Improve performance of regexp matching in the sqlite adapter

[paddor]

Nothing much, just what the title says. Theoretically, if an infinite amount of different regexp are used in SQL queries, it could of course fill up RAM, but I think that's uncommon.

In my case, this brought the runtime from ~300ms down to ~70ms for a query that uses one single regexp in a table with around ~5700 rows.

[jeremyevans]

I'm OK with supporting this, but it must be conditional and off by default for backwards compatibility. This introduces a potential memory leak for code that does not expect the regexps to be cached.

[paddor]

I've changed the implementation. It now only caches Regexp objects if setup_regexp_function => :cached was given.

[paddor]

Thinking about it a little longer, maybe the option setup_regexp_function should just support taking a block. That way, the user is able to use modern features like Regexp#match? and Regexp.new(x, timeout: y) or just periodically clear the cache hash to avoid memleaks.

[paddor]

This is how I plan to use it now:

regexp_cache = Hash.new { |h, k| h[k] = Regexp.new(k, timeout: 0.005) }
opts = {
  setup_regexp_function: proc { |re_str, str| regexp_cache[re_str].match? str }
}
Sequel.connect('sqlite:///path/db.sqlite3', opts)

Let me know what you think. I left the :cached functionality in, with appropriate changes to the documentation. Let me know if I should remove it.

[jeremyevans]

Squash merged at c824824

[paddor]

Thanks!