Issue Description

In the context of utilizing Foreign Data Wrappers (FDW) in PostgreSQL, there's a unique requirement to specify certain columns as mandatory in every SQL query, as well as to restrict specific columns to a subset of SQL operators. This enhancement request aims to address these needs by proposing the addition of two new fields to the ColumnDescription class: is_required and operators.

Proposed Fields

1. is_required: bool - This field indicates whether a specific field must always be included in an SQL statement. This is particularly relevant for ensuring compliance with certain constraints when working with FDWs in PostgreSQL.

2. operators: list[str] - This field specifies the subset of operators that are permissible for a given column. For example, a role_id column might only accept the equality operator (=). This flexibility is crucial for accurately reflecting the capabilities and constraints of underlying data sources accessed via FDWs.

Use Case

Consider a table wrapped by an FDW where certain columns are required for query execution due to the remote data source's constraints, or where specific columns only support a limited range of operations. The proposed enhancements would allow for more granular control over query construction, ensuring that generated SQL queries are both valid and optimized for execution against such data sources.

Proposed Implementation

The following outlines a preliminary approach to implementing these enhancements:

• Modify the ColumnDescription class to include is_required and operators fields, ensuring that these fields are appropriately serialized/deserialized and included in any relevant data structures or APIs.

• Update the SQL query generation logic to:

  • Ensure that columns marked as is_required are always included in SELECT queries.
  • Restrict the use of operators for each column to those specified in the operators list, particularly when constructing WHERE clauses or other conditional expressions.

Potential Challenges and Considerations

• Compatibility: Care must be taken to ensure that these enhancements do not introduce breaking changes for existing users of the library.
• Validation: Implementing robust validation for the operators field to ensure only valid SQL operators are included.
• Performance: Assessing the impact of these changes on query generation performance, especially in complex queries involving multiple constrained columns.

Request for Feedback

Feedback on the proposed enhancements, including any potential issues, alternative approaches, or considerations that might improve the implementation and utility of these new fields, is highly appreciated. Insights from developers with experience in PostgreSQL FDWs or similar technologies would be particularly valuable.