Driver specific behaviour. From DBD::mysql manual:

DBD::mysql has a "reconnect" feature that handles the so-called MySQL "morning bug":
If the server has disconnected, most probably due to a timeout, then by default the driver will reconnect and attempt to execute the same SQL statement again.

However, this behaviour is disabled when AutoCommit is off: Otherwise the transaction state would be completely unpredictable after a reconnect.

In reply to Re: DBI connection springs back to life - how? by pope
in thread DBI connection springs back to life - how? by mp

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