Re: More speedups for tuple deformation

Thanks for having a look.

On Sun, 8 Mar 2026 at 05:36, Junwang Zhao <zhjwpku(at)gmail(dot)com> wrote:
> I have some comments on v12-0004.
>
> 1.
>
> + off += cattr->attlen;
> + firstNonCachedOffsetAttr = i + 1;
> + }
> +
> + tupdesc->firstNonCachedOffsetAttr = firstNonCachedOffsetAttr;
> + tupdesc->firstNonGuaranteedAttr = firstNonGuaranteedAttr;
> +}
>
> The firstNonCachedOffsetAttr seems to be the first variable width
> attribute, but it seems that the offset of this attribute can be cached,
> for example, in a table defined as (int, text), the offset of
> firstNonCachedOffsetAttr should be 4, is that correct?

Yes.

> If TupleDescFinalize records the offset firstNonCachedOffsetAttr,
> it might save one iterator of the deforming loop. For example,
> add something like the following after the above mentioned code.
>
> if (firstNonCachedOffsetAttr < tupdesc->natts)
> {
> cattr = TupleDescCompactAttr(tupdesc, firstNonCachedOffsetAttr);
> cattr->attcacheoff = off;
> }

The problem is that short varlenas have 1 byte alignment and normal
varlenas have 4 byte alignment. It might be possible to do something
if the previous column is 4-byte aligned and has a length of 4 or 8,
since that means the offset must also be 1-byte aligned. The main
reason I don't want to do this is that the only positive is that
*maybe* 1 extra column can be deformed with a fixed offset. The
drawback is that the following code *has* to use
att_addlength_pointer(), *regardless* instead of "off += attlen;".
This means more deforming code and more complexity in
TupleDescFinalize(). I'd rather not do this.

> 2.
>
> in slot_deform_heap_tuple, there are multiple statements setting
> firstNonCacheOffsetAttr,
>
> + firstNonCacheOffsetAttr = tupleDesc->firstNonCachedOffsetAttr;
>
> + /* We can only use any cached offsets until the first NULL attr */
> + firstNonCacheOffsetAttr = Min(firstNonCacheOffsetAttr,
> + firstNullAttr);
>
> + /* We can only fetch as many attributes as the tuple has. */
> + firstNonCacheOffsetAttr = Min(firstNonCacheOffsetAttr, natts);
>
> Based on the logic, it seems the second one could be moved
> to the third position, and the third one could then be safely
> removed?

Yeah. Well spotted. I've done that in the attached.

I've also modified the 0006 patch to add a new deform_bench_select()
function which allows the benchmark to call the new selective deform
function. See the attached graphs comparing master to v13-0001-0005
and master to v13-0001-0006. It's good to see that there's still quite
a large speedup even from the tests that don't have an attcacheoff for
the column being deformed. Tests 1 and 5 do have a attcacheoff for the
column deformed, so they're a good bit faster again. To get the
0001-0006 results, I used the deform_test_run.sh script from [1] and
modified it to call deform_bench_select() instead of deform_bench().

I also noticed that when building with older gcc versions, I was
getting warnings about attlen and 'off' not being initialised. I ended
up switching back to the do/while loops to fix that rather than adding
needless initialisation, which would add overhead. 1 loop is
guaranteed, and the older compiler is not clever enough to work that
out.

David

[1] https://postgr.es/m/CAApHDvo1i-ycAcWnK3L7ZASTuM8mW46kvRqMaUHD46HSuJmx7A@mail.gmail.com