> **Custom & Foreign Scans** > <table> > <tr> > <td width="25%"><img src="assets/ex_customscanforeignscan.png"></td> > <td>Scans external data sources (Foreign Data Wrappers) or executes custom extension logic.</td> > </tr> > </table> > > ```sql > -- A self-contained file_fdw example using a tiny generated CSV stream > CREATE EXTENSION IF NOT EXISTS file_fdw; > CREATE SERVER lyap_local_files FOREIGN DATA WRAPPER file_fdw; > CREATE FOREIGN TABLE external_logs (id integer, message text) > SERVER lyap_local_files > OPTIONS ( > program 'printf ''1,started\n2,ready\n''', > format 'csv' > ); > > EXPLAIN (ANALYZE, COSTS, BUFFERS, VERBOSE) > SELECT * FROM external_logs; > ``` > >  > > <!-- literal-explain-plan > Captured EXPLAIN provenance for the adjacent reader-facing visual plan. > Canonical capture metadata lives in scratch/actual_operation_plans.json. > > Foreign Scan on public.external_logs (cost=0.00..138.00 rows=1280 width=36) (actual time=4.577..4.578 rows=2 loops=1) > Output: id, message > Foreign Program: printf '1,started\n2,ready\n' > Planning: > Buffers: shared hit=29 > Planning Time: 0.054 ms > Execution Time: 4.873 ms > --> > > > <!-- > Raw-capture provenance — separate run. > SQL, setup, dataset, settings, and scope: artifacts/chapter4_capture_matrix.json. > Target: PostgreSQL 18.x companion fixture. Cache state: uncontrolled. > Boundary: pg_wait_tracer backend execution root; client states are included when the chart shows them. > Fidelity: exact pg_wait_tracer export. Not the adjacent EXPLAIN run; compare state shape, not durations. > --> > >  > > <table> > <tr> > <td rowspan="2" width="25%"><img src="assets/ex_foreign_scan.svg"></td> > <td><b>Performance</b></td><td>Entirely dependent on the external source's latency and the efficiency of the Foreign Data Wrapper (FDW) or custom extension.</td> > </tr> > <tr><td><b>Cost</b></td><td>Defined by the FDW's custom cost model.</td></tr> > </table>