воскресенье, 2 апреля 2017 г.

Полезное по подсчетам Logical reads

Прочитал у Рендольфа

So here is an important point: If you want to understand the work Oracle has performed in terms of buffer visits you need to consider both, the number of logical I/Os as well as the number of buffers visited without involving logical I/O - this is represented by the “buffer is pinned count” statistics.
Quite often this fact is overlooked and people only focus on the logical I/Os - which is not unreasonable - but misses the point about pinned buffers re-visited without doing logical I/O.
Note that buffer pinning is not possible across fetch calls - if the control is returned to the client the buffers will no longer be kept pinned. This is the explanation why a the “fetchsize” or “arraysize” for bulk fetches can influence the number of logical I/Os required to process a result set.

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