Форум на Исходниках.RU https://forum.sources.ru Форум на Исходниках.RU Форум на Исходниках.RU https://forum.sources.ru/index.php?showtopic=41001&view=findpost&p=266013 Thu, 18 Dec 2003 14:57:25 +0000 Эмуляция таблицы БД в памяти https://forum.sources.ru/index.php?showtopic=41001&view=findpost&p=266013 Решение 1. Стандартная таблица TTable (BDE).

{  
  This is an InMemoryTable example. Free for anyone to use, modify and do  
  whatever else you wish.  

  Just like all things free it comes with no guarantees.  
  I cannot be responsible for any damage this code may cause.  
  Let me repeat this:  

   WARNING! THIS CODE IS PROVIDED AS IS WITH NO GUARANTEES OF ANY KIND!  
   USE THIS AT YOUR OWN RISK - YOU ARE THE ONLY PERSON RESPONSIBLE FOR  
   ANY DAMAGE THIS CODE MAY CAUSE - YOU HAVE BEEN WARNED!  

  THANKS to Steve Garland <72700.2407@compuserve.com> for his help.  
  He created his own variation of an in-memory table component and  
  I used it to get started.  

  InMemory tables are a feature of the Borland Database Engine (BDE).  
  InMemory tables are created in RAM and deleted when you close them.  
  They are much faster and are very useful when you need fast operations on  
  small tables. This example uses the DbiCreateInMemoryTable DBE function call.  

  This object should work just like a regular table, except InMemory  
  tables do not support certain features (like referntial integrity,  
  secondary indexes and BLOBs) and currently this code doesn't do anything to  
  prevent you from trying to use them. You will probably get some error if  
  you try to create a memo field.  
}  

unit Inmem;  

interface  

uses DBTables, WinTypes, WinProcs, DBITypes, DBIProcs, DB, SysUtils;  

type  
  TInMemoryTable = class(TTable)  
  private  
    hCursor: hDBICur;  
    procedure EncodeFieldDesc(var FieldDesc: FLDDesc;  
      const Name: string; DataType: TFieldType; Size: Word);  
    function CreateHandle: HDBICur; override;  
  public  
    procedure CreateTable;  
  end;  

implementation  

{  
  Luckely this function is virtual - so I could override it. In the  
  original VCL code for TTable this function actually opens the table -  
  but since we already have the handle to the table - we just return it  
}  

function TInMemoryTable.CreateHandle;  
begin  
  Result := hCursor;  
end;  

{  
  This function is cut-and-pasted from the VCL source code. I had to do  
  this because it is declared private in the TTable component so I had no  
  access to it from here.  
}  

procedure TInMemoryTable.EncodeFieldDesc(var FieldDesc: FLDDesc;  
  const Name: string; DataType: TFieldType; Size: Word);  
const  
  TypeMap: array[TFieldType] of Byte = (fldUNKNOWN, fldZSTRING, fldINT16,  
    fldINT32, fldUINT16, fldBOOL,  
    fldFLOAT, fldFLOAT, fldBCD, fldDATE, fldTIME, fldTIMESTAMP, fldBYTES,  
    fldVARBYTES, fldBLOB, fldBLOB, fldBLOB);  
begin  
  with FieldDesc do  
  begin  
    AnsiToNative(Locale, Name, szName, SizeOf(szName) - 1);  
    iFldType := TypeMap[DataType];  
    case DataType of  
      ftString, ftBytes, ftVarBytes, ftBlob, ftMemo, ftGraphic:  
        iUnits1 := Size;  
      ftBCD:  
        begin  
          iUnits1 := 32;  
          iUnits2 := Size;  
        end;  
    end;  
    case DataType of  
      ftCurrency:  
        iSubType := fldstMONEY;  
      ftBlob:  
        iSubType := fldstBINARY;  
      ftMemo:  
        iSubType := fldstMEMO;  
      ftGraphic:  
        iSubType := fldstGRAPHIC;  
    end;  
  end;  
end;  

{  
  This is where all the fun happens. I copied this function from the VCL  
  source and then changed it to use DbiCreateInMemoryTable instead of  
  DbiCreateTable.  

  Since InMemory tables do not support Indexes - I took all of the  
  index-related things out  
}  

procedure TInMemoryTable.CreateTable;  
var  
  I: Integer;  
  pFieldDesc: pFLDDesc;  
  szTblName: DBITBLNAME;  
  iFields: Word;  
  Dogs: pfldDesc;  
begin  
  CheckInactive;  
  if FieldDefs.Count = 0 then  
    for I := 0 to FieldCount - 1 do  
      with Fields[I] do  
        if not Calculated then  
          FieldDefs.Add(FieldName, DataType, Size, Required);  
  pFieldDesc := nil;  
  SetDBFlag(dbfTable, True);  
  try  
    AnsiToNative(Locale, TableName, szTblName, SizeOf(szTblName) - 1);  
    iFields := FieldDefs.Count;  
    pFieldDesc := AllocMem(iFields * SizeOf(FLDDesc));  
    for I := 0 to FieldDefs.Count - 1 do  
      with FieldDefs[I] do  
      begin  
        EncodeFieldDesc(PFieldDescList(pFieldDesc)^[I], Name,  
          DataType, Size);  
      end;  
    { the driver type is nil = logical fields }  
    Check(DbiTranslateRecordStructure(nil, iFields, pFieldDesc,  
      nil, nil, pFieldDesc));  
    { here we go - this is where hCursor gets its value }  
    Check(DbiCreateInMemTable(DBHandle, szTblName, iFields, pFieldDesc, hCursor));  
  finally  
    if pFieldDesc <> nil then FreeMem(pFieldDesc, iFields * SizeOf(FLDDesc));  
    SetDBFlag(dbfTable, False);  
  end;  
end;  

end.  

Взято с сайта http://www.swissdelphicenter.ch/en/tipsindex.php



Решение 2. Нестандартная таблица в памяти. Не требует BDE и других DB компонентов, библиотек и т.п. Впрочем все основные методы и свойства реализованы - с гридом и другими DB комонентами коннектится, поиск и фильтры работают, по-моему есть даже методы перекачки реальной таблицы в память с автоматическим воссозданием всех структур и типов полей

Компонент TMemoryData из библиотек RxLib или JVCL.


Кстати, во многих серьёзных серверах баз данных есть такая фишка: не обновляемые таблицы (reference-table), т.е. справочники, можно разместить в памяти на сервере, т.е. заставить сервер баз данных держать эти таблицы всё время в памяти и доступ к ним будет очень быстрый. ]]> Vit Базы данных, репортинг, печать