Re: PQfformat question and retrieving bytea data in C

Hey Jason,

2012/8/29 Jason Armstrong <ja(at)riverdrums(dot)com>

> I have a question regarding the return value of PQfformat()
>
> I have a 'data' column in my table, type bytea (postgresql 9.1.5).
>
> In postgresql.conf:
> bytea_output = 'escape'
>
> When I execute the query:
> PGresult *res = PQexec(db, "SELECT data::bytea FROM data_table WHERE
> id='xxx'")
>
PQexec() always returns data in the text format. You should use
PQexecParams() to obtain the data as binary.

>
> And I run through the results:
>
> int i, j;
> for (i = 0; i < PQntuples(res); i++) {
> for (j = 0; j < PQnfields(res); j++) {
> printf("Format %d: %d\n", j, PQfformat(res, j));
> printf("Type %d: %d\n", j, PQftype(res, j));
> }
> }
>
> This prints that the format is type 0, and the type is 17.
>
> Shouldn't the format be 1 (binary data)?
>
> I am getting a discrepancy between data that I put into the table and
> data I retrieve.
> When I dump the data, using:
>
> int di;
> char *val = PQgetvalue(res, i, j);
> for (di = 0; di < 16; di++) fprintf(stderr, "%2x ", val[di]);
>
> I see the following:
> 30 5c 33 33 32 5c 30 30 30 5c 30 31 31 5c 30 30
>
> But when I look at the same data in the database:
>
> psql> select encode(substr(data, 0, 16), 'hex') from data_table where
> id='xxx';
> encode
> --------------------------------
> 30da00090132420520203137323030
>
> This is the data I'm expecting to get back. Is the '00' (third byte)
> causing the problem?
>
> The data looks the same at a certain place (ie it starts with the same
> byte 30, then the C code has 22 bytes whereas the db hex dump has 7
> bytes, then the data is the same again. The 7/22 number of bytes isn't
> always the same, across the different data values).
>
> --
> Jason Armstrong
>
>
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--
// Dmitriy.