Re: BRIN page type identifier

Heikki Linnakangas wrote:
> The "special" area in a BRIN page looks like this:
> 
> >/* special space on all BRIN pages stores a "type" identifier */
> >#define        BRIN_PAGETYPE_META            0xF091
> >#define        BRIN_PAGETYPE_REVMAP        0xF092
> >#define        BRIN_PAGETYPE_REGULAR        0xF093
> >...
> >typedef struct BrinSpecialSpace
> >{
> >    uint16        flags;
> >    uint16        type;
> >} BrinSpecialSpace;
> 
> I believe this is supposed to follow the usual convention that the last two
> bytes of a page can be used to identify the page type. SP-GiST uses 0xFF82,
> while GIN uses values 0x00XX.
> 
> However, because the special size is MAXALIGNed, the 'type' field are not
> the last 2 bytes on the page, as intended. I'd suggest just adding "char
> padding[6]"  in BrinSpecialSpace, before 'flags'. That'll waste 4 bytes on
> 32-bit systems, but that seems acceptable.

Ouch.  You're right.  I don't understand why you suggest to use 6 bytes,
though -- that would make the struct size be 10 bytes, which maxaligns
to 16, and so we're back where we started.  Using 4 bytes does the
trick.

I wonder if this is permissible and whether it will do the right thing
on 32-bit systems:

/** Special area of BRIN pages.** We add some padding bytes to ensure that 'type' ends up in the last two* bytes of the
page,for consumption by pg_filedump and similar utilities.* (Special space is MAXALIGN'ed).*/
 
typedef struct BrinSpecialSpace
{char        padding[MAXALIGN(1) - 2 * sizeof(uint16)];uint16        flags;uint16        type;
} BrinSpecialSpace;


It's a bit ugly, but it seems to work for me on x86-64 ...

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
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