diff --git a/contrib/file_fdw/file_fdw.c b/contrib/file_fdw/file_fdw.c
index cf12710b71..dc035d72a2 100644
--- a/contrib/file_fdw/file_fdw.c
+++ b/contrib/file_fdw/file_fdw.c
@@ -131,6 +131,8 @@ static void fileEndForeignScan(ForeignScanState *node);
static bool fileAnalyzeForeignTable(Relation relation,
AcquireSampleRowsFunc *func,
BlockNumber *totalpages);
+static bool fileIsForeignScanParallelSafe(PlannerInfo *root, RelOptInfo *rel,
+ RangeTblEntry *rte);
/*
* Helper functions
@@ -170,6 +172,7 @@ file_fdw_handler(PG_FUNCTION_ARGS)
fdwroutine->ReScanForeignScan = fileReScanForeignScan;
fdwroutine->EndForeignScan = fileEndForeignScan;
fdwroutine->AnalyzeForeignTable = fileAnalyzeForeignTable;
+ fdwroutine->IsForeignScanParallelSafe = fileIsForeignScanParallelSafe;
PG_RETURN_POINTER(fdwroutine);
}
@@ -761,6 +764,18 @@ fileAnalyzeForeignTable(Relation relation,
return true;
}
+/*
+ * fileIsForeignScanParallelSafe
+ * Reading a file in a parallel worker should work just the same as
+ * reading it in the leader, so mark scans safe.
+ */
+static bool
+fileIsForeignScanParallelSafe(PlannerInfo *root, RelOptInfo *rel,
+ RangeTblEntry *rte)
+{
+ return true;
+}
+
/*
* check_selective_binary_conversion
*
diff --git a/doc/src/sgml/fdwhandler.sgml b/doc/src/sgml/fdwhandler.sgml
index c24ddfcb68..9eec3c8da7 100644
--- a/doc/src/sgml/fdwhandler.sgml
+++ b/doc/src/sgml/fdwhandler.sgml
@@ -988,6 +988,29 @@ ImportForeignSchema (ImportForeignSchemaStmt *stmt, Oid serverOid);
Size
+IsForeignScanParallelSafe(PlannerInfo *root, RelOptInfo *rel,
+ RangeTblEntry *rte);
+
+ Test whether a scan can be performed within a parallel worker. This
+ function will only be called when the planner believes that a parallel
+ plan might be possible, and should return true if it is safe for that scan
+ to run within a parallel worker. This will generally not be the case if
+ the remote data source has transaction semantics, unless the worker's
+ connection to the data can somehow be made to share the same transaction
+ context as the leader.
+
+
+
+ If this callback is not defined, it is assumed that the scan must take
+ place within the parallel leader. Note that returning true does not mean
+ that the scan itself can be done in parallel, only that the scan can be
+ performed within a parallel worker. Therefore, it can be useful to define
+ this method even when parallel execution is not supported.
+
+
+
+
+Size
EstimateDSMForeignScan(ForeignScanState *node, ParallelContext *pcxt);
Estimate the amount of dynamic shared memory that will be required
diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index bcb668fac5..870a46ce74 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -527,6 +527,23 @@ set_rel_consider_parallel(PlannerInfo *root, RelOptInfo *rel,
return;
return;
}
+
+ /*
+ * Ask FDWs whether they can support performing a ForeignScan
+ * within a worker. Most often, the answer will be no. For
+ * example, if the nature of the FDW is such that it opens a TCP
+ * connection with a remote server, each parallel worker would end
+ * up with a separate connection, and these connections might not
+ * be appropriately coordinated between workers and the leader.
+ */
+ if (rte->relkind == RELKIND_FOREIGN_TABLE)
+ {
+ Assert(rel->fdwroutine);
+ if (!rel->fdwroutine->IsForeignScanParallelSafe)
+ return;
+ if (!rel->fdwroutine->IsForeignScanParallelSafe(root, rel, rte))
+ return;
+ }
break;
case RTE_SUBQUERY:
diff --git a/src/include/foreign/fdwapi.h b/src/include/foreign/fdwapi.h
index 9fafab06e9..ae3c230d66 100644
--- a/src/include/foreign/fdwapi.h
+++ b/src/include/foreign/fdwapi.h
@@ -131,6 +131,10 @@ typedef void (*InitializeDSMForeignScan_function) (ForeignScanState *node,
typedef void (*InitializeWorkerForeignScan_function) (ForeignScanState *node,
shm_toc *toc,
void *coordinate);
+typedef bool (*IsForeignScanParallelSafe_function) (PlannerInfo *root,
+ RelOptInfo *rel,
+ RangeTblEntry *rte);
+
/*
* FdwRoutine is the struct returned by a foreign-data wrapper's handler
* function. It provides pointers to the callback functions needed by the
@@ -188,6 +192,7 @@ typedef struct FdwRoutine
ImportForeignSchema_function ImportForeignSchema;
/* Support functions for parallelism under Gather node */
+ IsForeignScanParallelSafe_function IsForeignScanParallelSafe;
EstimateDSMForeignScan_function EstimateDSMForeignScan;
InitializeDSMForeignScan_function InitializeDSMForeignScan;
InitializeWorkerForeignScan_function InitializeWorkerForeignScan;