Question

i am working wit h PostgreSQL 9 for an application, i have a database with a table ‘species’
where i store fish species details along with the image of the species.
the table is

       CREATE TABLE fishes
               (
                  fishes character varying(7) NOT NULL,
                  speciesimages oid,
                  CONSTRAINT species_pkey PRIMARY KEY (species)
                 )
                   WITH (
                       OIDS=TRUE
               );

i use

INSERT INTO species(fishes,fishesimages VALUES('01',lo_import('C://01.jpg'));

To store the images in the database.

to retrieve the images i use

       SELECT lo_export(fishes.fishesimages,'c://outimage.jpg') 
       FROM   fishes
       WHERE  fishes= '01'; 

This works fine when the host is Localhost but when it is the server i cannot use the path c:// as this may not exist on the server system and i dont have permissions anyways.

so i set out to use

\COPY

like this

"C:\Program Files\PostgreSQL\9.0\bin\psql.exe" -h 192.168.1.101 -p 5432 -d myDB -U DB_admin -c  "\COPY (SELECT lo_export(fishes.fishesimages,'01.jpg') FROM   fishes WHERE  species = '01') TO 'C://leeImage.jpeg' WITH BINARY";

but this create a image file but when i open it its invalid image

can anyone tell me how to use lo_export function from the server machine and create the image on client machine?

Answer 1

Basically:

1. lo_export will instruct the server to write a file locally (always)
2. \copy will be transformed by psql to a COPY ... TO STDOUT command and the output written to the specified file. (So what is written to that file is the result of the select statment you were doing before)

So, you can not use lo_export in this way, it will always write a file onto the server’s filesystem.

Of course, you can solve this simply by having the server write to a shared drive, and then read the file from that drive. Ugly, but effective IMHO.

For some recent versions of psql (not sure when this was introduced) there is a \lo_export psql command which takes an OID and filename, e.g.:

\lo_export 28914648 testfile

However you need to get the OID of the file into the script somehow…

You can write a PL/PGsql function like this to dump the file as a bytea:

CREATE OR REPLACE FUNCTION getfiledata(lobjid oid) RETURNS bytea
  STRICT STABLE LANGUAGE plpgsql AS $$
DECLARE
  fd int4;
  imgsize int4;
  INV_READ int4 := 262144;
  SEEK_SET int4 := 0;
  SEEK_END int4 := 2;
BEGIN
  SELECT lo_open(lobjid, INV_READ) INTO fd;
  PERFORM lo_lseek(fd, 0, SEEK_END);
  SELECT lo_tell(fd) INTO imgsize;
  PERFORM lo_lseek(fd, 0, SEEK_SET);
  RETURN loread(fd, imgsize);
END;
$$;

Now calling this function with the OID of the large object will return its content as a bytea value. You can thus call this function in a COPY command and it will return the file data… and by using \copy it will be sent to the client.

It’s generally recommended these days to use bytea columns directly rather than this large object interface (bytea was introduced a lot later). PostgreSQL will automatically move large values into out-of-line storage (“TOAST tables”) and will also compress them (unless the storage mode is set to “external” to suppress this, which is probably the right thing to do for JPEG images etc)

EDIT :
\lo_export
Try this from commandprompt.com

 `"C:\Program Files\PostgreSQL\9.0\bin\psql.exe" -h 192.168.1.101 -p 5432 -d myDB -U DB_admin -c  "\lo_export 19135 'C://leeImage.jpeg' ";`

where the number 19135 is the OID of the species whose image i want on the client system.. the OID you can get from the fishes table fishesimages OID
use the OID in the above code and you can use the OID get the images.