Question

i want to select a picture that save as a large object in a postgresql database.

i know i use lo_export to do this.

but there is a problem: i want to save this picture directly to my computer because i cant access the files that save on Server using lo_export

(i think that is best for me if picture transfer to my computer by a select query)

Answer 1

I don’t exactly know my way around C# but the Npgsql Manual has an example sort of like this of writing a bytea column to a file:

command = new NpgsqlCommand("select blob from t where id = 1);", conn);
Byte[] result = (Byte[])command.ExecuteScalar();

FileStream fs = new FileStream(args[0] + "database", FileMode.Create, FileAccess.Write);
BinaryWriter bw = new BinaryWriter(new BufferedStream(fs));

bw.Write(result);
bw.Flush();
fs.Close();
bw.Close();

So you just read it out of the database pretty much like any other column and write it to a local file. The example is about half way down the page I linked to, just search for “bytea” and you’ll find it.

UPDATE: For large objects, the process appears to be similar but less SQL-ish. The manual (as linked to above) includes a few large object examples:

NpgsqlTransaction t = Polacz.BeginTransaction();

LargeObjectManager lbm = new LargeObjectManager(Polacz);
LargeObject lo = lbm.Open(takeOID(idtowaru),LargeObjectManager.READWRITE); //take picture oid from metod takeOID

byte[] buf = new byte[lo.Size()];
buf = lo.Read(lo.Size());

MemoryStream ms = new MemoryStream();
ms.Write(buf,0,lo.Size());

// ...

Image zdjecie = Image.FromStream(ms);

Search the manual for “large object” and you’ll find it.