diff --git a/src/backend/executor/spi.c b/src/backend/executor/spi.c
index a97a7e3bd4..c93b6c192f 100644
--- a/src/backend/executor/spi.c
+++ b/src/backend/executor/spi.c
@@ -2425,12 +2425,17 @@ _SPI_execute_plan(SPIPlanPtr plan, const SPIExecuteOptions *options,
 	 * snapshot != InvalidSnapshot, read_only = false: use the given snapshot,
 	 * modified by advancing its command ID before each querytree.
 	 *
-	 * snapshot == InvalidSnapshot, read_only = true: use the entry-time
-	 * ActiveSnapshot, if any (if there isn't one, we run with no snapshot).
+	 * snapshot == InvalidSnapshot, read_only = true: do nothing for queries
+	 * that require no snapshot.  For those that do, ensure that a Portal
+	 * snapshot exists; then use that, or use the entry-time ActiveSnapshot if
+	 * that exists and is different.
 	 *
-	 * snapshot == InvalidSnapshot, read_only = false: take a full new
-	 * snapshot for each user command, and advance its command ID before each
-	 * querytree within the command.
+	 * snapshot == InvalidSnapshot, read_only = false: do nothing for queries
+	 * that require no snapshot.  For those that do, ensure that a Portal
+	 * snapshot exists; then, in atomic execution (!allow_nonatomic) take a
+	 * full new snapshot for each user command, and advance its command ID
+	 * before each querytree within the command.  In allow_nonatomic mode we
+	 * just use the Portal snapshot unmodified.
 	 *
 	 * In the first two cases, we can just push the snap onto the stack once
 	 * for the whole plan list.
diff --git a/src/pl/plpgsql/src/pl_exec.c b/src/pl/plpgsql/src/pl_exec.c
index 6947575b94..54ede9d85e 100644
--- a/src/pl/plpgsql/src/pl_exec.c
+++ b/src/pl/plpgsql/src/pl_exec.c
@@ -2218,12 +2218,12 @@ exec_stmt_call(PLpgSQL_execstate *estate, PLpgSQL_stmt_call *stmt)
 	 * for the plan.  This avoids refcount leakage complaints if the called
 	 * procedure ends the current transaction.
 	 *
-	 * Also, tell SPI to allow non-atomic execution.
+	 * Also, tell SPI to allow non-atomic execution if appropriate.
 	 */
 	memset(&options, 0, sizeof(options));
 	options.params = paramLI;
 	options.read_only = estate->readonly_func;
-	options.allow_nonatomic = true;
+	options.allow_nonatomic = !estate->atomic;
 	options.owner = estate->procedure_resowner;
 
 	rc = SPI_execute_plan_extended(expr->plan, &options);