Re: patch (for 9.1) string functions

2010/7/21 Itagaki Takahiro <itagaki.takahiro@gmail.com>:
> I reviewed the core changes of the patch. I don't think we need
> mb_string_info() at all. Instead, we can just call pg_mbxxx() functions.
>
> I rewrote the patch to use pg_mbstrlen_with_len() and pg_mbcharcliplen().
> What do you think the changes? It requires re-counting lengths of multi-byte
> strings in some cases, but the code will be much simpler and can avoid
> allocating length buffers.
>

It is a good idea. I see a problem only for "right" function, where
for most common use case a mblen will be called two times. I am not
able to say now, if this can be a performance issue or not. Highly
probably not - only for very large strings.

postgres=# create or replace function randomstr(int) returns text as
$$select string_agg(substring('abcdefghijklmnop' from
trunc(random()*13)::int+1 for 1),'') from generate_series(1,$1) $$
language sql;
CREATE FUNCTION
Time: 27,452 ms

postgres=# select count(*) from(select right(randomstr(1000),3) from
generate_series(1,10000))x;count
-------10000
(1 row)

Time: 5615,061 ms
postgres=# select count(*) from(select right(randomstr(1000),3) from
generate_series(1,10000))x;count
-------10000
(1 row)

Time: 5606,937 ms
postgres=# select count(*) from(select right(randomstr(1000),3) from
generate_series(1,10000))x;count
-------10000
(1 row)

Time: 5630,771 ms

postgres=# select count(*) from(select right(randomstr(1000),3) from
generate_series(1,10000))x;count
-------10000
(1 row)

Time: 5753,063 ms
postgres=# select count(*) from(select right(randomstr(1000),3) from
generate_series(1,10000))x;count
-------10000
(1 row)
Time: 5755,776 ms

It is about 2% slower for UTF8 encoding. So it isn't significant for me.

I agree with your changes. Thank You very much

Regards

Pavel Stehule

> I'd like to apply contrib/stringinfo apart from the core changes,
> because there seems to be still some idea to improve sprintf().
>
> --
> Itagaki Takahiro
>