Hi.
In some cases, the planner underestimates FULL JOIN.
Example:
postgres=# CREATE TABLE t AS SELECT x AS a, null AS b FROM
generate_series(1, 10) x;
postgres=# ANALYZE;
postgres=# EXPLAIN (ANALYZE, TIMING OFF) SELECT * FROM t t1 FULL JOIN t t2
ON t1.b = t2.b;
QUERY PLAN
-------------------------------------------------------------------------------------------
Hash Full Join (cost=1.23..2.37 rows=10 width=72) (actual rows=20 loops=1)
Hash Cond: (t1.b = t2.b)
-> Seq Scan on t t1 (cost=0.00..1.10 rows=10 width=36) (actual rows=10
loops=1)
-> Hash (cost=1.10..1.10 rows=10 width=36) (actual rows=10 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 9kB
-> Seq Scan on t t2 (cost=0.00..1.10 rows=10 width=36) (actual
rows=10 loops=1)
Planning Time: 0.067 ms
Execution Time: 0.052 ms
(8 rows)
Are these simple changes enough to improve this situation?
diff --git a/src/backend/optimizer/path/costsize.c
b/src/backend/optimizer/path/costsize.c
index ef475d95a18..9cd43b778f3 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -5259,6 +5259,8 @@ calc_joinrel_size_estimate(PlannerInfo *root,
break;
case JOIN_FULL:
nrows = outer_rows * inner_rows * fkselec * jselec;
+ if (2 * nrows < outer_rows + inner_rows)
+ nrows = outer_rows + inner_rows - nrows;
if (nrows < outer_rows)
nrows = outer_rows;
if (nrows < inner_rows)
There is no error in the above case:
postgres=# EXPLAIN (ANALYZE, TIMING OFF) SELECT * FROM t t1 FULL JOIN t t2
ON t1.b = t2.b;
QUERY PLAN
-------------------------------------------------------------------------------------------
Hash Full Join (cost=1.23..2.37 rows=20 width=72) (actual rows=20 loops=1)
Hash Cond: (t1.b = t2.b)
-> Seq Scan on t t1 (cost=0.00..1.10 rows=10 width=36) (actual rows=10
loops=1)
-> Hash (cost=1.10..1.10 rows=10 width=36) (actual rows=10 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 9kB
-> Seq Scan on t t2 (cost=0.00..1.10 rows=10 width=36) (actual
rows=10 loops=1)
Planning Time: 0.069 ms
Execution Time: 0.065 ms
(8 rows)
