Add new FDW API to test for parallel-safety.

This is basically a bug fix; the old code assumes that a ForeignScan
is always parallel-safe, but for postgres_fdw, for example, this is
definitely false.  It should be true for file_fdw, though, since a
worker can read a file from the filesystem just as well as any other
backend process.

Original patch by Thomas Munro.  Documentation, and changes to the
comments, by me.

1 files changed, 15 insertions, 0 deletions

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);
 }
@@ -762,6 +765,18 @@ fileAnalyzeForeignTable(Relation relation,
 }
 
 /*
+ * 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
  *
  * Check to see if it's useful to convert only a subset of the file's columns