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Commit87027cb

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Clarify the 'rows' parameter in create_append_path
This is extracted from a larger patch to improve the UNION planner.While working on that, I found myself having to check what the 'rows'parameter is for. It's not obvious that passing a negative number is theway to have the rows estimate calculated and to find that out you needto read code in create_append_path() and in cost_append().Discussion:https://postgr.es/m/CAApHDvpb_63XQodmxKUF8vb9M7CxyUyT4sWvEgqeQU-GB7QFoQ@mail.gmail.com
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‎src/backend/optimizer/util/pathnode.c

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@@ -1237,6 +1237,10 @@ create_tidrangescan_path(PlannerInfo *root, RelOptInfo *rel,
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*
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* Note that we must handle subpaths = NIL, representing a dummy access path.
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* Also, there are callers that pass root = NULL.
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*
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* 'rows', when passed as a non-negative number, will be used to overwrite the
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* returned path's row estimate. Otherwise, the row estimate is calculated
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* by totalling the row estimates from the 'subpaths' list.
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*/
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AppendPath*
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create_append_path(PlannerInfo*root,

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