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Thread: Retrieving points, arrays, ... with libpq

Retrieving points, arrays, ... with libpq

From
Date:
Hello,
 
Sorry to eventually bother you with questions coming from a newcomer to Postgres: I wonder how to retrieve data of such types as Points, enum, arrays (and all those fancy datatypes that have no directly equivalent representation in C) using the libpq library.
 
For example, imagine a table like this one, filed with the relevant information:
 
create type badluck as enum ( 'no more gas', 'flat tire', 'no keys' );
 
create table myTable(
    id SERIAL,
    carStatus badluck,
    whereIsIt Point [,
    and other columns]
);
   
I have read the most recent documentation from the first to the very last line, and I guess I can retreive data of any type with the PQgetValue() function,  something like:
 
char* chunkOfMemory = PQgetvalue( (const PGresult*) myresult, row_number, column_number);
 
But then, is it possible to cast the char* pointer to some meaningfull C structure if column_number relates to some datatype other than text or integer ?
 
For example, with regards to enum values, the documentation states that the length takes 4 bytes and the label at most NAMEDATALEN bytes, so is it equivalent to (or laid out as) something like
struct pgenum {
unsigned char length[4];
char label[NAMEDATALEN];
};
 
(This is only one example, I have not been able to find the layout of datatypes like Point and Array).
 
Do you know of any document that describe all those data types in term of the equivalent C structure (if there is any such structure of course).
 
Thank you again for your help and your patience.
 
Georges Brefort
 

Re: Retrieving points, arrays, ... with libpq

From
Jeroen Vermeulen
Date:
Georges.Brefort@sanofi-aventis.com wrote:

> I have read the most recent documentation from the first to the very
> last line, and I guess I can retreive data of any type with the
> PQgetValue() function,  something like: 
>  
> char* chunkOfMemory = PQgetvalue( (const PGresult*) myresult,
> row_number, column_number);
>  
> But then, is it possible to cast the char* pointer to some meaningfull C
> structure if column_number relates to some datatype other than text or
> integer ?

By default, you get your data in a textual representation.  So you need 
to parse the string that that char* points to.  (It's also possible to 
work in binary mode, but then you've got a lot more worries about 
portability and such for not much gain in speed.)


Jeroen