Re: [PR] Improve worst-case performance of LIKE filters by 20x (druid)

["gianm (via GitHub)" <gi...@apache.org>]

gianm commented on PR #16153:
URL: https://github.com/apache/druid/pull/16153#issuecomment-2030550437

   For `LIKE` it seems reasonable to have an optimized matcher specifically geared towards the needs of `LIKE`. I am disappointed that the JDK regexp library doesn't handle these relatively simple cases well. I would have hoped that it could realize that `.*.*` is equivalent to `.*` and not blow up runtime with naive matching.
   
   For `REGEXP_LIKE`, the possibility of runtime blowup has always been there, as you point out. I'm not aware of a regexp library for Java that is equivalent to `java.util.Pattern` but does not have any possibility of runtime blowup. I am not even sure if it's possible. As I understand it, regexp libraries that guarantee no runtime blowup tend to achieve that by limiting the pattern syntax somewhat. (Does `joni` guarantee no runtime blowup, btw? I don't see a note about that on its page.)
   
   Assuming such a library doesn't exist, probably good approaches would be optionally using an alternative library, or @abhishekagarwal87's suggestion (detecting runtime blowup and throwing an error). But that's out of scope for this particular PR anyway, so let's focus on the `LIKE` situation.


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