Greg,
Only if your sample is random and independent. The existing mechanism tries fairly hard to ensure that every record has an equal chance of being selected. If you read the entire block and not appropriate samples then you'll introduce systematic sampling errors. For example, if you read an entire block you'll be biasing towards smaller records.
Did you read any of the papers on block-based sampling? These sorts of issues are specifically addressed in the algorithms.
--Josh ---------------------------(end of broadcast)--------------------------- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly