Adjust batch size in postgres_fdw to not use too many parameters

The FE/BE protocol identifies parameters with an Int16 index, which
limits the maximum number of parameters per query to 65535. With
batching added to postges_fdw this limit is much easier to hit, as
the whole batch is essentially a single query, making this error much
easier to hit.

The failures are a bit unpredictable, because it also depends on the
number of columns in the query. So instead of just failing, this patch
tweaks the batch_size to not exceed the maximum number of parameters.

Reported-by: Hou Zhijie <houzj.fnst@cn.fujitsu.com>
Reviewed-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Discussion: https://postgr.es/m/OS0PR01MB571603973C0AC2874AD6BF2594299%40OS0PR01MB5716.jpnprd01.prod.outlook.com

Author: tvondra

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -9680,6 +9680,17 @@ SELECT COUNT(*) FROM ftable;
     34
 (1 row)
 
+TRUNCATE batch_table;
+DROP FOREIGN TABLE ftable;
+-- try if large batches exceed max number of bind parameters
+CREATE FOREIGN TABLE ftable ( x int ) SERVER loopback OPTIONS ( table_name 'batch_table', batch_size '100000' );
+INSERT INTO ftable SELECT * FROM generate_series(1, 70000) i;
+SELECT COUNT(*) FROM ftable;
+ count 
+-------
+ 70000
+(1 row)
+
 TRUNCATE batch_table;
 DROP FOREIGN TABLE ftable;
 -- Disable batch insert

contrib/postgres_fdw/postgres_fdw.c

@@ -2030,7 +2030,7 @@ postgresGetForeignModifyBatchSize(ResultRelInfo *resultRelInfo)
 	Assert(fmstate == NULL || fmstate->aux_fmstate == NULL);
 
 	/*
-	 * In EXPLAIN without ANALYZE, ri_fdwstate is NULL, so we have to lookup
+	 * In EXPLAIN without ANALYZE, ri_FdwState is NULL, so we have to lookup
 	 * the option directly in server/table options. Otherwise just use the
 	 * value we determined earlier.
 	 */
@@ -2045,7 +2045,14 @@ postgresGetForeignModifyBatchSize(ResultRelInfo *resultRelInfo)
 		 resultRelInfo->ri_TrigDesc->trig_insert_after_row))
 		return 1;
 
-	/* Otherwise use the batch size specified for server/table. */
+	/*
+	 * Otherwise use the batch size specified for server/table. The number of
+	 * parameters in a batch is limited to 65535 (uint16), so make sure we
+	 * don't exceed this limit by using the maximum batch_size possible.
+	 */
+	if (fmstate && fmstate->p_nums > 0)
+		batch_size = Min(batch_size, PQ_QUERY_PARAM_MAX_LIMIT / fmstate->p_nums);
+
 	return batch_size;
 }
 

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -3026,6 +3026,13 @@ SELECT COUNT(*) FROM ftable;
 TRUNCATE batch_table;
 DROP FOREIGN TABLE ftable;
 
+-- try if large batches exceed max number of bind parameters
+CREATE FOREIGN TABLE ftable ( x int ) SERVER loopback OPTIONS ( table_name 'batch_table', batch_size '100000' );
+INSERT INTO ftable SELECT * FROM generate_series(1, 70000) i;
+SELECT COUNT(*) FROM ftable;
+TRUNCATE batch_table;
+DROP FOREIGN TABLE ftable;
+
 -- Disable batch insert
 CREATE FOREIGN TABLE ftable ( x int ) SERVER loopback OPTIONS ( table_name 'batch_table', batch_size '1' );
 EXPLAIN (VERBOSE, COSTS OFF) INSERT INTO ftable VALUES (1), (2);

doc/src/sgml/postgres-fdw.sgml

@@ -372,6 +372,17 @@ OPTIONS (ADD password_required 'false');
        overrides an option specified for the server.
        The default is <literal>1</literal>.
       </para>
+
+      <para>
+       Note the actual number of rows <filename>postgres_fdw</filename> inserts at
+       once depends on the number of columns and the provided
+       <literal>batch_size</literal> value. The batch is executed as a single
+       query, and the libpq protocol (which <filename>postgres_fdw</filename>
+       uses to connect to a remote server) limits the number of parameters in a
+       single query to 65535. When the number of columns * <literal>batch_size</literal>
+       exceeds the limit, the <literal>batch_size</literal> will be adjusted to
+       avoid an error.
+      </para>
      </listitem>
     </varlistentry>
 

src/interfaces/libpq/fe-exec.c

@@ -1403,10 +1403,11 @@ PQsendQueryParams(PGconn *conn,
 							 libpq_gettext("command string is a null pointer\n"));
 		return 0;
 	}
-	if (nParams < 0 || nParams > 65535)
+	if (nParams < 0 || nParams > PQ_QUERY_PARAM_MAX_LIMIT)
 	{
-		appendPQExpBufferStr(&conn->errorMessage,
-							 libpq_gettext("number of parameters must be between 0 and 65535\n"));
+		appendPQExpBuffer(&conn->errorMessage,
+						  libpq_gettext("number of parameters must be between 0 and %d\n"),
+						  PQ_QUERY_PARAM_MAX_LIMIT);
 		return 0;
 	}
 
@@ -1451,10 +1452,11 @@ PQsendPrepare(PGconn *conn,
 							 libpq_gettext("command string is a null pointer\n"));
 		return 0;
 	}
-	if (nParams < 0 || nParams > 65535)
+	if (nParams < 0 || nParams > PQ_QUERY_PARAM_MAX_LIMIT)
 	{
-		appendPQExpBufferStr(&conn->errorMessage,
-							 libpq_gettext("number of parameters must be between 0 and 65535\n"));
+		appendPQExpBuffer(&conn->errorMessage,
+						  libpq_gettext("number of parameters must be between 0 and %d\n"),
+						  PQ_QUERY_PARAM_MAX_LIMIT);
 		return 0;
 	}
 
@@ -1548,10 +1550,11 @@ PQsendQueryPrepared(PGconn *conn,
 							 libpq_gettext("statement name is a null pointer\n"));
 		return 0;
 	}
-	if (nParams < 0 || nParams > 65535)
+	if (nParams < 0 || nParams > PQ_QUERY_PARAM_MAX_LIMIT)
 	{
-		appendPQExpBufferStr(&conn->errorMessage,
-							 libpq_gettext("number of parameters must be between 0 and 65535\n"));
+		appendPQExpBuffer(&conn->errorMessage,
+						  libpq_gettext("number of parameters must be between 0 and %d\n"),
+						  PQ_QUERY_PARAM_MAX_LIMIT);
 		return 0;
 	}
 

src/interfaces/libpq/libpq-fe.h

@@ -429,6 +429,8 @@ extern PGresult *PQexecPrepared(PGconn *conn,
 								int resultFormat);
 
 /* Interface for multiple-result or asynchronous queries */
+#define PQ_QUERY_PARAM_MAX_LIMIT 65535
+
 extern int	PQsendQuery(PGconn *conn, const char *query);
 extern int	PQsendQueryParams(PGconn *conn,
 							  const char *command,