Trait ForeignDataWrapper 

pub trait ForeignDataWrapper: Send + Sync {
    // Required methods
    fn kind(&self) -> &'static str;
    fn scan(
        &self,
        server_state: Option<&Arc<dyn WrapperState>>,
        table_options: &FdwOptions,
    ) -> Result<Vec<UnifiedRecord>, FdwError>;

    // Provided methods
    fn build_server_state(
        &self,
        _options: &FdwOptions,
    ) -> Result<Option<Arc<dyn WrapperState>>, FdwError> { ... }
    fn supports_pushdown(&self) -> bool { ... }
    fn estimated_row_count(
        &self,
        _server_state: Option<&Arc<dyn WrapperState>>,
        _table_options: &FdwOptions,
    ) -> Option<usize> { ... }
}

Core trait every FDW implements.

Phase 3.2 is read-only — scan is mandatory; insert / delete are defaulted to “not supported” so wrappers can opt-in. Filter pushdown is also opt-in via supports_pushdown; the runtime checks that before handing the wrapper the predicate AST.

Required Methods

fn kind(&self) -> &'static str

Unique identifier for the wrapper kind (e.g. “csv”, “postgres”). Matches the FOREIGN DATA WRAPPER <name> clause in DDL.

fn scan( &self, server_state: Option<&Arc<dyn WrapperState>>, table_options: &FdwOptions, ) -> Result<Vec<UnifiedRecord>, FdwError>

Stream rows from the foreign table. table_options merges the server’s options with the table’s options (table takes priority on conflicts). filter is opaque to the wrapper unless it advertises pushdown via supports_pushdown.

Provided Methods

fn build_server_state( &self, _options: &FdwOptions, ) -> Result<Option<Arc<dyn WrapperState>>, FdwError>

Validate + materialise a server’s options. Called once on CREATE SERVER and cached on ForeignServer. Wrappers that don’t need per-server state return Ok(None).

fn supports_pushdown(&self) -> bool

Whether the wrapper can evaluate a SQL WHERE predicate natively. When true, the planner will hand the AST to scan_with_filter; when false, the runtime applies the filter after the scan.

fn estimated_row_count( &self, _server_state: Option<&Arc<dyn WrapperState>>, _table_options: &FdwOptions, ) -> Option<usize>

Estimated row count — drives planner cost. None means the wrapper has no cheap way to estimate and the planner falls back to its default assumption.

Implementors

impl ForeignDataWrapper for CsvForeignWrapper