In physics, one important limitation of any experiment is the fact that an act of observation inevitably interferes with the observed process (“observer effect”). Same thing is true about databases. It is a well known fact that, for example, turning on tracing can significantly slow down the process for which it’s enabled. But there exist even nastier forms of this effect: for example, when you try to trace a SQL statement using nested loop batching mechanism, apparently, the batching gets disabled! This is extremely confusing, because plan statistics in the trace file indicate that the batching takes place (i.e. the plan shows the “double nested loop” shape characteristic for NLJ batching), and if you look at the plan using dbms_xplan.display_cursor using “outline” option, you’ll see NLJ_BATCHING among other hints.
This effect was discovered by Randolf Geist, and published in the part 3 of his wonderful series on Logical I/O evolution (see here, here and here). It is very thorough and very detailed. Because of the level of detail in there it’s rather lengthy, which is why not everyone gets to that particular paragraph, so let me quote it in here:
…be aware of an odd behaviour I’ve experienced during my tests: If any kind of row source statistics sampling was enabled by either using STATISTICS_LEVEL = ALL, the GATHER_PLAN_STATISTICS hint or even enabling (extended) SQL trace, the optimized, batched form of physical I/O could not be reproduced. You could tell this from the session statistics that start with “Batched IO%” – these all stayed at 0. Only when disabling all these things the effects were visible and the corresponding statistics where non-zero. I don’t know why this is the case, but it is an important detail when testing this feature.
I think it is important that the community be aware of this behavior, because not knowing about it made me waste several days of work, and I know at least one other person who also had difficulties of the same type. But aside from this particular oddity, this is also a reminder of importance of the “observer effect” in general. So any findings obtained with intrusive methods (like extended SQL trace or statistlcs_level = ALL etc.) need to be cross-checked with less intrusive methods (e.g. dumps from V$MYSTAT, V$SESSION_EVENT, V$SESS_TIME_MODEL etc., or SQL real-time monitor reports).
The obvious downside for using V$ views is necessity to take the baseline, but there is a simple workaround if taking a baseline is inconvenient: one can simply establish a new session, thus resetting all counters and statistics to zero.