Re: Speed up lpad() and rpad() for one-byte padding strings - Mailing list pgsql-hackers

On 23/09/2026 20:19, Sehrope Sarkuni wrote:
> On Wed, Sep 23, 2026 at 1:09 PM Nathan Bossart <nathandbossart@gmail.com> 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