How to compress Blob Fields
lazarus / 16-Databases

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DeBritto · 2016-05-27 18:33

Hi folks,

I would like to know if it's possible to compress (using Zlib
functions or something else) a string before stores the value in a
Firebird Blob field? I'm asking because I'm developing an remote
server-cliente application  that needs to share lots of information
through internet and this strategy could save some time.

Best regards

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Cyr · 2016-05-27 19:00

This is my code (not the best decision may be) to save some compressed
data in to the BLOB field:

uses ...., ZStream

procedure TForm1.CargoGrid3DButtonClick(Sender: TObject; aCol, aRow:
Integer);
var
 OutVertexStream,OutIndexStream: TMemoryStream;
 GzStream1,GzStream2 : Tcompressionstream;
 i:longint;
 s: string;
 d: Char;
begin
  if loadfromstl=false then exit;
  OutVertexStream := TMemoryStream.Create;
  OutIndexStream := TMemoryStream.Create;
   try
      GzStream1 := Tcompressionstream.create(clmax,OutVertexStream);
      GzStream2 := Tcompressionstream.create(clmax,OutIndexStream);
      try
        GzStream1.WriteBuffer(vertex[0],length(vertex)*12);
        GzStream2.WriteBuffer(vertexindex[0],length(vertexindex)*sizeo
f(vertexindex));
      finally
         GzStream1.free;
         GzStream2.free;
      end;

      OutVertexStream.Position:=0;
      OutIndexStream.Position:=0;


      SQLQuery1.Close;
      SQLQuery1.SQL.Clear;
      s:='select * from CargoTypeSizes where
CargoTypeSizeID='+CargoGrid3D.Cells[0,CargoGrid3D.Row];
      SQLQuery1.SQL.Text :=s ;
      SQLQuery1.Open;
      SQLQuery1.Edit;
      s:='';
      for i := 1 to OutVertexStream.Size do
       begin
         OutVertexStream.Read(d, 1);
         s := s + d;
       end;
      SQLQuery1.FieldByName('vertexes').AsString:=s;
      s:='';
      for i := 1 to OutIndexStream.Size do
       begin
         OutIndexStream.Read(d, 1);
         s := s + d;
       end;
      SQLQuery1.FieldByName('indexes').AsString:=s;
      SQLQuery1.Post;
      SQLQuery1.Upda]"]>Blockedde := upWhereAll;
      SQLQuery1.ApplyUpdates;
      SQLTransaction1.Commit;
   finally
      CargoGrid3D.Cells[2,CargoGrid3D.Row]:='loaded';
      CargoGrid3D.Cells[3,CargoGrid3D.Row]:=inttostr(OutVertexStream.S
ize+OutIndexStream.Size);
      OutVertexStream.free;
      OutIndexStream.Free;
   end;

 
end;

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DeBritto · 2016-05-28 03:02

Hi Cyr,
Thank you so much to share your code! I'll try it. But, let me ask you
one more thing, you said that maybe this is not the best decision (to
compress data into the Blob field). Why? Can you share some thoughts?

Best regards