When running UPDATE ... datetime = NOW(); will all rows updated have the same date/time?
Absolutely, using UPDATE with datetime = NOW() stamps the same timestamp on every row involved in the update. The function NOW() computes only once at the launch of the statement execution.
-- Don't you worry about time travel paradoxes, because...
UPDATE your_table SET your_datetime_column = NOW();
Each row in your_table receives the same NOW() timestamp, preserving the statement execution moment.
Digesting the NOW() function
The NOW() function acts as a timestamp generator at the start of the statement execution and stays fixed throughout the statement execution. Thus, it serves a consistent timestamp across all affected rows.
Key scenarios impacted by NOW()
Understanding NOW()'s behavior is pivotal in situations when:
- Your transactions affect numerous rows and you need a common timestamp for record tracking or auditing.
- The operation demands synchronization across multiple rows, such as in a replication scenario.
NOW() vs. SYSDATE(): Timestamp precision
NOW() might be mistaken for SYSDATE() when the situation requires a timestamp corresponding to the exact second of each individual row update. Unlike NOW(), SYSDATE() computes at the precise moment it's invoked, potentially different for each affected row in a sizeable or complex update operation.
Pondering over large table updates
Even when NOW() is implemented on vast tables, the timestamp remains consistent across all rows. However, it would be wise to consider performance:
- Be mindful of indexing and locking, which can possibly stretch the execution timeline.
- In a heavy transaction system, extensive write locks might trigger contention.
- Keep an eye on the performance overhead for smooth operations. You might need to update in batches or program them during low activity periods.
Consistency of NOW() since MySQL 5.0.13
The uniform behavior of NOW() has been a cornerstone of MySQL's performance since version 5.0.13. Any version before might not exhibit this level of predictability.
Golden rules for precise data management
To successfully handle accurate date-time values, you must:
- Leverage
NOW()for homogenous timestamps when consistency is vital. - Resort to
SYSDATE()when the exact time of each row update is crucial. - Gauge your choices whether allotting a uniform timestamp for a batch operation suits your case.
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