Re: Optimize UUID parse using SIMD

On Sun, Jun 28, 2026 at 7:20 PM Haibo Yan <tristan(dot)yim(at)gmail(dot)com> wrote:
>
> On Thu, Jun 25, 2026 at 3:16 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Thu, Jun 25, 2026 at 2:31 PM Haibo Yan <tristan(dot)yim(at)gmail(dot)com> wrote:
> > >
> > >
> > >
> > > On Thu, Jun 25, 2026 at 11:28 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > >>
> > >> Hi all,
> > >>
> > >> I'd like to propose the $subject.
> > >>
> > >> Since commit ec8719ccbfcd made hex_decode_safe() SIMD-aware, decoding
> > >> a run of hex digits is now fast. The attached patch reuses
> > >> hex_decode_safe() in the UUID input function to speed up parsing.
> > >>
> > >> We accept several textual forms of a UUID[1]. The fast path handles
> > >> the common ones: 32 hex digits, the canonical 8x-4x-4x-4x-12x form
> > >> (where "nx" means n hex digits), and either of those wrapped in
> > >> braces. Otherwise, it falls back to the ordinary scalar UUID parse.
> > >>
> > >> I've benchmarked the parse speed using the following query:
> > >>
> > >> CREATE TEMP TABLE u AS SELECT gen_random_uuid()::text AS t FROM
> > >> generate_series(1, 1000000);
> > >> EXPLAIN (ANALYZE, TIMING OFF) SELECT t::uuid FROM u;
> > >>
> > >> I compared the execution time of the second query, which measures
> > >> uuid_in() alone, with/without SIMD optimization. Here are results (the
> > >> median of 5 runs):
> > >>
> > >> HEAD: 208.879 ms
> > >> Patched: 40.983 ms
> > >>
> > >> The improvements look promising to me. But in a realistic pipeline the
> > >> parse is a small fraction of the work, so end-to-end gains could be
> > >> much smaller.
> > >>
> > >> Feedback is very welcome.
> > >>
> > > I may be missing something, but I wonder whether the fast path is relying on
> > > slightly different input semantics from the existing UUID parser.
> > >
> > > In particular, hex_decode_safe() is not a strict “32 hex characters only”
> > > decoder. It skips whitespace, which is fine for its existing callers, but I
> > > don’t think UUID input should treat whitespace inside the UUID body as
> > > ignorable.
> >
> > Good catch! hex_decode_safe() skips whitespaces so the patch accepts
> > the following UUID value, which is bad:
> >
> > select '019f00b5-7f8a-722f-b707-59f0ed25cd '::uuid;
> > uuid
> > --------------------------------------
> > 019f00b5-7f8a-722f-b707-59f0ed25cd00
> > (1 row)
> >
> > > Also, since hex_decode_safe() returns void, the UUID fast path
> > > cannot verify that exactly UUID_LEN bytes were produced.
> >
> > IIUC hex_decode_safe() does return the output length in bytes. So I
> > think we can fallback to the scalar UUID parser if
> > esctx.error_occurred is true or if the returned value is not 16.
> >
>
> You’re right, I misread that part. Checking both esctx.error_occurred and
> the returned length sounds good to me.
>
> > >
> > > So I think it would be safer either to pre-validate that the 32 source
> > > characters are all hex digits before calling hex_decode_safe(), or to use a
> > > UUID-specific strict hex decoder for this path. After that, a comment
> > > explaining why hex_decode_safe() is safe here would make the invariant much
> > > clearer.
> >
> > IIUC hex_decode_simd_helper() accepts only hex digits so we could
> > re-use it for UUID parsing. Let me check if the above idea of using
> > the return value works for us first.
> >
>
> That sounds reasonable. My main concern was to keep the fast path’s accepted
> input set identical to the scalar UUID parser. Falling back when the decoded
> length is not UUID_LEN, together with regression tests for whitespace cases,
> should address that.
>
> > >
> > > Could you also add a few regression tests for invalid inputs that contain
> > > whitespace inside otherwise fast-path-looking UUID strings? For example:
> > >
> > > ---------------------------------------------------------------
> > >
> > > SELECT 'a0eebc99 9c0b4ef8bb6d6bb9bd380a11'::uuid;
> > > SELECT 'a0eebc999c0b4ef8bb6d6bb9bd380a1 '::uuid;
> > > SELECT '{a0eebc999c0b4ef8bb6d6bb9bd380a1 }'::uuid;
> > > SELECT 'a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a1 '::uuid;
> > > ---------------------------------------------------------------
> > >
> > > These should continue to be rejected in the same way as the scalar parser.
> > > Regards,
> >
> > Agreed.
> >

I've attached the updated patch.

Regards,

--
Masahiko Sawada
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