Re: Multi-Block read count

The second issue which you mention is extremely important, as it can make a dramatic difference to performance.

Oracle has taken note of this in Oracle 9 by allowing you to collect "system statistics" over a typical time period so that it can incorporate I/O rates into its costing calculations. The interesting thing to note is that it records three things for I/O

Average time for a single block read over the period.

Average time for a multi block read over the period.

Average ACTUAL size requested for a multiblock read over the period.

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Herman de Boer wrote in message ...


>There is much to say about this topic. I think it's worth mentioning

>two additional related issues:

>

>2. When performing a full table scan, needed blocks in the buffer

>   cache can cause splits in the reading.

>   For instance, if the RDBMS needs blocks 11 to 20, each of the block

>   in the range is checked whether it exists in the buffer cache.

>   If block 15 is cached (the only one in the range), there will

>   be 2 reads: one for block 11 till 14, and one read for 16 - 20.

>