How to diagnose and fix the HV091 fdw_invalid_descriptor_field_identifier error code in Postgres.

by

The HV091 error code in PostgreSQL is related to a foreign data wrapper (FDW) and indicates that there is an invalid descriptor field identifier. This error typically occurs when an FDW function is trying to access a field of a tuple descriptor that does not exist or is otherwise invalid. The tuple descriptor is a data structure that describes the layout of a database row (a tuple).

Diagnosing and fixing this issue generally involves reviewing the FDW code to ensure that all references to tuple descriptor fields are correct and that the fields being accessed are valid for the given context.

Here are some examples and sample code to explain and address the possibilities that might trigger the HV091 error code:

Example 1: Incorrect Field Name

If the FDW code attempts to access a field in a tuple using an incorrect field name, it may raise the HV091 error.

// Hypothetical C function within an FDW implementation
Datum my_fdw_function(PG_FUNCTION_ARGS)
{
    // ... other code ...

    // Attempt to access a non-existent field in the tuple descriptor
    Datum field_value = GetAttributeByName(tuple, "non_existent_field", &isnull);

    // ... other code ...
}

To fix this, ensure that the field name is spelled correctly and exists in the tuple descriptor:

// Corrected field access
Datum field_value = GetAttributeByName(tuple, "existing_field", &isnull);

Example 2: Incorrect Field Index

An FDW might also use a numeric index to access fields in a tuple. If the index is out of bounds or otherwise incorrect, this can result in the HV091 error.

// Hypothetical C function within an FDW implementation
Datum my_fdw_function(PG_FUNCTION_ARGS)
{
    // ... other code ...

    // Attempt to access a field with an invalid index
    Datum field_value = GetAttributeByNum(tuple, invalid_index, &isnull);

    // ... other code ...
}

To resolve this, ensure that the index is within the valid range for the tuple descriptor:

// Corrected field access with a valid index
Datum field_value = GetAttributeByNum(tuple, valid_index, &isnull);

Example 3: Schema Changes

Sometimes, the underlying schema of a foreign table might change (e.g., a column is removed or renamed), and the FDW code may not have been updated to reflect these changes, leading to the HV091 error.

To diagnose this, check if there have been recent schema changes to the foreign data source. If so, update the FDW code or mapping to align with the new schema.

-- Correct the foreign table definition to match the updated schema
ALTER FOREIGN TABLE my_foreign_table DROP COLUMN old_column;
ALTER FOREIGN TABLE my_foreign_table ADD COLUMN new_column data_type;

When dealing with HV091 errors, reviewing the FDW’s implementation code and the foreign table’s schema is crucial. If you are using a third-party FDW, consult the FDW’s documentation for guidance on field access and ensure that you are using the most up-to-date version of the FDW that is compatible with your foreign data source’s schema.

Remember, the key to resolving HV091 errors is to ensure that all field accesses within the FDW are valid and correctly reference existing fields in the tuple descriptor. If the issue persists after these checks, consider reaching out to the community or support resources for the specific FDW for further assistance.

Leave a Comment