Guidelines for Avoiding PL/SQL Performance Problems

the use of more efficient hardware arithmetic. See "Use PLS_INTEGER for Integer Arithmetic" on page 11-6.

Use PLS_INTEGER for Integer Arithmetic

When you need to declare a local integer variable, use the datatype PLS_INTEGER, which is the most efficient integer type. PLS_INTEGER values require less storage than INTEGER or NUMBER values, and PLS_INTEGER operations use hardware arithmetic. Note that the BINARY_INTEGER datatype is identical to PLS_INTEGER.

The datatype NUMBER and its subtypes are represented in a special internal format, designed for portability and arbitrary scale and precision, not performance. Even the subtype INTEGER is treated as a floating-point number with nothing after the decimal point. Operations on NUMBER or INTEGER variables require calls to library routines.

Avoid constrained subtypes such as INTEGER, NATURAL, NATURALN, POSITIVE, POSITIVEN, and SIGNTYPE in performance-critical code. Variables of these types require extra checking at run time, each time they are used in a calculation.

Use BINARY_FLOAT and BINARY_DOUBLE for Floating-Point Arithmetic

The datatype NUMBER and its subtypes are represented in a special internal format, designed for portability and arbitrary scale and precision, not performance. Operations on NUMBER or INTEGER variables require calls to library routines.

The BINARY_FLOAT and BINARY_DOUBLE types can use native hardware arithmetic instructions, and are more efficient for number-crunching applications such as scientific processing. They also require less space in the database.

These types do not always represent fractional values precisely, and handle rounding differently than the NUMBER types. These types are less suitable for financial code where accuracy is critical.

Avoiding Memory Overhead in PL/SQL Code

This sections discusses how you can avoid excessive memory overhead in the PL/SQL code.

Be Generous When Declaring Sizes for VARCHAR2 Variables

You might need to allocate large VARCHAR2 variables when you are not sure how big an expression result will be. You can actually conserve memory by declaring VARCHAR2 variables with large sizes, such as 32000, rather than estimating just a little on the high side, such as by specifying 256 or 1000. PL/SQL has an optimization that makes it easy to avoid overflow problems and still conserve memory. Specify a size of more than 4000 characters for the VARCHAR2 variable; PL/SQL waits until you assign the variable, then only allocates as much storage as needed.

Group Related Subprograms into Packages

When you call a packaged subprogram for the first time, the whole package is loaded into the shared memory pool. Subsequent calls to related subprograms in the package require no disk I/O, and your code executes faster. If the package is aged out of memory, it must be reloaded if you reference it again.

You can improve performance by sizing the shared memory pool correctly. Make sure it is large enough to hold all frequently used packages but not so large that memory is wasted.

11-6 Oracle Database PL/SQL User’s Guide and Reference