diff --git a/src/pl/plpgsql/src/pl_exec.c b/src/pl/plpgsql/src/pl_exec.c
index fd799b724da..4a9ed83030a 100644
--- a/src/pl/plpgsql/src/pl_exec.c
+++ b/src/pl/plpgsql/src/pl_exec.c
@@ -6079,6 +6079,7 @@ exec_eval_simple_expr(PLpgSQL_execstate *estate,
 	LocalTransactionId curlxid = MyProc->lxid;
 	CachedPlan *cplan;
 	void	   *save_setup_arg;
+	bool		need_snapshot;
 	MemoryContext oldcontext;
 
 	/*
@@ -6150,12 +6151,19 @@ exec_eval_simple_expr(PLpgSQL_execstate *estate,
 
 	/*
 	 * We have to do some of the things SPI_execute_plan would do, in
-	 * particular advance the snapshot if we are in a non-read-only function.
-	 * Without this, stable functions within the expression would fail to see
-	 * updates made so far by our own function.
+	 * particular push a new snapshot so that stable functions within the
+	 * expression can see updates made so far by our own function.  However,
+	 * we can skip doing that (and just invoke the expression with the same
+	 * snapshot passed to our function) in some cases, which is useful because
+	 * it's quite expensive relative to the cost of a simple expression.  We
+	 * can skip it if the expression contains no stable or volatile functions;
+	 * immutable functions shouldn't need to see our updates.  Also, if this
+	 * is a read-only function, we haven't made any updates so again it's okay
+	 * to skip.
 	 */
 	oldcontext = MemoryContextSwitchTo(get_eval_mcontext(estate));
-	if (!estate->readonly_func)
+	need_snapshot = (expr->expr_simple_mutable && !estate->readonly_func);
+	if (need_snapshot)
 	{
 		CommandCounterIncrement();
 		PushActiveSnapshot(GetTransactionSnapshot());
@@ -6180,7 +6188,7 @@ exec_eval_simple_expr(PLpgSQL_execstate *estate,
 
 	estate->paramLI->parserSetupArg = save_setup_arg;
 
-	if (!estate->readonly_func)
+	if (need_snapshot)
 		PopActiveSnapshot();
 
 	MemoryContextSwitchTo(oldcontext);
@@ -8051,6 +8059,8 @@ exec_save_simple_expr(PLpgSQL_expr *expr, CachedPlan *cplan)
 	/* Also stash away the expression result type */
 	expr->expr_simple_type = exprType((Node *) tle_expr);
 	expr->expr_simple_typmod = exprTypmod((Node *) tle_expr);
+	/* We also want to remember if it is immutable or not */
+	expr->expr_simple_mutable = contain_mutable_functions((Node *) tle_expr);
 }
 
 /*
diff --git a/src/pl/plpgsql/src/plpgsql.h b/src/pl/plpgsql/src/plpgsql.h
index f66b2baa70a..6b40fcd1963 100644
--- a/src/pl/plpgsql/src/plpgsql.h
+++ b/src/pl/plpgsql/src/plpgsql.h
@@ -234,6 +234,7 @@ typedef struct PLpgSQL_expr
 	int			expr_simple_generation; /* plancache generation we checked */
 	Oid			expr_simple_type;	/* result type Oid, if simple */
 	int32		expr_simple_typmod; /* result typmod, if simple */
+	bool		expr_simple_mutable;	/* true if simple expr is mutable */
 
 	/*
 	 * if expr is simple AND prepared in current transaction,