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Oracle hardly ever executes your query in its original form. If the CBO determines that a different SQL formulation will achieve the same results more efficiently, it transforms the statement before executing it. A good example is where you submit a query with an OR condition, and the CBO transforms it into a statement using UNION or UNION ALL. Or your statement may include an index, but the CBO might transform the statement so it can do a full table scan, which can be more efficient under some circumstances. In any case, it s good to remember that the query a user wishes to be executed may not be executed in the same form by Oracle, but the query s results are still the same. Here are some common transformations performed by the Oracle CBO: Transform IN into OR statements. Transform OR into UNION or UNION ALL statements. Transform noncorrelated nested selects into more efficient joins. Transform outer joins into more efficient inner joins. Transform complex subqueries into joins, semi joins, and anti joins. Star transformation for data warehouse tables based on the star schema. Transform BETWEEN to greater than or equal to and less than or equal to statements.

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Oracle can often access the same data through different paths. For each query, the Optimizer evaluates all the available paths and picks the least expensive one in terms of resource usage. The following sections present a summary of the common access methods available to the Optimizer. If joins are involved, then the join order and the join method are evaluated to finally arrive at the best execution plan. You ll take a brief look at the steps the Optimizer goes through before deciding on its choice of execution path.

Oracle scans the entire table during a full table scan. Oracle reads each block in the table sequentially, so the full table scan can be efficient if the MULTI_BLOCK_READ_COUNT initialization parameter is set high enough. However, for large tables, full table scans are inefficient in general.

Here is a method of extracting a substring using expr. It uses the substr() function of expr. As before, the first argument is the string, the second is the position of the desired substring s starting character, and the last is the number of characters in the substring. The example gets 4 characters from the string stored in VAR, starting at character number 12.

Accessing a table by ROWID retrieves rows using unique ROWIDs. ROWIDs in Oracle specify the exact location in the data file and the data block where the row resides, so ROWID access is the fastest way to retrieve a row in Oracle. Often, Oracle obtains the ROWID through an index scan of the table s indexes. Using these ROWIDs, Oracle swiftly fetches the rows.

As member functions of managed types are often called methods, data members of managed types are often called fields. For most fields, you will likely use exactly the same syntax as for data members in native C++ classes. However, there are additional options for defining fields with read-only semantics. In C++/CLI, you can declare a non-static field as initonly. This allows you to initialize the field in either the member initialization list of the constructor or the constructor s body, but not outside. An initonly field is language interoperable. Other languages also have support for this construct. As an example, the equivalent C# keyword is readonly. Furthermore, the read-only character of an initonly field can be ensured not only by the compiler, but also by the JIT compiler. C++/CLI and most other .NET languages return compiler errors if you try to modify an initonly field out of the constructor s code. However, using tools like the IL Assembler (ILASM.EXE) or the System::Reflection::Emit API, you can easily bypass this compiler-level safety and generate code that tries to modify an initonly field. Even if you do this, the JIT compiler can detect that illegal attempt and throw a System::Security::VerificationException instead of blindly executing this illegal operation. This aspect can be important in the context of sandboxed execution. For static fields, there are two read-only concepts. These are literal fields and static initonly fields. A typical usage of these managed concepts would be with the value PI from the MathConstants class shown earlier.

An index stores two things: the column value of the column on which the index is based and the ROWID of the rows in the table that contain that column value. An index scan retrieves data from an index using the values of the index columns. If the query requests only the indexed column values, Oracle will return those values. If the query requests other columns outside the indexed column, Oracle will use the ROWIDs to get the rows of the table.

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