Fix "force_parallel_mode = regress" to work with ANALYZE + VERBOSE.

force_parallel_mode = regress is supposed to force use of a Gather
node without having any impact on EXPLAIN output.  But it failed to
accomplish that if both ANALYZE and VERBOSE are given, because that
enables per-worker output data that you wouldn't see if the Gather
hadn't been inserted.  Improve the logic so that we suppress the
per-worker data too.

This allows putting the new test case added by commit 5935917ce
back into the originally intended form (cf. 776a2c887, 22864f6e0).
We can also get rid of a kluge in subselect.sql, which previously
had to clean up after force_parallel_mode's failure to do what it
said on the tin.

Discussion: https://postgr.es/m/18445.1576177309@sss.pgh.pa.us

src/backend/commands/explain.c

@@ -695,13 +695,19 @@ ExplainPrintPlan(ExplainState *es, QueryDesc *queryDesc)
 
 	/*
 	 * Sometimes we mark a Gather node as "invisible", which means that it's
-	 * not displayed in EXPLAIN output.  The purpose of this is to allow
+	 * not to be displayed in EXPLAIN output.  The purpose of this is to allow
 	 * running regression tests with force_parallel_mode=regress to get the
 	 * same results as running the same tests with force_parallel_mode=off.
+	 * Such marking is currently only supported on a Gather at the top of the
+	 * plan.  We skip that node, and we must also hide per-worker detail data
+	 * further down in the plan tree.
 	 */
 	ps = queryDesc->planstate;
 	if (IsA(ps, GatherState) &&((Gather *) ps->plan)->invisible)
+	{
 		ps = outerPlanState(ps);
+		es->hide_workers = true;
+	}
 	ExplainNode(ps, NIL, NULL, NULL, es);
 
 	/*
@@ -806,6 +812,10 @@ ExplainPrintJIT(ExplainState *es, int jit_flags,
 	if (!ji || ji->created_functions == 0)
 		return;
 
+	/* don't print per-worker info if we're supposed to hide that */
+	if (for_workers && es->hide_workers)
+		return;
+
 	/* calculate total time */
 	INSTR_TIME_SET_ZERO(total_time);
 	INSTR_TIME_ADD(total_time, ji->generation_counter);
@@ -1877,7 +1887,8 @@ ExplainNode(PlanState *planstate, List *ancestors,
 		show_buffer_usage(es, &planstate->instrument->bufusage);
 
 	/* Show worker detail */
-	if (es->analyze && es->verbose && planstate->worker_instrument)
+	if (es->analyze && es->verbose && !es->hide_workers &&
+		planstate->worker_instrument)
 	{
 		WorkerInstrumentation *w = planstate->worker_instrument;
 		bool		opened_group = false;
@@ -2574,6 +2585,12 @@ show_sort_info(SortState *sortstate, ExplainState *es)
 		}
 	}
 
+	/*
+	 * You might think we should just skip this stanza entirely when
+	 * es->hide_workers is true, but then we'd get no sort-method output at
+	 * all.  We have to make it look like worker 0's data is top-level data.
+	 * Currently, we only bother with that for text-format output.
+	 */
 	if (sortstate->shared_info != NULL)
 	{
 		int			n;
@@ -2596,9 +2613,11 @@ show_sort_info(SortState *sortstate, ExplainState *es)
 			if (es->format == EXPLAIN_FORMAT_TEXT)
 			{
 				appendStringInfoSpaces(es->str, es->indent * 2);
+				if (n > 0 || !es->hide_workers)
+					appendStringInfo(es->str, "Worker %d:  ", n);
 				appendStringInfo(es->str,
-								 "Worker %d:  Sort Method: %s  %s: %ldkB\n",
-								 n, sortMethod, spaceType, spaceUsed);
+								 "Sort Method: %s  %s: %ldkB\n",
+								 sortMethod, spaceType, spaceUsed);
 			}
 			else
 			{

src/include/commands/explain.h

@@ -46,6 +46,7 @@ typedef struct ExplainState
 	List	   *rtable_names;	/* alias names for RTEs */
 	List	   *deparse_cxt;	/* context list for deparsing expressions */
 	Bitmapset  *printed_subplans;	/* ids of SubPlans we've printed */
+	bool		hide_workers;	/* set if we find an invisible Gather */
 } ExplainState;
 
 /* Hook for plugins to get control in ExplainOneQuery() */

src/test/regress/expected/partition_prune.out

@@ -3163,12 +3163,12 @@ execute mt_q1(35);
 deallocate mt_q1;
 prepare mt_q2 (int) as select * from ma_test where a >= $1 order by b limit 1;
 -- Ensure output list looks sane when the MergeAppend has no subplans.
-explain (verbose, costs off) execute mt_q2 (35);
-           QUERY PLAN           
---------------------------------
- Limit
+explain (analyze, verbose, costs off, summary off, timing off) execute mt_q2 (35);
+                 QUERY PLAN                 
+--------------------------------------------
+ Limit (actual rows=0 loops=1)
    Output: ma_test.a, ma_test.b
-   ->  Merge Append
+   ->  Merge Append (actual rows=0 loops=1)
          Sort Key: ma_test.b
          Subplans Removed: 3
 (5 rows)

src/test/regress/expected/subselect.out

@@ -1166,8 +1166,6 @@ begin
         select * from (select pk,c2 from sq_limit order by c1,pk) as x limit 3
     loop
         ln := regexp_replace(ln, 'Memory: \S*',  'Memory: xxx');
-        -- this case might occur if force_parallel_mode is on:
-        ln := regexp_replace(ln, 'Worker 0:  Sort Method',  'Sort Method');
         return next ln;
     end loop;
 end;

src/test/regress/sql/partition_prune.sql

@@ -841,7 +841,7 @@ deallocate mt_q1;
 prepare mt_q2 (int) as select * from ma_test where a >= $1 order by b limit 1;
 
 -- Ensure output list looks sane when the MergeAppend has no subplans.
-explain (verbose, costs off) execute mt_q2 (35);
+explain (analyze, verbose, costs off, summary off, timing off) execute mt_q2 (35);
 
 deallocate mt_q2;
 

src/test/regress/sql/subselect.sql

@@ -631,8 +631,6 @@ begin
         select * from (select pk,c2 from sq_limit order by c1,pk) as x limit 3
     loop
         ln := regexp_replace(ln, 'Memory: \S*',  'Memory: xxx');
-        -- this case might occur if force_parallel_mode is on:
-        ln := regexp_replace(ln, 'Worker 0:  Sort Method',  'Sort Method');
         return next ln;
     end loop;
 end;