Re: More efficient truncation of pg_stat_activity query strings

On 2017-09-12 16:53:49 +0900, Tatsuo Ishii wrote:
> > read side. I think that should work because all *server side* encodings
> > store character lengths in the *first* byte of a multibyte character
>
> What do you mean? I don't see such data in a multibyte string.

Check the information the pg_*_mblen use / how the relevant encodings
work. Will be something like
int
pg_utf_mblen(const unsigned char *s)
{
int len;

if ((*s & 0x80) == 0)
len = 1;
else if ((*s & 0xe0) == 0xc0)
len = 2;
else if ((*s & 0xf0) == 0xe0)
len = 3;
else if ((*s & 0xf8) == 0xf0)
len = 4;
#ifdef NOT_USED
else if ((*s & 0xfc) == 0xf8)
len = 5;
else if ((*s & 0xfe) == 0xfc)
len = 6;
#endif
else
len = 1;
return len;
}

As you can see, only the first character (*s) is accessed to determine
the length/width of the multibyte-character. That's afaict the case for
all server-side encodings.

Greetings,

Andres Freund