Slow parallel query on Oracle

[Naoyuki]

I'm sure it's on the same physical disk.
The data is in cache and there is no physical access anyway.

[Stan_17]

hi,

foremost, parallel slaves do direct reads to fetch data, it doesn't rely on buffer cached data. now, you say there is no phyiscal io.

do this and attach the trace file it generates.

alter session set events '10046 trace name context forever, level 8';

run your parallel query;

alter session set events '10046 trace name context off';

you can find the trace file under udump directory.

This is should help to clear things once for all.