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Fastest Way To Check How Many Posts Are In A Thread With MySQL And PHP

I was wondering if it which is faster when trying to, for example, check how many posts are in a particular thread on a forum. Should I...

(a) Go through each post in the database with the particular thread ID, and count how many rows

or

(b) Add one to a separate column in the threads database every time a thread is made, and then query that single row

Thanks.

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Answer

What's wrong with having an index for the thread ID? Wouldn't a simple COUNT expression grouped by the thread ID field suffice?

With any caching at all, this would be plenty fast from what I can tell.

--This will provide counts for all threads
SELECT COUNT(threadID)
FROM Posts
GROUP BY threadID;


--This will provide count for one thread
SELECT COUNT(threadID)
FROM Posts
WHERE threadID=123
GROUP BY threadID;
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source: stackoverflow.com
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