AMPS expressions are designed to work exactly as expected if you are familiar with XPath path specifiers and SQL-92 predicates. This section describes in detail how AMPS evaluates the syntax, operators, and functions available in the AMPS expression language.

AMPS expressions combine the following elements:

  • Identifiers specify a field in a message. When evaluating an expression, AMPS replaces identifiers with values from the message or set of messages being evaluated.

  • Literal values are explicit values in an AMPS expression, such as 'IBM' or 42.

  • Operators and functions such as =, <, >, *, and UNIX_TIMESTAMP().

Every AMPS expression produces a value. The way that AMPS uses the value depends on the context in which AMPS evaluates the expression. For example, if the expression is used for a filter, the message is considered to match the filter when the expression returns true. When an expression is used to project a field, the result of the expression is used as the value of the projected field.

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