fdwReason

fdwReason is a term used in the error reporting mechanisms of foreign data wrappers (FDWs) in database systems, most prominently PostgreSQL. It refers to a descriptive reason string that accompanies an error raised during a foreign-data operation, providing additional context beyond the standard error code.

In the FDW interface, when an operation involving a remote server fails, the FDW can supply a reason string that describes the problem. Common themes include connectivity issues, authentication failures, permission problems, data type or schema mismatches, and missing remote objects. The fdwReason is intended to complement the SQLSTATE code by offering more specific guidance to administrators and client applications.

The exact form and accessibility of fdwReason vary by implementation. Some FDWs expose the reason in error detail fields that clients can read through APIs or client libraries, while others may record it mainly in server logs. Because it is typically a concise, human-readable phrase, it is designed for quick interpretation rather than for exhaustive machine parsing; it should be considered alongside the primary error code and any detailed message.

Examples and applicability: The postgres_fdw extension, as a widely used FDW in PostgreSQL, demonstrates how FDWs propagate remote error information, including user-facing reasons that help diagnose issues such as network timeouts, authentication failures, or missing remote tables. Other FDWs may define their own set of standard reasons or pass through messages from the remote system.

See also: Foreign data wrapper, PostgreSQL, FDW error reporting.