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From Gopal Vijayaraghavan <gop...@apache.org>
Subject Re: Hive LIKE predicate. '_' wildcard decrease perfomance
Date Thu, 04 Aug 2016 19:15:16 GMT
> where res_url like '%mts.ru%'
...
> where res_url like '%mts_ru%'
...
> Why '_' wildcard decrease perfomance?

Because it misses the fast path by just one "_".

ORC vectorized reader has a zero-copy check for 3 patterns - prefix,
suffix and middle.

That means "https://%", "%.html", "%mts.ru%" will hit the fast path -
which uses StringExpr::equal() which JITs into the following.

https://issues.apache.org/jira/secure/attachment/12748720/string-intrinsic-
sse.png


In Hive-2.0, you can mix these up too to get "https:%mts%.html" in a
ChainedChecker.


Anything other than these 3 cases becomes a Regex and takes the slow path.

The pattern you mentioned gets rewritten into ".*mts.ru.*" and the inner
loop has a new String() as the input to the matcher + matcher.matches() in
it.

I've put in some patches recently which rewrite it Lazy regexes like
".?*mts.ru.?*", so the regex DFA will be smaller (HIVE-13196).

That improves the case where the pattern is found, but does nothing to
improve the performance of the new String() GC garbage.

Cheers,
Gopal



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