v9-0002-Clarify-display-of-rows-as-decimal-fractions.patch

From a30ee71240d731c386f9dd4eb37b2817e3238807 Mon Sep 17 00:00:00 2001
From: Ilia Evdokimov <ilya.evdokimov@tantorlabs.ru>
Date: Thu, 13 Feb 2025 11:24:03 +0300
Subject: [PATCH v9] Clarify display of rows as decimal fractions

When loops > 1, the average rows value is now displayed as a decimal fraction
with two digits after the decimal point in EXPLAIN ANALYZE.

Previously, the average rows value was always rounded to an integer,
which could lead to misleading results when estimating total rows
by multiplying rows by loop. This change ensures that users
get a more accurate representation of the data flow through execution nodes.
---
 src/backend/commands/explain.c                | 55 +++++++++++++------
 src/test/regress/expected/partition_prune.out | 18 +++---
 src/test/regress/sql/partition_prune.sql      |  2 +-
 3 files changed, 49 insertions(+), 26 deletions(-)

diff --git a/src/backend/commands/explain.c b/src/backend/commands/explain.c
index c24e66f82e..ae81aeea13 100644
--- a/src/backend/commands/explain.c
+++ b/src/backend/commands/explain.c
@@ -13,6 +13,8 @@
  */
 #include "postgres.h"
 
+#include <math.h>
+
 #include "access/xact.h"
 #include "catalog/pg_type.h"
 #include "commands/createas.h"
@@ -69,6 +71,9 @@ typedef struct SerializeMetrics
  */
 #define BYTES_TO_KILOBYTES(b) (((b) + 1023) / 1024)
 
+/* Check if float/double has any decimal number */
+#define HAS_DECIMAL(x) (floor(x) != x)
+
 static void ExplainOneQuery(Query *query, int cursorOptions,
 							IntoClause *into, ExplainState *es,
 							ParseState *pstate, ParamListInfo params);
@@ -1981,14 +1986,15 @@ ExplainNode(PlanState *planstate, List *ancestors,
 
 		if (es->format == EXPLAIN_FORMAT_TEXT)
 		{
+			appendStringInfo(es->str, " (actual ");
+
 			if (es->timing)
-				appendStringInfo(es->str,
-								 " (actual time=%.3f..%.3f rows=%.0f loops=%.0f)",
-								 startup_ms, total_ms, rows, nloops);
+				appendStringInfo(es->str, "time=%.3f..%.3f ", startup_ms, total_ms);
+
+			if (nloops > 1 && HAS_DECIMAL(rows))
+				appendStringInfo(es->str, "rows=%.2f loops=%.0f)", rows, nloops);
 			else
-				appendStringInfo(es->str,
-								 " (actual rows=%.0f loops=%.0f)",
-								 rows, nloops);
+				appendStringInfo(es->str, "rows=%.0f loops=%.0f)", rows, nloops);
 		}
 		else
 		{
@@ -1999,8 +2005,16 @@ ExplainNode(PlanState *planstate, List *ancestors,
 				ExplainPropertyFloat("Actual Total Time", "ms", total_ms,
 									 3, es);
 			}
-			ExplainPropertyFloat("Actual Rows", NULL, rows, 0, es);
-			ExplainPropertyFloat("Actual Loops", NULL, nloops, 0, es);
+			if (nloops > 1 && HAS_DECIMAL(rows))
+			{
+				ExplainPropertyFloat("Actual Rows", NULL, rows, 2, es);
+				ExplainPropertyFloat("Actual Loops", NULL, nloops, 0, es);
+			}
+			else
+			{
+				ExplainPropertyFloat("Actual Rows", NULL, rows, 0, es);
+				ExplainPropertyFloat("Actual Loops", NULL, nloops, 0, es);
+			}
 		}
 	}
 	else if (es->analyze)
@@ -2052,14 +2066,14 @@ ExplainNode(PlanState *planstate, List *ancestors,
 			if (es->format == EXPLAIN_FORMAT_TEXT)
 			{
 				ExplainIndentText(es);
+				appendStringInfo(es->str, "actual ");
 				if (es->timing)
-					appendStringInfo(es->str,
-									 "actual time=%.3f..%.3f rows=%.0f loops=%.0f\n",
-									 startup_ms, total_ms, rows, nloops);
+					appendStringInfo(es->str, "time=%.3f..%.3f", startup_ms, total_ms);
+
+				if (nloops > 1 && HAS_DECIMAL(rows))
+					appendStringInfo(es->str, "rows=%.2f loops=%.0f\n", rows, nloops);
 				else
-					appendStringInfo(es->str,
-									 "actual rows=%.0f loops=%.0f\n",
-									 rows, nloops);
+					appendStringInfo(es->str, "rows=%.0f loops=%.0f\n", rows, nloops);
 			}
 			else
 			{
@@ -2070,8 +2084,17 @@ ExplainNode(PlanState *planstate, List *ancestors,
 					ExplainPropertyFloat("Actual Total Time", "ms",
 										 total_ms, 3, es);
 				}
-				ExplainPropertyFloat("Actual Rows", NULL, rows, 0, es);
-				ExplainPropertyFloat("Actual Loops", NULL, nloops, 0, es);
+
+				if (nloops > 1 && HAS_DECIMAL(rows))
+				{
+					ExplainPropertyFloat("Actual Rows", NULL, rows, 2, es);
+					ExplainPropertyFloat("Actual Loops", NULL, nloops, 0, es);
+				}
+				else
+				{
+					ExplainPropertyFloat("Actual Rows", NULL, rows, 0, es);
+					ExplainPropertyFloat("Actual Loops", NULL, nloops, 0, es);
+				}
 			}
 
