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Commit9d6af73

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Use fuzzy path cost tiebreaking rule in our oldest supported branches.
We've been doing it that way since 9.2, cf commit33e9915,but some recently-added regression test cases are making a few buildfarmmembers fail (ie choose the "wrong" plan) in 9.0 and 9.1 due toplatform-specific roundoff differences in cost calculations. To fix,back-port the patch that made add_path treat cost difference ratios ofless than 1e-10 as equal.
1 parent24906bb commit9d6af73

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+29
-21
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1 file changed

+29
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‎src/backend/optimizer/util/pathnode.c

Lines changed: 29 additions & 21 deletions
Original file line numberDiff line numberDiff line change
@@ -92,44 +92,45 @@ compare_path_costs(Path *path1, Path *path2, CostSelector criterion)
9292
* same if they agree to within a "fuzz factor". This is used by add_path
9393
* to avoid keeping both of a pair of paths that really have insignificantly
9494
* different cost.
95+
*
96+
* The fuzz_factor argument must be 1.0 plus delta, where delta is the
97+
* fraction of the smaller cost that is considered to be a significant
98+
* difference. For example, fuzz_factor = 1.01 makes the fuzziness limit
99+
* be 1% of the smaller cost.
95100
*/
96101
staticint
97-
compare_fuzzy_path_costs(Path*path1,Path*path2,CostSelectorcriterion)
102+
compare_fuzzy_path_costs(Path*path1,Path*path2,CostSelectorcriterion,
103+
doublefuzz_factor)
98104
{
99-
/*
100-
* We use a fuzz factor of 1% of the smaller cost.
101-
*
102-
* XXX does this percentage need to be user-configurable?
103-
*/
104105
if (criterion==STARTUP_COST)
105106
{
106-
if (path1->startup_cost>path2->startup_cost*1.01)
107+
if (path1->startup_cost>path2->startup_cost*fuzz_factor)
107108
return+1;
108-
if (path2->startup_cost>path1->startup_cost*1.01)
109+
if (path2->startup_cost>path1->startup_cost*fuzz_factor)
109110
return-1;
110111

111112
/*
112113
* If paths have the same startup cost (not at all unlikely), order
113114
* them by total cost.
114115
*/
115-
if (path1->total_cost>path2->total_cost*1.01)
116+
if (path1->total_cost>path2->total_cost*fuzz_factor)
116117
return+1;
117-
if (path2->total_cost>path1->total_cost*1.01)
118+
if (path2->total_cost>path1->total_cost*fuzz_factor)
118119
return-1;
119120
}
120121
else
121122
{
122-
if (path1->total_cost>path2->total_cost*1.01)
123+
if (path1->total_cost>path2->total_cost*fuzz_factor)
123124
return+1;
124-
if (path2->total_cost>path1->total_cost*1.01)
125+
if (path2->total_cost>path1->total_cost*fuzz_factor)
125126
return-1;
126127

127128
/*
128129
* If paths have the same total cost, order them by startup cost.
129130
*/
130-
if (path1->startup_cost>path2->startup_cost*1.01)
131+
if (path1->startup_cost>path2->startup_cost*fuzz_factor)
131132
return+1;
132-
if (path2->startup_cost>path1->startup_cost*1.01)
133+
if (path2->startup_cost>path1->startup_cost*fuzz_factor)
133134
return-1;
134135
}
135136
return0;
@@ -277,9 +278,11 @@ add_path(RelOptInfo *parent_rel, Path *new_path)
277278
/*
278279
* As of Postgres 8.0, we use fuzzy cost comparison to avoid wasting
279280
* cycles keeping paths that are really not significantly different in
280-
* cost.
281+
* cost. We use a 1% fuzziness limit. (XXX does this percentage need
282+
* to be user-configurable?)
281283
*/
282-
costcmp=compare_fuzzy_path_costs(new_path,old_path,TOTAL_COST);
284+
costcmp=compare_fuzzy_path_costs(new_path,old_path,
285+
TOTAL_COST,1.01);
283286

284287
/*
285288
* If the two paths compare differently for startup and total cost,
@@ -292,7 +295,7 @@ add_path(RelOptInfo *parent_rel, Path *new_path)
292295
*/
293296
if (costcmp==0||
294297
costcmp==compare_fuzzy_path_costs(new_path,old_path,
295-
STARTUP_COST))
298+
STARTUP_COST,1.01))
296299
{
297300
switch (compare_pathkeys(new_path->pathkeys,old_path->pathkeys))
298301
{
@@ -305,11 +308,16 @@ add_path(RelOptInfo *parent_rel, Path *new_path)
305308
{
306309
/*
307310
* Same pathkeys, and fuzzily the same cost, so keep
308-
* just one --- but we'll do an exact cost comparison
309-
* to decide which.
311+
* just one; to decide which, do a fuzzy total-cost
312+
* comparison with very small fuzz limit. (We used to
313+
* do an exact cost comparison, but that results in
314+
* annoying platform-specific plan variations due to
315+
* roundoff in the cost estimates.) If things are
316+
* still tied, arbitrarily keep only the old path.
310317
*/
311-
if (compare_path_costs(new_path,old_path,
312-
TOTAL_COST)<0)
318+
if (compare_fuzzy_path_costs(new_path,old_path,
319+
TOTAL_COST,
320+
1.0000000001)<0)
313321
remove_old= true;/* new dominates old */
314322
else
315323
accept_new= false;/* old equals or dominates new */

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