I was reading your article on building a search engine here, http://www.onlamp.com/pub/a/php/2002/...
You can improve this on your occurrence table, remove the id, there's no need for it, you can use a composite key with pageID and wordID. count is a running total of the occurrences of a word (as opposed to multiple redundant entries).
You can also very easily search for multiple words by modifying your search query to this,
SELECT p.page_url AS url,
SUM(`count`) AS occurrences
FROM page p, word w, occurrence o
WHERE p.page_id = o.page_id AND
w.word_id = o.word_id AND
w.word_word IN ("$keyword1", "$keyword2", "$keywordN")
GROUP BY p.page_id
ORDER BY occurrences DESC
That can work with one keyword or an infinite number, and the change in the structure for indexing will greatly reduce space with your bridging table.
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