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Support flattening of empty-FROM subqueries and one-row VALUES tables.
We can't handle this in the general case due to limitations of the planner's data representations; but we can allow it in many useful cases, by being careful to flatten only when we are pulling a single-row subquery up into a FROM (or, equivalently, inner JOIN) node that will still have at least one remaining relation child. Per discussion of an example from Kyotaro Horiguchi.
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9 files changed

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-71
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9 files changed

+466
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‎src/backend/nodes/outfuncs.c

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@@ -1762,6 +1762,7 @@ _outPlannerInfo(StringInfo str, const PlannerInfo *node)
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WRITE_BOOL_FIELD(hasInheritedTarget);
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WRITE_BOOL_FIELD(hasJoinRTEs);
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WRITE_BOOL_FIELD(hasLateralRTEs);
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WRITE_BOOL_FIELD(hasDeletedRTEs);
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WRITE_BOOL_FIELD(hasHavingQual);
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WRITE_BOOL_FIELD(hasPseudoConstantQuals);
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WRITE_BOOL_FIELD(hasRecursion);

‎src/backend/optimizer/plan/planner.c

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@@ -352,8 +352,7 @@ subquery_planner(PlannerGlobal *glob, Query *parse,
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* Check to see if any subqueries in the jointree can be merged into this
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* query.
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*/
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parse->jointree = (FromExpr *)
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pull_up_subqueries(root, (Node *) parse->jointree);
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pull_up_subqueries(root);
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/*
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* If this is a simple UNION ALL query, flatten it into an appendrel. We

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