  // kreiranje transakcije
     tTrans := TSQLTransaction.Create(Nil);
     tTrans.Database := tConn;
  // kreiranje Query-a
     tQ := TSQLQuery.Create(nil);
     tQ.DataBase    := tConn;
     tQ.Transaction := tTrans;

     tConn.Transaction := tTrans;
  // Otvaranje konekcije i tabele
     tConn.Connected:=true;
     tQ.SQL.Add(tSQL);
end;


constructor TGetComboItems.Create(sSQL, sField, sFieldID: String;
                                  ComboBox: TComboBox; sIndex: Integer);
begin
     FreeOnTerminate:=true;
     StrList := TStringList.Create;
     tField  := sField;
     tFieldID:= sFieldID;
     tIndex  := sIndex;
     tCombo  := ComboBox;
     tSQL    := sSQL;
     IsFinish:= false;
     inherited Create(true);
end;

destructor TGetComboItems.Destroy;
begin
     FreeAndNil(StrList);
     tConn.Connected:=false;
     FreeAndNil(tConn);
     FreeAndNil(tQ);
     FreeAndNil(tTrans);
     inherited Destroy;
end;

procedure TGetComboItems.Execute;
begin
   if not Terminated then
    begin
     Synchronize(@SetDisable);
     Synchronize(@SetCursorWait);
     s:='Učitavanje podataka je u toku...';
     Synchronize(@ShowStatus);
     PropertyQuery;
     with tQ do
      Begin
        Open;
        while not eof do
          begin
            if (not IsFinish) then
              begin
                StrList.AddObject(FieldByName(tField).AsString
                                     ,TObject(FieldByName(tFieldID).AsInteger));
             //   Synchronize(@ShowStatus);
                Next;
              end
            else
               Last;
          end;
         close;
         tConn.Connected:=false;
      end;
     IsFinish:=true;
     s:='Učitavanje podataka je izvršeno.';
     Synchronize(@ShowStatus);
     Synchronize(@AddToCombo);
     Synchronize(@SetCursorDefault);
     Synchronize(@SetEnable);
    end;
end;

procedure TGetComboItems.AddToCombo;
begin
     tCombo.Clear;
     tCombo.Items.AddStrings(StrList);
     tCombo.ItemIndex:=tCombo.Items.IndexOfObject(TObject(tIndex));
     tCombo.Enabled:=true;
end;

procedure TGetComboItems.ShowStatus;
begin
     Form1.Status.SimpleText:=s;
end;

procedure TGetComboItems.SetCursorWait;
begin
     Screen.Cursor:=crSQLWait;
end;

procedure TGetComboItems.SetCursorDefault;
begin
     Screen.Cursor:=crDefault;
end;

end.

Code:

type
  { TCusField }

  TCusField = record
     Index    : Integer;
     Value    : variant;
     FieldName: String;
  end;  
...
type
  PData = ^TData;
  TData = object  
     Field      : array of TCusField;
     RecNo      : Integer;
     inicialize : Boolean;
     public
       function ValueOfFiledName(sName: String): variant;
       function IndexOfName(sName: String): integer;
       procedure SetValue(sName: String; Value: variant);
       procedure SetIndexValue(i: Integer; Value: variant);

  end; 
...
procedure TSetTable.SetLocalTable;
var
  Data      : PData;
  Node      : PVirtualNode;
  CustField : TCusField;
  i         : Integer;
begin
      with TVirtualStringTree(fSender) do
         begin
            BeginUpdate;
            Clear;

            RootNodeCount := tQ.RecordCount;
            NodeDataSize  := SizeOf(TCusField);   // <  mislim da je ovde negde problem
            Node          := GetFirst;

            while not tQ.EOF do
              begin
                Data := GetNodeData(Node);
                SetLength(Data^.Field,tQ.FieldCount);
                Data^.RecNo := tQ.RecNo;
                for i := 0 to High(Data^.Field) do
                  begin
                     CustField.Value     := tQ.Fields[i].Value;
                     CustField.FieldName := tQ.Fields[i].FieldName;
                     CustField.Index     := i;
                     Data^.Field[i]      := CustField;
                  end;
                Data^.inicialize    := true;
               Node:= GetNext(Node);
               end;
            end;
            EndUpdate;
         end;
end; 

Code:
with TVirtualStringTree(fSender) do

Code:
       for i:=0 to 20 do
                 begin
                  MyThread  := MyThread.Create(history.TAB_HISTORY,'',PConnect);
                  MyThread.Start;
                  MyThread.WaitFor;
                  history.ShowModal;
                 end;