Wednesday, January 4, 2017

12.2: Avoid hard-coding maximum length of VARCHAR2 (and more)

Starting with Oracle Database 12c Release 2 (12.2), we can now use static expressions* where previously only literal constants were allowed. Here are some examples (also available in this LiveSQL script):

   c_max_length constant integer := 32767;
   SUBTYPE maxvarchar2 IS VARCHAR2 (c_max_length);

   l_big_string1 VARCHAR2 (pkg.c_max_length) := 'So big....';
   l_big_String2 pkg.maxvarchar2 := 'So big via packaged subtype....';
   l_half_big VARCHAR2 (pkg.c_max_length / 2) := 'So big....';
   DBMS_OUTPUT.PUT_LINE (l_big_string1);
   DBMS_OUTPUT.PUT_LINE (l_big_string2);

As you can see from this code, static expressions can now be used in subtype declarations.

The definition of static expressions is expanded to include all the PL/SQL scalar types and a much wider range of operators. Character operands are restricted to a safe subset of the ASCII character set. Operators whose results depend on any implicit NLS parameter are disallowed

Expanded and generalized expressions have two primary benefits for PL/SQL developers:
  • Programs are much more adaptable to changes in their environment
  • Programs are more compact, clearer, and substantially easier to understand and maintain
* A static expression is an expression whose value can be determined at compile time. This means the expression cannot include character comparisons, variables, or function invocations. An expression is static if it is any of the following:
  • the NULL literal
  • a character, numeric, or boolean literal
  • a reference to a static constant
  • a reference to a conditional compilation variable begun with $$ 
  • an operator is allowed in static expressions, if all of its operands are static, and if the operator does not raise an exception when it is evaluated on those operands
Read more in the doc

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