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

On Thu, Mar 14, 2024 at 12:06 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Thu, Mar 14, 2024 at 1:29 PM John Naylor <johncnaylorls(at)gmail(dot)com> wrote:
> > Okay, here's an another idea: Change test_lookup_tids() to be more
> > general and put the validation down into C as well. First we save the
> > blocks from do_set_block_offsets() into a table, then with all those
> > blocks lookup a sufficiently-large range of possible offsets and save
> > found values in another array. So the static items structure would
> > have 3 arrays: inserts, successful lookups, and iteration (currently
> > the iteration output is private to check_set_block_offsets(). Then
> > sort as needed and check they are all the same.
>
> That's a promising idea. We can use the same mechanism for randomized
> tests too. If you're going to work on this, I'll do other tests on my
> environment in the meantime.

Some progress on this in v72 -- I tried first without using SQL to
save the blocks, just using the unique blocks from the verification
array. It seems to work fine. Some open questions on the test module:

- Since there are now three arrays we should reduce max bytes to
something smaller.
- Further on that, I'm not sure if the "is full" test is telling us
much. It seems we could make max bytes a static variable and set it to
the size of the empty store. I'm guessing it wouldn't take much to add
enough tids so that the contexts need to allocate some blocks, and
then it would appear full and we can test that. I've made it so all
arrays repalloc when needed, just in case.
- Why are we switching to TopMemoryContext? It's not explained -- the
comment only tells what the code is doing (which is obvious), but not
why.
- I'm not sure it's useful to keep test_lookup_tids() around. Since we
now have a separate lookup test, the only thing it can tell us is that
lookups fail on an empty store. I arranged it so that
check_set_block_offsets() works on an empty store. Although that's
even more trivial, it's just reusing what we already need.