access path

The SQL compiler uses them to transform a statement into a more optimized form, thus improving the access path to the required data.

Data flows from the bottom up (as displayed in the graph) and is processed by operation nodes in the access path graph.

As you'll see shortly, DB2 can use both of these indexes to generate an efficient access path for our query.

From a performance point of view, a good performing access path should touch the least amount of data with respect to the final result set.

So, any interruption to one of the access path does not affect storage access from the host.

This type of access path is referred to as a nested loop join (NLJOIN) with sort composite.

Keep in mind that this catalog data is what the optimizer uses to make access path decisions.

Having the DBMS determine the optimal access path to the data lifts a heavy burden off the programmer's shoulders.

An index scan is typically shown in Query Tuner as a group of four nodes in the access path graph.

For simplicity, this article refers to a particular way of satisfying an SQL statement as the access path.

This means the optimizer will always attempt to formulate an access path for each query that reduces overall cost .

Therefore, if the optimizer's estimate is accurate, this access path should be a very efficient access path.

By right clicking on the SQL from the pureQuery outline, you can run Visual Explain to see the access path of the SQL (Figure 13).

This section illustrates how the Query Tuner query annotation and access path graph can help you analyze access paths and SQL performance.

For SQL statements with variables, the optimizer uses a default filter factor to determine the best access path at bind time.

Assuming this to be the case, the underlying access path is considered optimal with high confidence.

The description of each operation includes a Query Tuner access path graph that explains its operational semantics.

In that case, reorganizing the index might lead to its selection as part of a better-performing query access path.

The developer can now improve the SQL (or work with the DBA to do so) and repeat the cycle until the SQL uses a desired access path.

Statistics advisor: recommendations for collecting the required statistics to facilitate access path selection.

Next, the Publishing Wizard prompts you for an alternative access path.

This tells me that I may have an inefficient access path, where DB2 reads more data than it should.

API cursor query contains an indexed view with a unique access path, but not at the base-table level.

This implementation style is useful to remove duplicates and provide (in many cases federated) a consistent access path to master data.

The statistics in the DB2 catalog tables are used to estimate the cost of the access path.

Similarly, the last leg in the access path graph is evaluated eight times, once for each record produced by the first NLJOIN node.

With the appropriate access path, the ORDER BY or GROUP BY requirement can be met without sorting.

The inconsistent statistics may also mislead the optimizer into choosing a bad access path.

By default, this alternative access path is set to the same location that you just specified.

Access plan describes the access path that the DB2 optimizer has chosen for accessing data for an SQL statement.

An access path graph not only describes the "operational details" of query execution, it also describes how the data flows.

To see the table descriptor, right-click the table node in the access path graph.

There is no FETCH or table node shown in the access path graph because it is an index-only scan.

However, for this example a different access path is selected, which is shown in Figure 7.

That simple step could lead to a new access path that might dramatically improve query performance.

As shown in Figure 1, the DB2 optimizer chooses the best access path.

Class with the specified permission access and permission access path.

It analyzes SQL statements and determines the most efficient access path available for satisfying each statement (see Figure 1).

WebSphere Federation Server uses its cost based optimizer in order to determine the optimal access path to retrieve and process the data.

Figure 1 is the access path graph generated by Optim Query Tuner for the query shown in Listing 1.

The SQL statement in Listing 3 creates the access path graph shown in Figure 3.

You can obtain the statistics from the table descriptor of the table node in the underlying access path graph.

An appropriate access path means that an index is available and used so that the index drives the access to the data.

Once you have the EXPLAIN output, look at how the query's result set is generated, and then consider alternative access path possibilities.

Figure 6 is the access path graph generated by Query Tuner for the sample query in this article.

This article described the basic concepts of access paths and how to read an access path graph.

Using an index hint, force the optimizer to use a different access path on the table in question.

Index design has a critical impact on access path selection.

In Figure 2, the descriptor for the table node and the descriptor for the TBSCAN node are shown to the left of the access path graph.

The SQL statement in Listing 2 creates the access path graph shown on the right side of the Optim Query Tuner screenshot in Figure 2.

Then it will either pick the cheapest access path or (as of DB2 10) a slightly more costly but less risky path.

Access path selection: What did DB2 decide, and why?

It does this by performing some work ahead of time, such as determining the database access path.

You can use the REOPT bind options to reoptimize the access path either at bind time or at run time.

The access path shows that a TBSCAN operation is performed over the table CUST_ORDER_HEADER.

IBM does not publish the actual, in-depth details of how the optimizer determines the best access path, but the optimizer is a

the second memory controller is used for accessing and operating the second memory module through a second memory access path;

Intuitively, an access path is a procedural description of query execution that consists of three components

Association rule-based similarity analysis for web access path

Other considerations for access path selection

Create a temporary keyed access path during execution

Research on Web User Access Path's Difference Measurement

A Cost Model for Data Placement and Access Path Selection Problem in FC-SAN

Research on Algorithm of Frequent Web Access Path Mining Based on Access Path Tree

Web access path analysis based on fuzzy clustering

The optimizer compares the cost of each candidate access path based on the information from multiple inputs, for example, see the following