decoding BLOB's

On 02/22/2016 11:08 AM, CS DBA wrote:

Hi All;

we've found a post about dumping blobs:
/
/

/I wrote: [fanlijing wants to write bytea to file] > A simple > COPY
(SELECT byteacol WROM mytab WHERE ...) TO 'filename' (FORMAT binary) >
should do the trick. Corrections: a) "binary" must be surrounded by
single quotes. b) *that won't dump just the binary data - you would have
to remove the first 25 bytes and the last 2 bytes...* So maybe using the
functions I mentioned would be the best way after all. You could also
write your own user defined function in C. /

Can someone point me in the right direction per how I would remove the
first 25 bytes and the last 2 bytes from a bytea column?

http://www.postgresql.org/docs/9.3/static/functions-binarystring.html

Substring might do it for you.

JD

Thanks in advance

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On 2/22/2016 11:23 AM, Joshua D. Drake wrote:

Can someone point me in the right direction per how I would remove the
first 25 bytes and the last 2 bytes from a bytea column?

http://www.postgresql.org/docs/9.3/static/functions-binarystring.html

Substring might do it for you.

won't doing it in SQL still result in a BYTEA result which will be
wrapped when dumped via COPY ?

instead, I think this needs to be done externally to sql, like by piping
the resulting file through something like ...

tail -c +25 infile.dat | head -c -2 > outfile.dat

or doing something in perl or whatever.

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john r pierce, recycling bits in santa cruz

Can someone point me in the right direction per how I would remove the
first 25 bytes and the last 2 bytes from a bytea column?

http://www.postgresql.org/docs/9.3/static/functions-binarystring.html

Substring might do it for you.

won't doing it in SQL still result in a BYTEA result which will be wrapped when dumped via COPY ?

instead, I think this needs to be done externally to sql, like by piping the resulting file through something like ...

tail -c +25 infile.dat | head -c -2 > outfile.dat

or doing something in perl or whatever.

Hi,

I think it's easier if you go via base64 and then use the shell command "base64" to
decode it back to binary.

I start from this sample blob with 5 bytes (values 0, 1, 2, 4, 5), just a sample
(I actually failed to count to 4 correctly ;):

chris=# select * from t;
id | blob
----+--------------
1 | \x0001020405
(1 row)

In the shell now I can do:

chris$ psql -A -t -c "select encode(blob, 'base64') from t where id = 1"
AAECBAU=

To save this in binary I add base64 -d (Linux) or base64 -D (OS X, FreeBSD?):

chris$ psql -A -t -c "select encode(blob, 'base64') from t where id = 1" | base64 -D > blob.dat

blob.dat is now the 5 bytes, nothing else:

chris$ od -tx1 blob.dat
0000000 00 01 02 04 05
0000005

Bye,
Chris.

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