Re: Variable length custom data types help

On Thu, Mar 31, 2005 at 07:45:12PM -0800, Morgan Kita wrote:
>
> Ok I am creating some important custom data types for my DB. The data
> should be stored as an array of strucs, which themselves contain a few
> different data fields. Anyways I wrote the input function in C when I
> realized I had a problem.
>
> The arrays will not always contain the same number of structures, and
> as such should be variable length. I read the pages in the manual on
> custom data type creation, and I noticed that you can do this by
> setting the first word of the stored bytes to be the total length in
> bytes of the stored object. So my question is, given that this is a
> typed array, what is the best way to represent this in the c code?

The usual way is to create a structure:

typedef struct foo {
int32 total_length;
/* rest of structure here */
} foo;

What follows the length is irrelevant to PostgreSQL, so define it
however makes sense in your code. Just make sure that all data is
in a contiguous chunk of memory that begins with the 4-byte length
(don't embed pointers in the data, because PostgreSQL won't know
anything about them).

> In the end I am returning a pointer correct?

Correct. Use palloc() to allocate the memory and PG_RETURN_POINTER()
to return the pointer.

> What is the type of the pointer I should be returning?

It doesn't matter; it's just a memory address. With the structure
above you could declare a foo *.

> I guess I really don't understand how the memory returned by the
> pointer is going to be parsed by Postgres.

PostgreSQL isn't going to parse the data beyond looking at the
4-byte length. You're returning a pointer to "length + stuff" and
PostgreSQL treats "stuff" as an opaque bunch of bytes.

> I thought of making the first member of the array a dummy structure whose
> first data field would contain the length. However, that doesn't sound
> implementation independant and I think it would be better if I actually
> understood how the DB will actually parse the pointer that my function
> returns.

PostgreSQL only cares about the length in the first 4 bytes, so if
the data begins with an int32 then you should be okay.

> Also I have a related question... from what I read in the manual; by
> creating the type as variable length that automatically makes it toasted
> in the DB. Is that the case, and if so, do I have to do anything else
> other than untoast it in my output function?

CREATE TYPE's "storage" parameter determines the storage strategy,
but you should probably use PG_DETOAST_DATUM() in any case (I think
it's a no-op if the data isn't actually toasted). I can't think
of any other special handling you'd need to do.

--
Michael Fuhr
http://www.fuhr.org/~mfuhr/