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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 09:36:30 +05:30
parent 0ba06e0bfb
commit 75f9c4ca5a
5 changed files with 40 additions and 2 deletions
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3
src/backend/optimizer/plan/planner.c
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@ -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);
@ -2870,7 +2869,7 @@ 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;