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Thread: Size of TOAST pointer

Size of TOAST pointer

From
Vignesh Raghunathan
Date:
Hello,

I was looking at the documentation for TOAST (http://www.postgresql.org/docs/devel/static/storage-toast.html) and it's specified that the toast pointer occupies 18 bytes. However, the struct representing the toast pointer is defined as follows 

typedef struct varatt_external
{
        int32           va_rawsize;             /* Original data size (includes header) */
        int32           va_extsize;             /* External saved size (doesn't) */
        Oid                     va_valueid;             /* Unique ID of value within TOAST table */
        Oid                     va_toastrelid;  /* RelID of TOAST table containing it */
}       varatt_external;


Assuming that the pointer is aligned to a 4 byte boundary, doesn't it sum up to 16 bytes?

Could someone please explain why the documentation specifies it as 18 bytes?

I am pretty new to the source code and the C language in general so I apologize if this question has an obvious answer.

Thanks,
Vignesh.


Re: Size of TOAST pointer

From
Tom Lane
Date:
Vignesh Raghunathan <vignesh.pgsql@gmail.com> writes:
> I was looking at the documentation for TOAST (
> http://www.postgresql.org/docs/devel/static/storage-toast.html) and it's
> specified that the toast pointer occupies 18 bytes. However, the struct
> representing the toast pointer is defined as follows
> typedef struct varatt_external

The data that actually ends up on disk is a varattrib_1b_e wrapping a
varatt_external, and that header is where the extra 2 bytes come from.
        regards, tom lane