Just noticed this.

We have a column in the links table called Private_Notes.
Search weight for this column is zero.
The search_blocked string is Contact_Email,Contact_Name,Hits,isChanged,isNew,isPopular,isValidated,LinkOwner,Private_Notes,Rating,Votes.

So we expect the Private_Notes column to be invisible to the search engine. But it isn't blocked!. Search.cgi always returns links with keywords in the blocked columns -- not just our new column, but also regular columns like LinkOwner, etc.

Of course, the content of the Private_Notes notes column is not shown because it is not in the link.html template. But it does mean that right now the search engine returns a large proportion of meaningless links and is therefore unusable.

GT Staffers: Have you met this one before? What can we do? Could this be a Search.pm bug?

Ok -- solved this one -- not a bug, just one misunderstanding + one obscurity :

Misunderstanding: search_blocked merely determines which column names will be removed from a list of arguments to search.cgi, so is actually irrelevant to the perceived problem.

Obscurity: the only indexing scheme that uses search weights is internal. Other indexing schemes apparently return results based on all columns. We fixed the problem by switching to internal for the link and category tables, then running Rebuild Search.

Lesson: if you want to use search weights, you must use internal indexing.