Which two are true about Smart Scan?
A. a query rewrite may occur to a container table stored in Exadata but will never benefit From Smart scan.
B. Column projection does not contribute to the performance benefit of Smart Scan
C. It is possible to offload single row functions to the storage servers.
D. Some joins can be offloaded to the storage servers.
E. A query rewrite may occur to a container table stored Exadata, and it will always benefit from Smart Scan.
F. All joins can be offloaded to the storage servers.
Correct Answer: CD
Explanation/Reference:
C: With Exadata storage, database operations are handled much more efficiently. Queries that perform table scans can be processed within Exadata storage with only the required subset of data returned to the database server. Row filtering, column filtering and some join processing (among other functions) are performed within the Exadata storage cells. When this takes place only the relevant and required data is returned to the database server.
D (not F):
* Exadata performs joins between large tables and small lookup tables, a very common scenario for data warehouses with star schemas. Joining large tables and small lookup tables is implemented using Bloom Filters, which are a very efficient probabilistic method to determine whether a row is a member of the desired result set.
* If storage indexes are so great, why doesn’t Oracle Exadata use them all the time? The short answer is that they are created and used only when they will be beneficial.
* To use storage indexes, Oracle Exadata queries must use smart scans, so not all types of applications can benefit from storage indexes. Applications with queries that include predicates and perform a lot of full table scans or fast full scans of indexes—typically those used in data warehousing environments—will benefit greatly from storage indexes. Online transaction processing (OLTP) applications, on the other hand, typically access a small number of rows through standard indexes and do not perform full table scans, so they may not benefit from storage indexes.
Note:
* Storage indexes reside in the memory of the storage servers—also called storage cells—and significantly reduce unnecessary I/O by excluding irrelevant database blocks in the storage cells.
* To use storage indexes, Oracle Exadata queries must use smart scans, so not all types of applications can benefit from storage indexes.
Incorrect:
Not B: Exadata provides column filtering, also called column projection, for table scans. Only the columns requested are returned to the database server rather than all columns in a table. For example, when the following SQL is issued, only the employee_name and employee_number columns are returned from Exadata to the database kernel.
SELECT employee_name, employee_number FROM employee_table.
For tables with many columns, or columns containing LOBs (Large Objects), the I/O bandwidth saved can be very large. Using both predicate and column filtering dramatically improves performance and reduces I/O bandwidth consumption. In addition, column filtering also applies to indexes, allowing for even faster query performance.
Reference: Oracle Communications Data Model Implementation and Operations Guide, Exadata Smart Scan Processing and Storage Index
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