On Thu, 9 Feb 2023 at 14:49, Tom Lane <t...@sss.pgh.pa.us> wrote: > > David Rowley <dgrowle...@gmail.com> writes: > > Tom's argument seems to think it's impossible, so if you find that > > it's definitely not impossible, then you can assume he's wrong about > > that. > > My point was that it seems like you'd need a separate BMH engine for > each %-separated segment of the LIKE pattern. I'm not quite clear on > whether BMH can handle '_' (single-char wildcard) conveniently by > itself, although my gut feel is that you can probably make that part > work. Maybe you can even extend the idea to embedded %, but that > seems more difficult.
Yeah, I think to make it work with more complex patterns like '%some%string%' or '%some_string%' you'd need to break that into multiple independent searches for each portion between a wildcard character. For the former pattern, you'd need to do some final check that ensures that the 2nd pattern was found in some position >= the position of the 1st pattern + its length. For the latter, the final check would need to validate that the 2nd pattern was found at a position of the first pattern + its length + 1. It's probably also possible to make those patterns work when they don't contain the leading and trailing % by checking that the first pattern is found at position 0 and the end of the 2nd pattern is found at the end of the search string. However, I imagine going to the trouble of trying to make it work for more complex patterns initially would be a bad idea. I imagine there are just too many cases where we could demonstrate performance regressions and that would cause us to reject the patch. David
