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1. Although these access paths can be changed at a later date, this can usually be achieved only with considerable difficulty.
2. However, for this example a different access path is selected, which is shown in Figure 7.
3. Access plan describes the access path that the DB2 optimizer has chosen for accessing data for an SQL statement.
4. DAFS can make it possible to shorten data access path effectively through RDMA technology, make data access faster and improve the access speed performance in NAS appliance.
5. In some cases, the access path does not perform well at runtime if the statement contains host variables, parameter markers, or special registers.
6. This type of access path is referred to as a nested loop join (NLJOIN) with sort composite.
7. Having the DBMS determine the optimal access path to the data lifts a heavy burden off the programmer's shoulders.
8. So, any interruption to one of the access path does not affect storage access from the host.
9. Before access path selection, the DB2 optimizer transforms SQL statements into forms that are semantically identical, such as by using predicate pushdown or predicate transitive closure.
10. Without column data-value distribution information in the catalog, DB2 will assume an even distribution of values, which may lead the optimizer to a sub-optimal access path.
11. In addition, the database can have a better understanding of the state of the data it stores, and thereby can produce a more efficient and dynamic access path to the data.
12. Without correct column distribution statistics, DB2 may not have enough information to make the best choice when choosing an access path to the data—especially if data is heavily skewed.
13. This tells me that I may have an inefficient access path, where DB2 reads more data than it should.
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14. For SQL statements with variables, the optimizer uses a default filter factor to determine the best access path at bind time.
15. If the SQL statement is badly coded, it is difficult for the optimizer to transform the query, leaving fewer options available for choosing an optimal access path.
16. When you and DB2 agree If you and DB2 agree on the best available access path but that path does not deliver satisfactory performance, you need to provide DB2 with a new access path choice.
17. For simplicity, this article refers to a particular way of satisfying an SQL statement as the access path.
18. Note that we categorically argue against setting up multiple SPNs so that SSO access to the application will work, regardless of the access path.
19. You can obtain the statistics from the table descriptor of the table node in the underlying access path graph.
20. Without the FETCH FIRST (or similarly, but not exactly, without an OPTIMIZE FOR clause), then DB2 would almost certainly prefer some other access path.
21. WebSphere Federation Server uses its cost based optimizer in order to determine the optimal access path to retrieve and process the data.
22. The description of each operation includes a Query Tuner access path graph that explains its operational semantics.
23. IBM does not publish the actual, in-depth details of how the optimizer determines the best access path, but the optimizer is a cost-based optimizer.
24. Although the DB2 optimizer is helpful for selecting the best access path, the result depends on the input data, which is often not accessed or controlled by the optimizer.
25. Figure 2 shows an example of how you can display detailed descriptions of two nodes (the CUST_ORDER_HEADER table and the table scan (TBSCAN) operation) from the access path graph.
26. There is no FETCH or table node shown in the access path graph because it is an index-only scan.
27. However, when new statistics are applied, DB2 will now realize data may not be clustered as previously thought, therefore causing a change to an access path that is less desirable.
28. The universal relation model aims at achieving the complete access path independence.
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