On 23/09/2026 20:19, Sehrope Sarkuni wrote:
On Wed, Sep 23, 2026 at 1:09 PM Nathan Bossart <[email protected]> wrote:
On Wed, Sep 23, 2026 at 09:41:36AM -0500, Nathan Bossart wrote:
On Wed, Sep 23, 2026 at 10:10:09AM -0400, Sehrope Sarkuni wrote:
lpad() and rpad() pad one character at a time, calling
pg_mblen_range() and memcpy() once per padding char. When the padding
string is a single byte, e.g., lpad(x, n, '0') or rpad(x, n, ' '),
the padding is that byte repeated, so the attached patch fills it with
one memset().
I wonder if we could expand these gains by using SIMD whenever the vector
length is divisible by the padding string length. My hunch is that's where
a lot of the memset() gains come from.
Actually, I think we can expand this to any padding string length by
copying the padding string once, and then copying from the beginning of the
padding to the end repeatedly so that we write double the padding each
time. This is a bit like what commit c60e520 added for pglz_decompress().
I've attached some proof-of-concept grade patches. This doesn't quite
match the performance of your 1-byte fast-path, but it's pretty close and
applies to many more cases.
Ah! just saw this after I hit send.
I think my version does the rest of what you're describing!
Exact conclusion reached from testing it too.
See also similar thread on REPEAT():
https://www.postgresql.org/message-id/tencent_C5BBECF985A270FBC49463EDAF722CD5E005%40qq.com.
Whatever we do here, let's use the same implementation for REPEAT(),
LPAD(), and RPAD().
- Heikki
