When do checkpoints happen?
At regular intervals depending on the number and frequency of data modifications.
Can they be invoked programmatically?
Yes. The command is CHECKPOINT. It's not something that's often needed/
Besides simple recovery, are there other techniques to minimize the size of the transaction log?
Keep transactions short. The tran log can only be truncated to the beginning of the oldest active transaction. If you have transactions that remain open for long periods of time, it will prevent log truncation.
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SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
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