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Fix incorrect row estimates used for Memoize costing

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In order to estimate the cache hit ratio of a Memoize node, one of the
inputs we require is the estimated number of times the Memoize node will
be rescanned.  The higher this number, the large the cache hit ratio is
likely to become.  Unfortunately, the value being passed as the number of
"calls" to the Memoize was incorrectly using the Nested Loop's
outer_path->parent->rows instead of outer_path->rows.  This failed to
account for the fact that the outer_path might be parameterized by some
upper-level Nested Loop.

This problem could lead to Memoize plans appearing more favorable than
they might actually be.  It could also lead to extended executor startup
times when work_mem values were large due to the planner setting overly
large MemoizePath->est_entries resulting in the Memoize hash table being
initially made much larger than might be required.

Fix this simply by passing outer_path->rows rather than
outer_path->parent->rows.  Also, adjust the expected regression test
output for a plan change.

Reported-by: Pavel Stehule
Author: David Rowley
Discussion: https://postgr.es/m/CAFj8pRAMp%3DQsMi6sPQJ4W3hczoFJRvyXHJV3AZAZaMyTVM312Q%40mail.gmail.com
Backpatch-through: 14, where Memoize was introduced
This commit is contained in:
David Rowley 2022-05-16 16:07:56 +12:00
parent 5bcc4d0933
commit 1e731ed12a
2 changed files with 11 additions and 17 deletions
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2
src/backend/optimizer/path/joinpath.c
View File

@ -610,7 +610,7 @@ get_memoize_path(PlannerInfo *root, RelOptInfo *innerrel,
hash_operators,
extra->inner_unique,
binary_mode,
outer_path->parent->rows);
outer_path->rows);
}
return NULL;

26
src/test/regress/expected/join.out
View File

@ -3673,8 +3673,8 @@ select * from tenk1 t1 left join
(tenk1 t2 join tenk1 t3 on t2.thousand = t3.unique2)
on t1.hundred = t2.hundred and t1.ten = t3.ten
where t1.unique1 = 1;
QUERY PLAN
--------------------------------------------------------------
QUERY PLAN
--------------------------------------------------------
Nested Loop Left Join
-> Index Scan using tenk1_unique1 on tenk1 t1
Index Cond: (unique1 = 1)
@ -3684,20 +3684,17 @@ where t1.unique1 = 1;
Recheck Cond: (t1.hundred = hundred)
-> Bitmap Index Scan on tenk1_hundred
Index Cond: (hundred = t1.hundred)
-> Memoize
Cache Key: t2.thousand
Cache Mode: logical
-> Index Scan using tenk1_unique2 on tenk1 t3
Index Cond: (unique2 = t2.thousand)
(14 rows)
-> Index Scan using tenk1_unique2 on tenk1 t3
Index Cond: (unique2 = t2.thousand)
(11 rows)
explain (costs off)
select * from tenk1 t1 left join
(tenk1 t2 join tenk1 t3 on t2.thousand = t3.unique2)
on t1.hundred = t2.hundred and t1.ten + t2.ten = t3.ten
where t1.unique1 = 1;
QUERY PLAN
--------------------------------------------------------------
QUERY PLAN
--------------------------------------------------------
Nested Loop Left Join
-> Index Scan using tenk1_unique1 on tenk1 t1
Index Cond: (unique1 = 1)
@ -3707,12 +3704,9 @@ where t1.unique1 = 1;
Recheck Cond: (t1.hundred = hundred)
-> Bitmap Index Scan on tenk1_hundred
Index Cond: (hundred = t1.hundred)
-> Memoize
Cache Key: t2.thousand
Cache Mode: logical
-> Index Scan using tenk1_unique2 on tenk1 t3
Index Cond: (unique2 = t2.thousand)
(14 rows)
-> Index Scan using tenk1_unique2 on tenk1 t3
Index Cond: (unique2 = t2.thousand)
(11 rows)
explain (costs off)
select count(*) from