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Don't allow LIMIT/OFFSET clause within sub-selects to be pushed to workers.

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Allowing sub-select containing LIMIT/OFFSET in workers can lead to
inconsistent results at the top-level as there is no guarantee that the
row order will be fully deterministic.  The fix is to prohibit pushing
LIMIT/OFFSET within sub-selects to workers.

Reported-by: Andrew Fletcher
Bug: 15324
Author: Amit Kapila
Reviewed-by: Dilip Kumar
Backpatch-through: 9.6
Discussion: https://postgr.es/m/153417684333.10284.11356259990921828616@wrigleys.postgresql.org
This commit is contained in:
Amit Kapila
2018-09-14 09:51:47 +05:30
parent a2afcdab8a
commit 830d756590
5 changed files with 40 additions and 2 deletions
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13
src/backend/optimizer/path/allpaths.c
View File

@ -620,7 +620,20 @@ set_rel_consider_parallel(PlannerInfo *root, RelOptInfo *rel,
* the SubqueryScanPath as not parallel-safe. (Note that
* set_subquery_pathlist() might push some of these quals down
* into the subquery itself, but that doesn't change anything.)
*
* We can't push sub-select containing LIMIT/OFFSET to workers as
* there is no guarantee that the row order will be fully
* deterministic, and applying LIMIT/OFFSET will lead to
* inconsistent results at the top-level. (In some cases, where
* the result is ordered, we could relax this restriction. But it
* doesn't currently seem worth expending extra effort to do so.)
*/
{
Query *subquery = castNode(Query, rte->subquery);
if (limit_needed(subquery))
return;
}
break;
case RTE_JOIN:

3
src/backend/optimizer/plan/planner.c
View File

@ -123,7 +123,6 @@ static void preprocess_rowmarks(PlannerInfo *root);
static double preprocess_limit(PlannerInfo *root,
double tuple_fraction,
int64 *offset_est, int64 *count_est);
static bool limit_needed(Query *parse);
static void remove_useless_groupby_columns(PlannerInfo *root);
static List *preprocess_groupclause(PlannerInfo *root, List *force);
static List *extract_rollup_sets(List *groupingSets);
@ -2871,7 +2870,7 @@ preprocess_limit(PlannerInfo *root, double tuple_fraction,
* a key distinction: here we need hard constants in OFFSET/LIMIT, whereas
* in preprocess_limit it's good enough to consider estimated values.
*/
static bool
bool
limit_needed(Query *parse)
{
Node *node;