"Etsuro Fujita" <fujita.etsuro@lab.ntt.co.jp> writes:
> 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.
This is a bug fix, not a performance improvement (any improvement would
be welcome, but that's not the point). There's absolutely nothing
guaranteeing that WORDS_PER_CHUNK is less than WORDS_PER_PAGE, and if
it were the other way around, the code would be outright broken. It's
pure luck that it was merely inefficient.
Committed, thanks for finding it!
regards, tom lane