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if we can commit several transactions at the same time – RDBMS needs to fsync() our DB logs to physical disks only once

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if going our preferred way of single-writing-DB connection – make sure NOT to use RDBMS-level Group Commit

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with our app-level cache, we’re guaranteed to get exactly the same results as if we’d be querying the underlying database.

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why settle for invalidation when we can modify cache accordingly, saving on the extra DB request when we’re dealing with the same USER again?

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Have I already mentioned that you DO need to understand execution plans?

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DO keep in mind that maintaining indexes has its own cost

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A correlated sub-query is a type of query, where inner query depends upon the outcome of the outer query in order to perform its execution.

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Correlated sub-queries are slower.