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Avoid sharing subpath list structure when flattening nested AppendRels.

In some situations the original coding led to corrupting the child AppendRel'ssubpaths list, effectively adding other members of the parent's list to it.This was usually masked because we never made any further use of the child'slist, but given the right combination of circumstances, we could do so. Thevisible symptom would be a relation getting scanned twice, as in bug #5673from David Schmitt.Backpatch to 8.2, which is as far back as the risky coding appears. Theexample submitted by David only fails in 8.4 and later, but I'm not convincedthat there aren't any even-more-obscure cases where 8.2 and 8.3 would fail.

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src/backend/optimizer/path
- allpaths.c

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`‎src/backend/optimizer/path/allpaths.c`

Lines changed: 1 addition & 1 deletion

Original file line number	Diff line number	Diff line change
`@@ -425,7 +425,7 @@ set_append_rel_pathlist(PlannerInfo root, RelOptInfo rel,`
`425`	`425`	`childpath=childrel->cheapest_total_path;`
`426`	`426`	`if (IsA(childpath,AppendPath))`
`427`	`427`	`subpaths=list_concat(subpaths,`
`428`		`- ((AppendPath*)childpath)->subpaths);`
	`428`	`+list_copy(((AppendPath*)childpath)->subpaths));`
`429`	`429`	`else`
`430`	`430`	`subpaths=lappend(subpaths,childpath);`
`431`	`431`

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Commitee63981

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`‎src/backend/optimizer/path/allpaths.c`

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