Minor improvements in Incremental Sort explain

Some places still used "Maximum" instead of "Peak" when displaying info
about sort space, so fix that. Also, add a comment clarifying why it's
correct to check the number of full/prefix sort groups.

Author: James Coleman
Discussion: https://postgr.es/m/CAPpHfds1waRZ=NOmueYq0sx1ZSCnt+5QJvizT8ndT2=etZEeAQ@mail.gmail.com

src/backend/commands/explain.c

@@ -2841,7 +2841,7 @@ show_incremental_sort_group_info(IncrementalSortGroupInfo *groupInfo,
 			ExplainOpenGroup("Sort Space", memoryName.data, true, es);
 
 			ExplainPropertyInteger("Average Sort Space Used", "kB", avgSpace, es);
-			ExplainPropertyInteger("Maximum Sort Space Used", "kB",
+			ExplainPropertyInteger("Peak Sort Space Used", "kB",
 								   groupInfo->maxMemorySpaceUsed, es);
 
 			ExplainCloseGroup("Sort Spaces", memoryName.data, true, es);
@@ -2858,7 +2858,7 @@ show_incremental_sort_group_info(IncrementalSortGroupInfo *groupInfo,
 			ExplainOpenGroup("Sort Space", diskName.data, true, es);
 
 			ExplainPropertyInteger("Average Sort Space Used", "kB", avgSpace, es);
-			ExplainPropertyInteger("Maximum Sort Space Used", "kB",
+			ExplainPropertyInteger("Peak Sort Space Used", "kB",
 								   groupInfo->maxDiskSpaceUsed, es);
 
 			ExplainCloseGroup("Sort Spaces", diskName.data, true, es);
@@ -2883,6 +2883,15 @@ show_incremental_sort_info(IncrementalSortState *incrsortstate,
 	if (!es->analyze)
 		return;
 
+	/*
+	 * Since we never have any prefix groups unless we've first sorted a full
+	 * groups and transitioned modes (copying the tuples into a prefix group),
+	 * we don't need to do anything if there were 0 full groups.
+	 *
+	 * We still have to continue after this block if there are no full groups,
+	 * though, since it's possible that we have workers that did real work even
+	 * if the leader didn't participate.
+	 */
 	if (fullsortGroupInfo->groupCount > 0)
 	{
 		show_incremental_sort_group_info(fullsortGroupInfo, "Full-sort", true, es);
@@ -2914,6 +2923,13 @@ show_incremental_sort_info(IncrementalSortState *incrsortstate,
 			 */
 			fullsortGroupInfo = &incsort_info->fullsortGroupInfo;
 			prefixsortGroupInfo = &incsort_info->prefixsortGroupInfo;
+
+			/*
+			 * Since we never have any prefix groups unless we've first sorted
+			 * a full groups and transitioned modes (copying the tuples into a
+			 * prefix group), we don't need to do anything if there were 0 full
+			 * groups.
+			 */
 			if (fullsortGroupInfo->groupCount == 0 &&
 				prefixsortGroupInfo->groupCount == 0)
 				continue;

src/test/regress/expected/incremental_sort.out

@@ -109,7 +109,7 @@ begin
     for group_key in select unnest(array['Full-sort Groups', 'Presorted Groups']::text[]) t loop
       for space_key in select unnest(array['Sort Space Memory', 'Sort Space Disk']::text[]) t loop
         node := jsonb_set(node, array[group_key, space_key, 'Average Sort Space Used'], '"NN"', false);
-        node := jsonb_set(node, array[group_key, space_key, 'Maximum Sort Space Used'], '"NN"', false);
+        node := jsonb_set(node, array[group_key, space_key, 'Peak Sort Space Used'], '"NN"', false);
       end loop;
     end loop;
     nodes := nodes || node;
@@ -131,7 +131,7 @@ begin
     for group_key in select unnest(array['Full-sort Groups', 'Presorted Groups']::text[]) t loop
       group_stats := node->group_key;
       for space_key in select unnest(array['Sort Space Memory', 'Sort Space Disk']::text[]) t loop
-        if (group_stats->space_key->'Maximum Sort Space Used')::bigint < (group_stats->space_key->'Maximum Sort Space Used')::bigint then
+        if (group_stats->space_key->'Peak Sort Space Used')::bigint < (group_stats->space_key->'Peak Sort Space Used')::bigint then
           raise exception '% has invalid max space < average space', group_key;
         end if;
       end loop;
@@ -546,7 +546,7 @@ select explain_analyze_without_memory('select * from (select * from t order by a
 
