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Commit4a8a5dd

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author
Etsuro Fujita
committed
Improve comments for trivial_subqueryscan().
This function can be called from mark_async_capable_plan(), a helperfunction for create_append_plan(), before set_subqueryscan_references(),to determine the triviality of a SubqueryScan that is a child of anAppend plan node, which is done before doing finalize_plan() on theSubqueryScan (if necessary) and set_plan_references() on the subplan,unlike when called from set_subqueryscan_references(). The reason whythis is safe wouldn't be that obvious, so add comments explaining this.Follow-up for commitc2bb02b.Reviewed by Zhihong Yu.Discussion:https://postgr.es/m/CAPmGK17%2BGiJBthC6va7%2B9n6t75e-M1N0U18YB2G1B%2BE5OdrNTA%40mail.gmail.com
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‎src/backend/optimizer/plan/setrefs.c

Lines changed: 18 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -1349,8 +1349,23 @@ set_subqueryscan_references(PlannerInfo *root,
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* We can delete it if it has no qual to check and the targetlist just
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* regurgitates the output of the child plan.
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*
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* This might be called repeatedly on a SubqueryScan node, so we cache the
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* result in the SubqueryScan node to avoid repeated computation.
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* This can be called from mark_async_capable_plan(), a helper function for
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* create_append_plan(), before set_subqueryscan_references(), to determine
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* triviality of a SubqueryScan that is a child of an Append node. So we
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* cache the result in the SubqueryScan node to avoid repeated computation.
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*
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* Note: when called from mark_async_capable_plan(), we determine the result
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* before running finalize_plan() on the SubqueryScan node (if needed) and
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* set_plan_references() on the subplan tree, but this would be safe, because
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* 1) finalize_plan() doesn't modify the tlist or quals for the SubqueryScan
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* node (or that for any plan node in the subplan tree), and
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* 2) set_plan_references() modifies the tlist for every plan node in the
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* subplan tree, but keeps const/resjunk columns as const/resjunk ones and
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* preserves the length and order of the tlist, and
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* 3) set_plan_references() might delete the topmost plan node like an Append
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* or MergeAppend from the subplan tree and pull up the child plan node,
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* but in that case, the tlist for the child plan node exactly matches the
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* parent.
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*/
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bool
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trivial_subqueryscan(SubqueryScan*plan)
@@ -1359,7 +1374,7 @@ trivial_subqueryscan(SubqueryScan *plan)
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ListCell*lp,
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*lc;
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1362-
/* We might have detected this already (see mark_async_capable_plan) */
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/* We might have detected this already; in which case reuse the result */
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if (plan->scanstatus==SUBQUERY_SCAN_TRIVIAL)
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return true;
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if (plan->scanstatus==SUBQUERY_SCAN_NONTRIVIAL)

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