Re: More efficient truncation of pg_stat_activity query strings

From: Andres Freund <andres(at)anarazel(dot)de>
To: Tatsuo Ishii <ishii(at)sraoss(dot)co(dot)jp>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: More efficient truncation of pg_stat_activity query strings
Date: 2017-09-12 08:19:29
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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
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;
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.


Andres Freund

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