Overhead to consistent gets?

A reader just asked me an interesting question, to which I can only answer "I don't know". But I'd sure like to know, so in the interests of gaining enlightnment here's the question:

Is there likely to be a performance difference between the following two situations:

1. Your query reads 1000 rows packed into 10 blocks. All blocks are already in the buffer cache (no physical reads). No other transaction has updated the blocks.
2. Your query reads the same 1000 rows, but this time they are scattered over, say, 50 blocks. Again, all blocks are in cache. No other transaction has updated the blocks.

Case b results in more consistent gets. But the same number of rows are returned in either case. This question seems to boil down to "what's the overhead to a consistent get?"

And, my own question that I'll is: do things change if another transaction has updated the blocks since your transaction began?

Best regards,

Jonathan Gennick --- Brighten the corner where you are http://Gennick.com * 906.387.1698 * mailto:jonathan@gennick.com

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