vb3 stores a list of similar threads along with each thread. This list is generated when the post is made/edited. Scanning the entire thread table on every view of showthread and then running checks on every thread is not the best way to go about this. The list is built from scanning the word table, NOT the thread table. Remember the word table is indexed so we can search for words efficiently. vB3 also only uses the title of the thread to determine something's value as being similar.
The function that searches for posts from search.php is reused for generating this search.
Enough information?
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