Diagnosing problems with expressions

• expressions that are available in the Fast Path Online Analyzer/EP online data extract function
• expressions that are available in Fast Path Reorg/EP DEDB change, unload, and reload functions

The operators and operands that are used within a script or an expression are converted by the command language parser into a sequence of small operational steps. These operational steps are encoded into binary codes (called pseudo-code ). This process is similar to the process that might be used by a programming language compiler to convert the source language into machine language. The generated pseudo-instructions are conceptually similar to actual machine instructions that might be executed by a CPU.

During execution of the product, the pseudo-code that is generated by the parser is executed (interpreted) as required. As the pseudo-code is executed, various conditions are tested to detect problems to ensure that invalid results are not generated. When one of these tests fails, an exception report is generated, and the command function terminates. The exception report is preceded by the following message:

BMC110300C <dbdname> <areaname> : Exception code <code (name)> occurred during processing of <keyword> expression coded for the <command name> command on row <row number> , column <column number>

The message contains the specific reason for the failure, identified by the exception code and its corresponding name.

The following table lists the exceptions that might be produced. The most common exceptions encountered pertain to the problems with data values (Data, Overflow, Divide).

Message BMC110300C also identifies the failing script or expression by the keyword that is used and the position of the command containing that keyword. The remainder of the exception report provides detailed information needed for diagnosing the exact problem.

The contents of the exception report will vary, depending on the nature of the error. When possible, the product provides a specific diagnosis of the problem in a short summary report, such as the the following example.

This exception report identifies the pseudo-instruction that was being processed when the error was detected. In the preceding example, an error occurred at offset 006C while executing a LOAD instruction for the data field coded as CUSTROOT.26:4P (a 4-byte packed field starting in column 26 of the root segment). Apparently, a root segment has been read that does not contain a valid packed value in this field. This is followed by a snap dump showing the contents of the segment being processed and the concatenated key of the target segment. In this example, the target segment for the expression is CURRSEGM, even though the error occurred in a field from the root segment.

For other exceptions, the product produces a complete exception analysis, such as the following example. In this example, a decimal divide exception has occurred during processing of the FIELDS keyword from some OUTPUT subcommand (not shown). This exception is caused when an attempt is made to divide by zero.

This exception report identifies the offset of the pseudo-instruction that was being processed when the error was detected, followed by a detailed listing of all pseudo-code that was generated for the script or expression in error. While it is not necessary to understand the specific details of the pseudo-code to diagnose the problem, being able to identify the operation involved with the error might provide all required information. The example indicates that the error occurred at offset 00A2. Looking at the pseudo-code listing, you find that a DIV instruction was being processed. This is preceded by LOAD instructions for the dividend (006C), the divisor (007E), and a CONV(-1) instruction that modifies the size of the dividend in preparation for the division. The divisor is the data field coded as 18:8P (an 8-byte packed field starting in column 18 of the default segment). Apparently, in one of the input segments selected this field contains a zero value.

The exception report lists the concatenated key of the current segment (CURRSEGM), with the I/O area for that segment. The report indicates that the 8-byte field that is used as the divisor (at offset X‘0011’) in this segment has a zero value.

The exception report shows the current contents of the working storage used in script or expression evaluation (if any). It also shows the current contents of the output area being generated (if any); this would not be present for a WHERE expression because it does not produce an output area.

The exception report shows the current I/O area for all other database segments used in the expression. In the example, no database segments other than the target segment are referenced.

The exception report snaps the contents of the run-time expression environment block(#EXE). The content of this storage might be useful in diagnosing certain types of exceptions but is not needed in this example.