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Re: Patch: add conversion from pg_wchar to multibyte

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Alexander Korotkov <aekorotkov(at)gmail(dot)com>
Cc: Erik Rijkers <er(at)xs4all(dot)nl>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Patch: add conversion from pg_wchar to multibyte
Date: 2012-05-02 13:48:33
Message-ID: CA+Tgmobi0hDCfq7cF91QOZBUF0KK09Q8dxydB1u6FsmisgtghQ@mail.gmail.com (view raw or flat)
Thread:
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On Wed, May 2, 2012 at 9:35 AM, Alexander Korotkov <aekorotkov(at)gmail(dot)com> wrote:
>> I was thinking you could perhaps do it just based on the *number* of
>> trigrams, not necessarily their frequency.
>
> Imagine we've two queries:
> 1) SELECT * FROM tbl WHERE col LIKE '%abcd%';
> 2) SELECT * FROM tbl WHERE col LIKE '%abcdefghijk%';
>
> The first query require reading posting lists of trigrams "abc" and "bcd".
> The second query require reading posting lists of trigrams "abc", "bcd",
> "cde", "def", "efg", "fgh", "ghi", "hij" and "ijk".
> We could decide to use index scan for first query and sequential scan for
> second query because number of posting list to read is high. But it is
> unreasonable because actually second query is narrower than the first one.
> We can use same index scan for it, recheck will remove all false positives.
> When number of trigrams is high we can just exclude some of them from index
> scan. It would be better than just decide to do sequential scan. But the
> question is what trigrams to exclude? Ideally we would leave most rare
> trigrams to make index scan cheaper.

True.  I guess I was thinking more of the case where you've got
abc|def|ghi|jkl|mno|pqr|stu|vwx|yza|....  There's probably some point
at which it becomes silly to think about using the index.

>> > Probably you have some comments on idea of conversion from pg_wchar to
>> > multibyte? Is it acceptable at all?
>>
>> Well, I'm not an expert on encodings, but it seems like a logical
>> extension of what we're doing right now, so I don't really see why
>> not.  I'm confused by the diff hunks in pg_mule2wchar_with_len,
>> though.  Presumably either the old code is right (in which case, don't
>> change it) or the new code is right (in which case, there's a bug fix
>> needed here that ought to be discussed and committed separately from
>> the rest of the patch).  Maybe I am missing something.
>
> Unfortunately I didn't understand original logic of pg_mule2wchar_with_len.
> I just did proposal about how it could be. I hope somebody more familiar
> with this code would clarify this situation.

Well, do you think the current code is buggy, or not?

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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