Re: [HACKERS] More efficient truncation of pg_stat_activity querystrings

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_USEDelse if ((*s & 0xfc) == 0xf8)    len = 5;else if ((*s & 0xfe) == 0xfc)    len = 6;
#endifelse    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


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