Not sure whether the do() is portable. The DBI code seems to suggest that some drivers return -1 untill all rows are fetched. Not sure if this would propagate back through the do() result.
I can confirm that it indeed does return -1, with a recent version of DBD::ODBC, and MS Access. It's a trick you should only use very cautiously.

In reply to Re: How to count rows using DBI & mysql - Performance results by bart
in thread How to count rows using DBI & mysql - Performance results by Brovnik

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