Query optimizer is a funny animal. I don't think anyone can tell you with
full certainty why it does some of the things it does.
I can tell you for sure it does "care" about keyed LFs!
One reason for redundant advice may be that having an SQL index with larger
logical page size (64K vs LF's 8k) might bias query optimizer toward using
that index. This is especially helpful in situations where index is being
scanned sequentially (i.e. read-by-key, read-next-key, next....). Both
native I/O program can do this as well as query engine.
This is by no means the only "quirk" query optimizer has. I'm sure you'll
run into more as you dig deeper into it.
Celebrating 11-Years of SQL Performance Excellence on IBM i5/OS and OS/400
Subject: Using the Index Advisor
I was reviewing the Index Advisor in iSeries Navigator today and see entries
advising me to create multiple indexes that I confirmed already exist via a
logical file. The logical file has existed for years in some cases yet the
advisor shows today as the last advised for query use date. Am I reading
this wrong or does the Index Advisor not care about DDS indexes?
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