fix up previous commit (after review of ioguix and gleu)

README.md

@@ -174,7 +174,10 @@ Caveats
 `pg_query_settings` doesn't work well with **prepared queries**. More specifically:
 for a given parameter `P`, if the generic plan is selected, and if the executor
 needs to fetch the value of `P`, then it won't get the value specified in the
-`pgqs_config` table. For example, the executor fetches the value of `work_mem`
+`pgqs_config` table: at this point the value would have been reset to its default.
+This will be fixed in future releases.
+
+For example, the executor fetches the value of `work_mem`
 in order to decide whether to perform an in-memory sort, while the parameter
 `max_parallel_workers_per_gather` is only used during the planification stage,
 so `pg_query_settings` only works for this last example.

pg_query_settings.c

@@ -298,6 +298,12 @@ execPlantuner(Query *parse, const char *query_st, int cursorOptions, ParamListIn
 static void
 PlanTuner_ExecutorEnd(QueryDesc *q)
 {
+  /* FIXME: here, we restore the value of each parameter, but that's a problem
+   * for prepared queries. For example, the executor fetches the value of work_mem
+   * in order to decide whether to perform an in-memory sort, but at this point the
+   * value would have been reset to its default (at the end of the PREPARE statement).
+   * However, there's no problem with parameters that only affect planification, such
+   * as max_parallel_workers_per_gather. */
   DestroyPRList(true);
 
   if (prev_ExecutorEnd)