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Thread: Handling Blobs with libpq

Handling Blobs with libpq

From
Stéphane Pinel
Date:
What is the best approach to handle blobs with libpq ?

I've read in documentation that there are 2 ways:

1- Bytea
2- Larges objects (lo_import)

My main need is storing pictures (like Jpegs etc) or small blobs. Doc
says that (2) is more
appropriate but I need to work with blobs in memory, not from files.

Thanks for your help.

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Stephane
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Re: Handling Blobs with libpq

From
Doug McNaught
Date:
Stéphane Pinel <spinel@noos.fr> writes:

> What is the best approach to handle blobs with libpq ?
>
> I've read in documentation that there are 2 ways:
>
> 1- Bytea
> 2- Larges objects (lo_import)
>
> My main need is storing pictures (like Jpegs etc) or small blobs. Doc
> says that (2) is more
> appropriate but I need to work with blobs in memory, not from files.

You don't have to use lo_import()--you can write from memory to a large
object using lo_write(), and read an LO (or part of one) into memory
using lo_read().  See the documentation.

-Doug

Re: Handling Blobs with libpq

From
"Reid Thompson"
Date:
I'd be appreciative if someone could give or point me to a working example of storing/retrieving images via bytea.

--
thanks,
reid


-----Original Message-----
From: Doug McNaught [mailto:doug@mcnaught.org]
Sent: Thursday, March 20, 2003 11:50 AM
To: Stéphane Pinel
Cc: PostgreSQL-general
Subject: Re: [GENERAL] Handling Blobs with libpq


Stéphane Pinel <spinel@noos.fr> writes:

> What is the best approach to handle blobs with libpq ?
>
> I've read in documentation that there are 2 ways:
>
> 1- Bytea
> 2- Larges objects (lo_import)
>
> My main need is storing pictures (like Jpegs etc) or small blobs. Doc
> says that (2) is more appropriate but I need to work with blobs in
> memory, not from files.

You don't have to use lo_import()--you can write from memory to a large object using lo_write(), and read an LO (or
partof one) into memory using lo_read().  See the documentation. 

-Doug

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Re: Handling Blobs with libpq

From
Darko Prenosil
Date:
On Thursday 20 March 2003 17:03, Reid Thompson wrote:
> I'd be appreciative if someone could give or point me to a working example
> of storing/retrieving images via bytea.

    I don not understand the question, "bytea" is the data type that can hold any
binary data, so the "picture" is just a way to interpret that data.

    If you are asking for sample how to read and write "bytea", there are
attached files from postgres sql driver for QT library. It is not the
original QT driver, because it does not know how to handle "bytea", and
"blobs", and have some serious errors in code, so I made my own plugin. From
the source you can see how "bytea" and "blobs" are read and written.

    Based on this sql plugin I wrote set of database objects, and one of those
objects is "PgPicture".
PgPicture simply takes "QByteArray" from driver, does some work on it and
shows the picture on the screen.

    For postgres version 7.3 it is working fine, but in previous releases
PQEscabeBytea had errors, and some pictures that I was using to test the
library could not be saved.
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