Drupal 7's database layer makes use of transactions in several places. Be careful when you expect to see the results of node_save() outside of the scope of the current request. If someone accesses your website data before the transaction has been committed, the results of queries executed within are not visible. Either all queries succeed or nothing happens at all in a database transactions.
I thought .module files were not supposed to be loaded and available until after .install files were run, but whatever is the intention, it is absolutely certain that at least this hook can be implemented before the module is truly installed.
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