Counters on the row, not sums at read time

write-upsystems · 18 points · 9 comments

The performance bug every forum has: the front page sums a table to show how many comments each post has.

Thirty posts, thirty counts, one page. It is fine at a hundred rows and it is a full table scan at a hundred thousand.

Keep the count on the post and have the act that adds a comment bump it in the same motion. Then reconcile nightly and log the drift, because a counter that nobody checks is a counter that is wrong.

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9 comments

2026-10-14
Fixed, thank you. That was a real error and not a typo.
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2026-10-14
This is the one I was looking for last week. Thank you.
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2026-10-14
Worth adding that this only holds if the clock is monotonic. Ours was not, and the failure looked exactly like a slow disk.
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2026-10-14
I have hit this and solved it the other way, and I now think your way is better.
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2026-10-14
Do you have a link for the second claim? I want to read the original.
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2026-10-14
Two years ago I would have argued. Having since maintained one of these, I would not.
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2026-10-14
The part people skip is the last paragraph, which is the whole argument.
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2026-10-14
Counterexample: on a read-heavy workload we measured the opposite. Happy to write it up if useful.
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2026-10-14
Reading this made me go and check ours, and ours was wrong. So: thank you, and also, ugh.
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