Table of Contents


  1. Preface
  2. Introduction to PowerExchange
  3. DBMOVER Configuration File
  4. Netport Jobs
  5. PowerExchange Message Logs and Destination Overrides
  6. SMF Statistics Logging and Reporting
  7. PowerExchange Security
  8. Secure Sockets Layer Support
  9. PowerExchange Alternative Network Security
  10. PowerExchange Nonrelational SQL
  11. DTLDESCRIBE Metadata
  12. PowerExchange Globalization
  13. Using the PowerExchange ODBC Drivers
  14. PowerExchange Datatypes and Conversion Matrix
  15. Appendix A: DTL__CAPXTIMESTAMP Time Stamps
  16. Appendix B: PowerExchange Glossary

Candidate Key Processing

Candidate Key Processing

Candidate key processing determines which column conditions can be used for optimized reading, and attaches a list of low and high values to the accepted columns.
The system does not know whether the column is actually part of a key, or if it is the primary column within the key. This information becomes available later. For example, after opening a VSAM KSDS file on z/OS, the offset and length of the key become available depending on whether the default index or an alternate index is used.
If candidate key processing encounters an unacceptable condition for a column, all conditions for the column and the optimization level are also excluded for optimized reading, but they are retained for post-read qualification processing. For a list of unacceptable conditions, see Conditions Skipped by the Optimized Read Process.
If all WHERE clause conditions for a column are accepted, a list of low and high value pairs are created using the "internal form." Numeric values are either 31 digits or 30 digits preceded by a negative sign (-).
Where only one of the boundaries have been set for the LE, LT, GE, or GT conditions, the system inserts the missing low or h condition, typically using binary zeros or X'FF'.


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