Re: Index Full Scan -- Strange Issue

Hi Ian,

Little correction.
*Sometimes* index full scan (IFS) also uses multiblock IOs. In this case number of blocks is controlled with _db_file_noncontig_mblock_read_count. If my memory servers me right the default value is 11 for 8.1.7 on Solaris. You can easily check out this by using 10046 event, level 8. My assumption is that IFS may use multiblock IOs for reading leaf blocks and it always use single block IOs for reading branch blocks.
I said may because I believe it somehow depends on index structure and on the other hand IFS *must* preserve index order.

Regards,
Ed

> You are right Fast Full Scans use multiblock IO. Other index scans do not. That had slipped by
me. So my speculation is moot. If it was an FFS, I could come up with a scenario where such a path would be better than range scan. But that's moot, and I'm pretty sure such scenarios are beyond Oracle's optimizer.
>
> Ian MacGregor
> Stanford Linear Accelerator Center
> ian_at_slac.stanford.edu
>

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