Ezio,
thanks for taking a look. As I wrote, those are old patches
and we need to evaluate if they at all provide a generic solution I
guess. To me there are two parts of it:
a) how would the user interface to the substring search look like?
Using the same as the "LIKE" operator?

Here is some describtion:
"""
string LIKE pattern 

If pattern does not contain percent signs or underscore, then the
pattern only represents the string itself; in that case LIKE acts like
the equals operator. An underscore (_) in pattern stands for (matches)
any single character; a percent sign (%) matches any string of zero or
more characters."""

Given that we want to use an index of some kind, I haven't looked into
solutions how to do a pattern match that is not directly supported
by the index engine (like "LIKE").