I'd like to do the complementary explanation of this.
In tbm_union_page() and tbm_intersect_page() in tidbitmap.c, WORDS_PER_PAGE
is used in the scan of a lossy chunk, instead of WORDS_PER_CHUNK, where
these macros are defined as:
/* number of active words for an exact page: */
#define WORDS_PER_PAGE ((MAX_TUPLES_PER_PAGE - 1) / BITS_PER_BITMAPWORD +
1)
/* number of active words for a lossy chunk: */
#define WORDS_PER_CHUNK ((PAGES_PER_CHUNK - 1) / BITS_PER_BITMAPWORD + 1)
Though the use of WORDS_PER_PAGE in the scan of a lossy chunk is logically
correct since these macros implicitly satisfy that WORDS_PER_CHUNK <
WORDS_PER_PAGE, I think WORDS_PER_CHUNK should be used in the scan of a
lossy chunk for code readability and maintenance. So, I submitted a patch
working in such a way in an earlier email.
I think that, as a secondary effect of the patch, the scan would be
performed a bit efficiently.
I'll add the patch to the 2013-11 CF. Any comments are welcome.
Thanks,
Best regards,
Etsuro Fujita