Syntax of COMPUTE_EXPR in target type metadata

Question

Anyone knows what's the syntax of that expression? I can do basic stuff there like calculating rate from cumulative counters. I need, however, to have something like DECODE or CASE to overcome huge negative values in case of counter overflow.
If I have transient column COL I put formula like:
(COL - _COL) / __intervalIf that expression is negative I want it it to be 0 or better yet empty (NULL).

Answer 1

COMPUTE_EXPR: This attribute specifies the formula for calculating the value of the column. Columns previously defined in the Table descriptor can participate in the calculation. Attaching a ‘_’ prefix to a column name denotes previous value of a column. Support for string expressions is introduced in version 10.2. Please refer to the Examples for details about the expression grammar and usage.
Predefined special values:
a) __interval: collect interval.
b) __sysdate: current system time.
c) __GMTdate: current GMT time.
d) __contains: tests a given string expression for presence of a string expression.
e) __beginswith: tests whether a given string expression begins with a specified string expression.
f) __endswith: testw whether a given string expression ends with the specified string expression.
g) __matches: tests whether a given string expression matches a specified string expression.
h) __delta: computes the difference between the current value and the previous value.
i) __leadingchars: returns the leading characters in the specified string.
j) __trailingchars: returns the trailing characters in the specified string.
k) __substringpos: returns the position of the occurrence of the pattern within a specified string.
l) __is_null: tests whether the expression is NULL
m) __length: returns the length of the string expression.
n) __to_upper: converts the string to upper case.
o) __to_lower: converst the string to lower_case.
p) __ceil: returns the smallest integral value not less than identifier.
q) __floor: returns the largest integral value not greater than the identifier.
r) __round: rounds to nearest integer, away from zero.
Compute Expression support:
Supported Grammar:
expression := (cond_expr | (cond_expr ? cond_expr : cond_expr)
cond_expr := (string_expr |
(string_expr == string_expr) |
(string_expr < string_expr) |
(string_expr > string_expr) |
(string_expr <= string_expr) |
(string_expr >= string_expr) |
(string_expr __contains string_expr) |
(string_expr __beginswith string_expr) |
(string_expr __endswith string_expr) |
(string_expr __matches string_expr) |
(string_expr __delta string_expr))
string_expr := (simple_expr |
(simple_expr __leadingchars simple_expr) |
(simple_expr __trailingchars simple_expr) |
(simple_expr __substringpos simple_expr))
simple_expr := (term |
(simple_expr + term ) |
(simple_expr - term) )
term := (unary_expr |
(term * unary_expr ) |
(term / unary_expr ) )
unary_expr := (factor |
(__is_null factor) |
(__length factor) |
(__to_upper factor) |
(__to_lower factor) |
(__ceil factor) |
(__floor factor) |
(__round factor) )
factor := ( identifier |
string_literal |
number |
'(' expression ')' )
string_literal := '\'' (character | "\\'" )* '\''