On our message boards each post is a row. The powers that be like to know how many posts there are total (In addition to 'today')- select count(*) from posts is how it has been done on our informix db. With our port to PG I instead select reltuples pg_class.
We have exactly the same situation, except we just added a 'num_replies' field to each thread and a 'num_posts' field to each forum, so that getting that information out is a very fast operation. Because, of course, there are hundreds of times more reads of that information than writes...
Chris
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