postgres_fdw: Remove redundancy in postgresAcquireSampleRowsFunc().

Previously, in the loop in postgresAcquireSampleRowsFunc() to iterate
fetching rows from a given remote table, we redundantly 1) determined the
fetch size by parsing the table's server/table-level options and then 2)
constructed the fetch command; remove that redundancy.

Author: Etsuro Fujita
Reviewed-by: Julien Rouhaud
Discussion: https://postgr.es/m/CAPmGK17_urk9qkLV65_iYMFw64z5qhdfhY=tMVV6Jg4KNYx8+w@mail.gmail.com

contrib/postgres_fdw/postgres_fdw.c

@@ -4490,31 +4490,20 @@ postgresAcquireSampleRowsFunc(Relation relation, int elevel,
 	/* In what follows, do not risk leaking any PGresults. */
 	PG_TRY();
 	{
+		char		fetch_sql[64];
+		int			fetch_size;
+		ListCell   *lc;
+
 		res = pgfdw_exec_query(conn, sql.data);
 		if (PQresultStatus(res) != PGRES_COMMAND_OK)
 			pgfdw_report_error(ERROR, res, conn, false, sql.data);
 		PQclear(res);
 		res = NULL;
 
-		/* Retrieve and process rows a batch at a time. */
-		for (;;)
-		{
-			char		fetch_sql[64];
-			int			fetch_size;
-			int			numrows;
-			int			i;
-			ListCell   *lc;
-
-			/* Allow users to cancel long query */
-			CHECK_FOR_INTERRUPTS();
-
 		/*
-			 * XXX possible future improvement: if rowstoskip is large, we
-			 * could issue a MOVE rather than physically fetching the rows,
-			 * then just adjust rowstoskip and samplerows appropriately.
+		 * Determine the fetch size.  The default is arbitrary, but shouldn't
+		 * be enormous.
 		 */
-
-			/* The fetch size is arbitrary, but shouldn't be enormous. */
 		fetch_size = 100;
 		foreach(lc, server->options)
 		{
@@ -4537,10 +4526,26 @@ postgresAcquireSampleRowsFunc(Relation relation, int elevel,
 			}
 		}
 
-			/* Fetch some rows */
+		/* Construct command to fetch rows from remote. */
 		snprintf(fetch_sql, sizeof(fetch_sql), "FETCH %d FROM c%u",
 				 fetch_size, cursor_number);
 
+		/* Retrieve and process rows a batch at a time. */
+		for (;;)
+		{
+			int			numrows;
+			int			i;
+
+			/* Allow users to cancel long query */
+			CHECK_FOR_INTERRUPTS();
+
+			/*
+			 * XXX possible future improvement: if rowstoskip is large, we
+			 * could issue a MOVE rather than physically fetching the rows,
+			 * then just adjust rowstoskip and samplerows appropriately.
+			 */
+
+			/* Fetch some rows */
 			res = pgfdw_exec_query(conn, fetch_sql);
 			/* On error, report the original query, not the FETCH. */
 			if (PQresultStatus(res) != PGRES_TUPLES_OK)