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Use fuzzy not exact cost comparison for the final tie-breaker in add_path.

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Instead of an exact cost comparison, use a fuzzy comparison with 1e-10
delta after all other path metrics have proved equal.  This is to avoid
having platform-specific roundoff behaviors determine the choice when
two paths are really the same to our cost estimators.  Adjust the
recently-added test case that made it obvious we had a problem here.
This commit is contained in:
Tom Lane 2012-04-21 00:51:14 -04:00
parent 09ff76fcdb
commit 33e99153e9
2 changed files with 38 additions and 22 deletions
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44
src/backend/optimizer/util/pathnode.c
View File

@ -125,8 +125,11 @@ compare_fractional_path_costs(Path *path1, Path *path2,
*
* We use fuzzy comparisons so that add_path() can avoid keeping both of
* a pair of paths that really have insignificantly different cost.
* The fuzz factor is 1% of the smaller cost. (XXX does this percentage
* need to be user-configurable?)
*
* The fuzz_factor argument must be 1.0 plus delta, where delta is the
* fraction of the smaller cost that is considered to be a significant
* difference. For example, fuzz_factor = 1.01 makes the fuzziness limit
* be 1% of the smaller cost.
*
* The two paths are said to have "equal" costs if both startup and total
* costs are fuzzily the same. Path1 is said to be better than path2 if
@ -138,16 +141,16 @@ compare_fractional_path_costs(Path *path1, Path *path2,
* dominates the other across the whole performance spectrum.
*/
static PathCostComparison
compare_path_costs_fuzzily(Path *path1, Path *path2)
compare_path_costs_fuzzily(Path *path1, Path *path2, double fuzz_factor)
{
/*
* Check total cost first since it's more likely to be different; many
* paths have zero startup cost.
*/
if (path1->total_cost > path2->total_cost * 1.01)
if (path1->total_cost > path2->total_cost * fuzz_factor)
{
/* path1 fuzzily worse on total cost */
if (path2->startup_cost > path1->startup_cost * 1.01)
if (path2->startup_cost > path1->startup_cost * fuzz_factor)
{
/* ... but path2 fuzzily worse on startup, so DIFFERENT */
return COSTS_DIFFERENT;
@ -155,10 +158,10 @@ compare_path_costs_fuzzily(Path *path1, Path *path2)
/* else path2 dominates */
return COSTS_BETTER2;
}
if (path2->total_cost > path1->total_cost * 1.01)
if (path2->total_cost > path1->total_cost * fuzz_factor)
{
/* path2 fuzzily worse on total cost */
if (path1->startup_cost > path2->startup_cost * 1.01)
if (path1->startup_cost > path2->startup_cost * fuzz_factor)
{
/* ... but path1 fuzzily worse on startup, so DIFFERENT */
return COSTS_DIFFERENT;
@ -167,12 +170,12 @@ compare_path_costs_fuzzily(Path *path1, Path *path2)
return COSTS_BETTER1;
}
/* fuzzily the same on total cost */
if (path1->startup_cost > path2->startup_cost * 1.01)
if (path1->startup_cost > path2->startup_cost * fuzz_factor)
{
/* ... but path1 fuzzily worse on startup, so path2 wins */
return COSTS_BETTER2;
}
if (path2->startup_cost > path1->startup_cost * 1.01)
if (path2->startup_cost > path1->startup_cost * fuzz_factor)
{
/* ... but path2 fuzzily worse on startup, so path1 wins */
return COSTS_BETTER1;
@ -354,7 +357,11 @@ add_path(RelOptInfo *parent_rel, Path *new_path)
p1_next = lnext(p1);
costcmp = compare_path_costs_fuzzily(new_path, old_path);
/*
* Do a fuzzy cost comparison with 1% fuzziness limit. (XXX does this
* percentage need to be user-configurable?)
*/
costcmp = compare_path_costs_fuzzily(new_path, old_path, 1.01);
/*
* If the two paths compare differently for startup and total cost,
@ -403,15 +410,24 @@ add_path(RelOptInfo *parent_rel, Path *new_path)
/*
* Same pathkeys and outer rels, and fuzzily
* the same cost, so keep just one; to decide
* which, first check rows and then do an
* exact cost comparison.
* which, first check rows and then do a fuzzy
* cost comparison with very small fuzz limit.
* (We used to do an exact cost comparison,
* but that results in annoying
* platform-specific plan variations due to
* roundoff in the cost estimates.) If things
* are still tied, arbitrarily keep only the
* old path. Notice that we will keep only
* the old path even if the less-fuzzy
* comparison decides the startup and total
* costs compare differently.
*/
if (new_path->rows < old_path->rows)
remove_old = true; /* new dominates old */
else if (new_path->rows > old_path->rows)
accept_new = false; /* old dominates new */
else if (compare_path_costs(new_path, old_path,
TOTAL_COST) < 0)
else if (compare_path_costs_fuzzily(new_path, old_path,
1.0000000001) == COSTS_BETTER1)
remove_old = true; /* new dominates old */
else
accept_new = false; /* old equals or dominates new */

16
src/test/regress/expected/join.out
View File

@ -2798,17 +2798,17 @@ select b.unique1 from
Sort Key: b.unique1
-> Nested Loop Left Join
-> Seq Scan on int4_tbl i2
-> Nested Loop Left Join
Join Filter: (b.unique1 = 42)
-> Nested Loop
-> Nested Loop
-> Seq Scan on int4_tbl i1
-> Nested Loop Left Join
Join Filter: (b.unique1 = 42)
-> Nested Loop
-> Seq Scan on int4_tbl i1
-> Index Scan using tenk1_thous_tenthous on tenk1 b
Index Cond: ((thousand = i1.f1) AND (i2.f1 = tenthous))
-> Index Scan using tenk1_unique1 on tenk1 a
Index Cond: (unique1 = b.unique2)
-> Index Only Scan using tenk1_thous_tenthous on tenk1 c
Index Cond: (thousand = a.thousand)
-> Index Scan using tenk1_unique1 on tenk1 a
Index Cond: (unique1 = b.unique2)
-> Index Only Scan using tenk1_thous_tenthous on tenk1 c
Index Cond: (thousand = a.thousand)
(15 rows)
select b.unique1 from