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Delphi Tips & Tricks

Making a Virtual Table (in Memory)

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!

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

{ luckily 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.

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