Fix tuple_fraction calculation in generate_orderedappend_paths()

6b94e7a adjusted generate_orderedappend_paths() to consider fractional
paths.  However, it didn't manage to interpret the tuple_fraction value
correctly.  According to the header comment of grouping_planner(), the
tuple_fraction >= 1 specifies the absolute number of expected tuples.  That
number must be divided by the expected total number of tuples to get the
actual fraction.

Even though this is a bug fix, we don't backpatch it.  The risks of the side
effects of plan changes on stable branches are too high.

Reported-by: Andrei Lepikhov <[email protected]>
Discussion: https://postgr.es/m/3ca271fa-ca5c-458c-8934-eb148622b270%40gmail.com
Author: Andrei Lepikhov <[email protected]>
Reviewed-by: Junwang Zhao <[email protected]>
Reviewed-by: Alvaro Herrera <[email protected]>
Reviewed-by: Tom Lane <[email protected]>

Author: akorotkov

src/backend/optimizer/path/allpaths.c

@@ -1891,7 +1891,17 @@ generate_orderedappend_paths(PlannerInfo *root, RelOptInfo *rel,
 			 */
 			if (root->tuple_fraction > 0)
 			{
-				double		path_fraction = (1.0 / root->tuple_fraction);
+				double		path_fraction = root->tuple_fraction;
+
+				/*
+				 * Merge Append considers only live children relations.  Dummy
+				 * relations must be filtered out before.
+				 */
+				Assert(childrel->rows > 0);
+
+				/* Convert absolute limit to a path fraction */
+				if (path_fraction >= 1.0)
+					path_fraction /= childrel->rows;
 
 				cheapest_fractional =
 					get_cheapest_fractional_path_for_pathkeys(childrel->pathlist,

src/test/regress/expected/partition_join.out

@@ -5260,6 +5260,24 @@ SELECT x.id, y.id FROM fract_t x LEFT JOIN fract_t y USING (id) ORDER BY x.id DE
                      Index Cond: (id = x_2.id)
 (11 rows)
 
+EXPLAIN (COSTS OFF) -- Should use NestLoop with parameterised inner scan
+SELECT x.id, y.id FROM fract_t x LEFT JOIN fract_t y USING (id)
+ORDER BY x.id DESC LIMIT 2;
+                                   QUERY PLAN                                   
+--------------------------------------------------------------------------------
+ Limit
+   ->  Merge Append
+         Sort Key: x.id DESC
+         ->  Nested Loop Left Join
+               ->  Index Only Scan Backward using fract_t0_pkey on fract_t0 x_1
+               ->  Index Only Scan using fract_t0_pkey on fract_t0 y_1
+                     Index Cond: (id = x_1.id)
+         ->  Nested Loop Left Join
+               ->  Index Only Scan Backward using fract_t1_pkey on fract_t1 x_2
+               ->  Index Only Scan using fract_t1_pkey on fract_t1 y_2
+                     Index Cond: (id = x_2.id)
+(11 rows)
+
 --
 -- Test Append's fractional paths
 --

src/test/regress/sql/partition_join.sql

@@ -1224,6 +1224,9 @@ SELECT x.id, y.id FROM fract_t x LEFT JOIN fract_t y USING (id) ORDER BY x.id AS
 
 EXPLAIN (COSTS OFF)
 SELECT x.id, y.id FROM fract_t x LEFT JOIN fract_t y USING (id) ORDER BY x.id DESC LIMIT 10;
+EXPLAIN (COSTS OFF) -- Should use NestLoop with parameterised inner scan
+SELECT x.id, y.id FROM fract_t x LEFT JOIN fract_t y USING (id)
+ORDER BY x.id DESC LIMIT 2;
 
 --
 -- Test Append's fractional paths