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If the database server has stellar buffer or caching size/capabiltiescapabilities, then sub-sequent subsequent viewing of each pages page will not be penalized by the expensive count query.
Conclusion
When you start to design your App, you will need to try to project the amount of data that will be pouring in over a considerable amount of time and evaluate if your App can still serve and hold up well by then. You can try to also make use of Performance Analyzer for continuous monitoring of your App's performance.
Only use a derived query when it is absolutely necessary. Try to have the data set computed elsewhere or beforehand when possible.
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