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From: Shinya Kato <shinya11.kato@gmail.com>
Date: Wed, 13 Aug 2025 15:21:06 +0900
Message-ID: 
 <CAOzEurSqgA69er9SzhPnXwmsVpO7-piUOuOy3dXcHOi__nSQcg@mail.gmail.com>
Subject: Re: Speed up COPY FROM text/CSV parsing using SIMD
To: Nazir Bilal Yavuz <byavuz81@gmail.com>
Cc: pgsql-hackers@postgresql.org
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On Tue, Aug 12, 2025 at 4:25=E2=80=AFPM Shinya Kato <shinya11.kato@gmail.co=
m> wrote:

> > +         * However, SIMD optimization cannot be applied in the followi=
ng cases:
> > +         * - Inside quoted fields, where escape sequences and closing =
quotes
> > +         *   require sequential processing to handle correctly.
> >
> > I think you can continue SIMD inside quoted fields. Only important
> > thing is you need to set last_was_esc to false when SIMD skipped the
> > chunk.
>
> That's a clever point that last_was_esc should be reset to false when
> a SIMD chunk is skipped. You're right about that specific case.
>
> However, the core challenge is not what happens when we skip a chunk,
> but what happens when a chunk contains special characters like quotes
> or escapes. The main reason we avoid SIMD inside quoted fields is that
> the parsing logic becomes fundamentally sequential and
> context-dependent.
>
> To correctly parse a "" as a single literal quote, we must perform a
> lookahead to check the next character. This is an inherently
> sequential operation that doesn't map well to SIMD's parallel nature.
>
> Trying to handle this stateful logic with SIMD would lead to
> significant implementation complexity, especially with edge cases like
> an escape character falling on the last byte of a chunk.

Ah, you're right. My apologies, I misunderstood the implementation. It
appears that SIMD can be used even within quoted strings.

I think it would be better not to use the SIMD path when last_was_esc
is true. The next character is likely to be a special character, and
handling this case outside the SIMD loop would also improve
readability by consolidating the last_was_esc toggle logic in one
place.

Furthermore, when inside a quote (in_quote) in CSV mode, the detection
of \n and \r can be disabled.

+               last_was_esc =3D false;

Regarding the implementation, I believe we must set last_was_esc to
false when advancing input_buf_ptr, as shown in the code below. For
this reason, I think it=E2=80=99s best to keep the current logic for toggli=
ng
last_was_esc.

+               int advance =3D pg_rightmost_one_pos32(mask);
+               input_buf_ptr +=3D advance;

I've attached a new patch that includes these changes. Further
modifications are still in progress.

--=20
Best regards,
Shinya Kato
NTT OSS Center

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