Re: [PoC] Improve dead tuple storage for lazy vacuum

On Thu, Apr 25, 2024 at 6:03 AM Noah Misch <noah(at)leadboat(dot)com> wrote:
>
> On Mon, Apr 15, 2024 at 04:12:38PM +0700, John Naylor wrote:
> > - Some paths for single-value leaves are not covered:
> >
> > https://anarazel.de/postgres/cov/16-vs-HEAD-2024-04-14/src/include/lib/radixtree.h.gcov.html#L904
> > https://anarazel.de/postgres/cov/16-vs-HEAD-2024-04-14/src/include/lib/radixtree.h.gcov.html#L954
> > https://anarazel.de/postgres/cov/16-vs-HEAD-2024-04-14/src/include/lib/radixtree.h.gcov.html#L2606
> >
> > However, these paths do get regression test coverage on 32-bit
> > machines. 64-bit builds only have leaves in the TID store, which
> > doesn't (currently) delete entries, and doesn't instantiate the tree
> > with the debug option.
> >
> > - In RT_SET "if (found)" is not covered:
> >
> > https://anarazel.de/postgres/cov/16-vs-HEAD-2024-04-14/src/include/lib/radixtree.h.gcov.html#L1768
> >
> > That's because we don't yet have code that replaces an existing value
> > with a value of a different length.
>
> I saw a SIGSEGV there when using tidstore to write a fix for something else.
> Patch attached.

Great find, thank you for the patch!

The fix looks good to me. I think we can improve regression tests for
better coverage. In TidStore on a 64-bit machine, we can store 3
offsets in the header and these values are embedded to the leaf page.
With more than 3 offsets, the value size becomes more than 16 bytes
and a single value leaf. Therefore, if we can add the test with the
array[1,2,3,4,100], we can cover the case of replacing a single-value
leaf with a different size new single-value leaf. Now we add 9 pairs
of do_gset_block_offset() and check_set_block_offsets(). If these are
annoying, we can remove the cases of array[1] and array[1,2].

I've attached a new patch. In addition to the new test case I
mentioned, I've added some new comments and removed an unnecessary
added line in test_tidstore.sql.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com