> but I would like to know/monitor how much memory matviews are using
Unfortunately we don't currently provide that. It's also somewhat complicated given the current design. It would be a mixture of heap estimates and actual byte storage sizes. Specifically as it relates to page usage of heap memory, that is mostly determined by the row count.
> to prevent excessive usage or adapt the "heap" to needed usage.
The heap is only an issue for an extremely large number of pages. The data itself is paged to disk as determined by the buffer manager settings.
> Or, just to know. What/where/when should I check to get this (an maybe others) system status info in my wildfly/teiid?
engine-statistics is exposed as an admin operation that shows a variety of stats on the buffermanager, but it doesn't specifically show the number of pages/batches held in memory. That could be added with an enhancement request.
> And what counts here as available "heap"
Available heap would be the heap size beyond the base footprint of Teiid/WildFly and any of the other deployments.