Re: pgsql: Compress GIN posting lists, for smaller index size.

Hi,
On 2014-01-22 17:28:48 +0000, Heikki Linnakangas wrote:
> Compress GIN posting lists, for smaller index size.
>
> GIN posting lists are now encoded using varbyte-encoding, which allows them
> to fit in much smaller space than the straight ItemPointer array format used
> before. The new encoding is used for both the lists stored in-line in entry
> tree items, and in posting tree leaf pages.
>
> To maintain backwards-compatibility and keep pg_upgrade working, the code
> can still read old-style pages and tuples. Posting tree leaf pages in the
> new format are flagged with GIN_COMPRESSED flag, to distinguish old and new
> format pages. Likewise, entry tree tuples in the new format have a
> GIN_ITUP_COMPRESSED flag set in a bit that was previously unused.

A new version of clang complains:

/home/andres/src/postgresql/src/backend/access/gin/ginvacuum.c:512:34: warning: signed shift result (0x80000000) sets the sign bit of the
shift expression's type ('int') and becomes negative [-Wshift-sign-overflow]
uncompressed = (ItemPointer) GinGetPosting(itup);
^~~~~~~~~~~~~~~~~~~
/home/andres/src/postgresql/src/include/access/gin_private.h:226:59: note: expanded from macro 'GinGetPosting'
#define GinGetPosting(itup) ((Pointer) ((char*)(itup) + GinGetPostingOffset(itup)))
^~~~~~~~~~~~~~~~~~~~~~~~~
/home/andres/src/postgresql/src/include/access/gin_private.h:224:85: note: expanded from macro 'GinGetPostingOffset'
#define GinGetPostingOffset(itup) (GinItemPointerGetBlockNumber(&(itup)->t_tid) & (~GIN_ITUP_COMPRESSED))
^~~~~~~~~~~~~~~~~~~
/home/andres/src/postgresql/src/include/access/gin_private.h:223:33: note: expanded from macro 'GIN_ITUP_COMPRESSED'
#define GIN_ITUP_COMPRESSED (1 << 31)

As far as I understand the code it should rather be
#define GIN_ITUP_COMPRESSED (1U << 31)

Is that right?

Greetings,

Andres Freund

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Andres Freund http://www.2ndQuadrant.com/
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