 			ExplainCloseWorker(n, es);
diff --git a/src/test/regress/expected/partition_prune.out b/src/test/regress/expected/partition_prune.out
index e667503c96..fde44db1a0 100644
--- a/src/test/regress/expected/partition_prune.out
+++ b/src/test/regress/expected/partition_prune.out
@@ -2367,7 +2367,7 @@ begin
             $1)
     loop
         ln := regexp_replace(ln, 'Workers Launched: \d+', 'Workers Launched: N');
-        ln := regexp_replace(ln, 'actual rows=\d+ loops=\d+', 'actual rows=N loops=N');
+        ln := regexp_replace(ln, 'actual rows=\d+(?:\.\d+)? loops=\d+', 'actual rows=N loops=N');
         ln := regexp_replace(ln, 'Rows Removed by Filter: \d+', 'Rows Removed by Filter: N');
         return next ln;
     end loop;
@@ -3049,14 +3049,14 @@ order by tbl1.col1, tprt.col1;
 insert into tbl1 values (1001), (1010), (1011);
 explain (analyze, costs off, summary off, timing off, buffers off)
 select * from tbl1 inner join tprt on tbl1.col1 > tprt.col1;
-                                QUERY PLAN                                
---------------------------------------------------------------------------
+                                 QUERY PLAN                                  
+-----------------------------------------------------------------------------
  Nested Loop (actual rows=23 loops=1)
    ->  Seq Scan on tbl1 (actual rows=5 loops=1)
-   ->  Append (actual rows=5 loops=5)
+   ->  Append (actual rows=4.60 loops=5)
          ->  Index Scan using tprt1_idx on tprt_1 (actual rows=2 loops=5)
                Index Cond: (col1 < tbl1.col1)
-         ->  Index Scan using tprt2_idx on tprt_2 (actual rows=3 loops=4)
+         ->  Index Scan using tprt2_idx on tprt_2 (actual rows=2.75 loops=4)
                Index Cond: (col1 < tbl1.col1)
          ->  Index Scan using tprt3_idx on tprt_3 (actual rows=1 loops=2)
                Index Cond: (col1 < tbl1.col1)
@@ -3070,16 +3070,16 @@ select * from tbl1 inner join tprt on tbl1.col1 > tprt.col1;
 
 explain (analyze, costs off, summary off, timing off, buffers off)
 select * from tbl1 inner join tprt on tbl1.col1 = tprt.col1;
-                                QUERY PLAN                                
---------------------------------------------------------------------------
+                                 QUERY PLAN                                  
+-----------------------------------------------------------------------------
  Nested Loop (actual rows=3 loops=1)
    ->  Seq Scan on tbl1 (actual rows=5 loops=1)
-   ->  Append (actual rows=1 loops=5)
+   ->  Append (actual rows=0.60 loops=5)
          ->  Index Scan using tprt1_idx on tprt_1 (never executed)
                Index Cond: (col1 = tbl1.col1)
          ->  Index Scan using tprt2_idx on tprt_2 (actual rows=1 loops=2)
                Index Cond: (col1 = tbl1.col1)
-         ->  Index Scan using tprt3_idx on tprt_3 (actual rows=0 loops=3)
+         ->  Index Scan using tprt3_idx on tprt_3 (actual rows=0.33 loops=3)
                Index Cond: (col1 = tbl1.col1)
          ->  Index Scan using tprt4_idx on tprt_4 (never executed)
                Index Cond: (col1 = tbl1.col1)
diff --git a/src/test/regress/sql/partition_prune.sql b/src/test/regress/sql/partition_prune.sql
index 730545e86a..58756e0b18 100644
--- a/src/test/regress/sql/partition_prune.sql
+++ b/src/test/regress/sql/partition_prune.sql
@@ -586,7 +586,7 @@ begin
             $1)
     loop
         ln := regexp_replace(ln, 'Workers Launched: \d+', 'Workers Launched: N');
-        ln := regexp_replace(ln, 'actual rows=\d+ loops=\d+', 'actual rows=N loops=N');
+        ln := regexp_replace(ln, 'actual rows=\d+(?:\.\d+)? loops=\d+', 'actual rows=N loops=N');
         ln := regexp_replace(ln, 'Rows Removed by Filter: \d+', 'Rows Removed by Filter: N');
         return next ln;
     end loop;
-- 
2.34.1