preserve ordinality in postgres when checking existence of documents (#5533)

Author: arrested-developer

libs/langchain-community/src/indexes/postgres.ts

@@ -136,7 +136,15 @@ export class PostgresRecordManager implements RecordManagerInterface {
       .join(", ");
 
     const query = `
-      SELECT k, (key is not null) ex from unnest(ARRAY[${arrayPlaceholders}]) k left join ${this.finalTableName} on k=key and namespace = $1;
+      WITH ordered_keys AS (
+        SELECT * FROM unnest(ARRAY[${arrayPlaceholders}]) WITH ORDINALITY as t(key, o)
+      )
+      SELECT ok.key, (r.key IS NOT NULL) ex
+      FROM ordered_keys ok 
+      LEFT JOIN ${this.finalTableName} r 
+      ON r.key = ok.key 
+      AND namespace = $1
+      ORDER BY ok.o;
       `;
     const res = await this.pool.query(query, [this.namespace, ...keys.flat()]);
     return res.rows.map((row: { ex: boolean }) => row.ex);