cell smart table scan等待事件

cell smart table scan

This is an Exadata wait event typically seen during full table scans that have been offloaded to the storage cells. This event replaces waits on “direct path read” in many cases. As with direct path reads, data is returned directly to the PGA rather than going through the buffer cache. When the storage cells process full table scans they can apply columns filters and perform column projection so that not all blocks are returned, only the ones that are needed.

Solutions

This event indicates that a full table scan is being performed. In some cases this could be faster than an index lookup, but is not a replacement for query tuning. If the query will return a small subset of the data, utilizing an index may be more efficient. Test the differences to understand any performance penalties incurred by doing a smart table scan vs. the index lookup.

Also, ensure the smart table scan is being done effectively by reviewing the Cell Smart Table Scan Latency metric on the Exadata tab under Resources in Ignite. The Objects tab in Ignite will also show response time information by object. This is critical for understanding which table is causing the majority of wait times.