 select jsonb_pretty(explain_analyze_inc_sort_nodes_without_memory('select * from (select * from t order by a) s order by a, b limit 55'));
                   jsonb_pretty                   
---------------------------------------------------
+-------------------------------------------------
  [                                              +
      {                                          +
          "Sort Key": [                          +
@@ -567,8 +567,8 @@ select jsonb_pretty(explain_analyze_inc_sort_nodes_without_memory('select * from
                  "quicksort"                    +
              ],                                 +
              "Sort Space Memory": {             +
-                 "Average Sort Space Used": "NN",+
+                 "Peak Sort Space Used": "NN",  +
-                 "Maximum Sort Space Used": "NN" +
+                 "Average Sort Space Used": "NN"+
              }                                  +
          },                                     +
          "Parent Relationship": "Outer"         +
@@ -720,7 +720,7 @@ select explain_analyze_without_memory('select * from (select * from t order by a
 
 select jsonb_pretty(explain_analyze_inc_sort_nodes_without_memory('select * from (select * from t order by a) s order by a, b limit 70'));
                   jsonb_pretty                   
---------------------------------------------------
+-------------------------------------------------
  [                                              +
      {                                          +
          "Sort Key": [                          +
@@ -740,8 +740,8 @@ select jsonb_pretty(explain_analyze_inc_sort_nodes_without_memory('select * from
                  "quicksort"                    +
              ],                                 +
              "Sort Space Memory": {             +
-                 "Average Sort Space Used": "NN",+
+                 "Peak Sort Space Used": "NN",  +
-                 "Maximum Sort Space Used": "NN" +
+                 "Average Sort Space Used": "NN"+
              }                                  +
          },                                     +
          "Presorted Groups": {                  +
@@ -750,8 +750,8 @@ select jsonb_pretty(explain_analyze_inc_sort_nodes_without_memory('select * from
                  "quicksort"                    +
              ],                                 +
              "Sort Space Memory": {             +
-                 "Average Sort Space Used": "NN",+
+                 "Peak Sort Space Used": "NN",  +
-                 "Maximum Sort Space Used": "NN" +
+                 "Average Sort Space Used": "NN"+
              }                                  +
          },                                     +
          "Parent Relationship": "Outer"         +

src/test/regress/sql/incremental_sort.sql

@@ -85,7 +85,7 @@ begin
     for group_key in select unnest(array['Full-sort Groups', 'Presorted Groups']::text[]) t loop
       for space_key in select unnest(array['Sort Space Memory', 'Sort Space Disk']::text[]) t loop
         node := jsonb_set(node, array[group_key, space_key, 'Average Sort Space Used'], '"NN"', false);
-        node := jsonb_set(node, array[group_key, space_key, 'Maximum Sort Space Used'], '"NN"', false);
+        node := jsonb_set(node, array[group_key, space_key, 'Peak Sort Space Used'], '"NN"', false);
       end loop;
     end loop;
     nodes := nodes || node;
@@ -108,7 +108,7 @@ begin
     for group_key in select unnest(array['Full-sort Groups', 'Presorted Groups']::text[]) t loop
       group_stats := node->group_key;
       for space_key in select unnest(array['Sort Space Memory', 'Sort Space Disk']::text[]) t loop
-        if (group_stats->space_key->'Maximum Sort Space Used')::bigint < (group_stats->space_key->'Maximum Sort Space Used')::bigint then
+        if (group_stats->space_key->'Peak Sort Space Used')::bigint < (group_stats->space_key->'Peak Sort Space Used')::bigint then
           raise exception '% has invalid max space < average space', group_key;
         end if;
       end loop;