A few comments. First it looks like your tkprof is suggesting that you read 43m rows to return 700k or so. I'd therefore be quite interested in seeing what the execution plan was, 50 reads per row isn't horrible, but imagine what might happen if it could be 1 read per row or fewer. Niall Litchfield On Jun 14, 2010 1:41 PM, "Oracle Dba Wannabe" <oracledbawannabe@xxxxxxxxx> wrote: Hi All, Thanks for the responses. The report itself is actually a problem - to resolve it I understand I need to address the number of i./o's issued by it. That said, I posed this question because I just wanted to try and understand how oracle issues reads. The reason the naming of the wait events confused me - was I've also understood them to mean physical reads. What didn't make sense to me was why after the report completed and I submitted it again there were still wait events for reads issued in awr - I would have thought the data was cached. I had traced each run of the session, but had just not looked into the trace file yet (my mistake) and was only looking at the awr file. Looking at the trace I see the following: call count cpu elapsed disk query current rows ------- ------ -------- ---------- ---------- ---------- ---------- ---------- Parse 1 0.00 0.00 0 0 0 0 Execute 1 0.00 0.00 0 0 0 0 Fetch 90966 2752.59 2690.45 0 43989168 0 727721 ------- ------ -------- ---------- ---------- ---------- ---------- ---------- total 90968 2752.59 2690.46 0 43989168 0 727721 Which appears to indicate zero disk reads. However AWR, shows the following: Event Waits Time(s) Avg Wait(ms) % Total Call Time Wait Class db file sequential read 1,191,745 1,368 1 45.3 User I/O There's nothing else running on this system other than this SQL (it was a weekend afterall). So although my trace file shows no physical read events - how do I account for the 1.1 million wait events - The load profile on awr looks like this: Per Second Per Transaction Logical reads: 15,445.20 19,592.96 Block changes: 38.26 48.54 Physical reads: 658.30 835.08 So are the db file sequential reads a result of waits from reads at the storage cache? the file system cache (have filemin_cache at 3% and filemax_cache 10% total memory =120GB), the oracle buffer cache or all three? Or is it just the first 2 and does not include the buffer cache requests as a call for data in the buffer cache does not translate to a read wait event? A bit more detail on this system: The server is a 2 Node RAC (10.2.0.4) on HP-UX 11.31, Super Dome, the storage is XP 24k, the db cache size is approx 60GB on each node. Thanks once again. ------------------------------ *From:* Greg Rahn <greg@xxxxxxxxxxxxxxxxxx> *To:* oracledbawannabe@xxxxxxxxx *Cc:* oracle-l@xxxxxxxxxxxxx *Sent:* Mon, June 14, 2010 11:23:24 AM *Subject:* Re: db file sequential/scattered read - physical or logical io or both? When you run the report via SQL*Plus with autotrace on, (or look at the session stats) what is the ...