Re: Exact meaning of TCH in X$BH

Hello Lothar,
the TCH can be way higher ... but ... quoting from Jonathan's book "Oracle Core: Essential Internals": -------8<-----------
Oracle Corp. added a counter (and a timestamp—called tim) to the buffer header, and every time someone visits the buffer, they increment the touch count and update the timestamp—provided that at least 3 seconds has passed since the last update ...
There is a commonly held theory that you can identify which block is causing latch contention on a given cache buffers chains latch by checking for very high touch counts (TCH) on all the buffers covered by that latch. Unfortunately this is not a very sound method. A buffer that is visited an average of once per second for half an hour will have a touch count around 600; a buffer that has been visited 10 million times in the last 5 minutes will have a touch count of around 100. The touch count can give you a clue, but it is not the final answer. ...
-------8<-----------

Hope this explains the behavior you are seeing.

Best Regards
Stefan Koehler

Independent Oracle performance consultant and researcher Website: http://www.soocs.de
Twitter: _at_OracleSK

> Lothar Flatz <l.flatz_at_bluewin.ch> hat am 30.12.2022 11:58 CET geschrieben:
>
>
> Hi guys,
>
> the official definition is : "X$BH.TCH is a touch count for the buffer. A high value for X$BH.TCH indicates a hot block."
> (Source: https://docs.oracle.com/en/database/oracle/oracle-database/19/tgdba/instance-tuning-using-performance-views.html#GUID-64F78817-8B4C-4392-B518-CA31CF728B69).
>
> But the biggest value for tch I have ever seen is 255 (unsigned seven bit integer) . Therefore I think it is not exactly a counter but rather reflects a range.
> Can somebody give a detailed explanation? Does the number wrap around? When?
>
> Thanks
>
> Lothar

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