8000 Teach reparameterize_path() to handle AppendPaths. · postgres/postgres@ae3699a · GitHub
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Teach reparameterize_path() to handle AppendPaths.
If we're inside a lateral subquery, there may be no unparameterized paths for a particular child relation of an appendrel, in which case we *must* be able to create similarly-parameterized paths for each other child relation, else the planner will fail with "could not devise a query plan for the given query". This means that there are situations where we'd better be able to reparameterize at least one path for each child. This calls into question the assumption in reparameterize_path() that it can just punt if it feels like it. However, the only case that is known broken right now is where the child is itself an appendrel so that all its paths are AppendPaths. (I think possibly I disregarded that in the original coding on the theory that nested appendrels would get folded together --- but that only happens *after* reparameterize_path(), so it's not excused from handling a child AppendPath.) Given that this code's been like this since 9.3 when LATERAL was introduced, it seems likely we'd have heard of other cases by now if there were a larger problem. Per report from Elvis Pranskevichus. Back-patch to 9.3. Discussion: https://postgr.es/m/5981018.zdth1YWmNy@hammer.magicstack.net
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‎src/backend/optimizer/util/pathnode.c

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@@ -3205,6 +3205,29 @@ reparameterize_path(PlannerInfo *root, Path *path,
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spath->path.pathkeys,
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required_outer);
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}
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case T_Append:
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{
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AppendPath *apath = (AppendPath *) path;
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List *childpaths = NIL;
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ListCell *lc;
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/* Reparameterize the children */
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foreach(lc, apath->subpaths)
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{
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Path *spath = (Path *) lfirst(lc);
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spath = reparameterize_path(root, spath,
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required_outer,
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loop_count);
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if (spath == NULL)
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return NULL;
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childpaths = lappend(childpaths, spath);
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}
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return (Path *)
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create_append_path(rel, childpaths,
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required_outer,
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apath->path.parallel_workers);
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}
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default:
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break;
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}

‎src/test/regress/expected/join.out

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@@ -5150,6 +5150,25 @@ select * from
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Output: 3
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(11 rows)
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-- check handling of nested appendrels inside LATERAL
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select * from
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((select 2 as v) union all (select 3 as v)) as q1
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cross join lateral
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((select * from
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((select 4 as v) union all (select 5 as v)) as q3)
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union all
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(select q1.v)
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) as q2;
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v | v
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---+---
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2 | 4
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2 | 5
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2 | 2
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3 | 4
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3 | 5
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3 | 3
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(6 rows)
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-- check we don't try to do a unique-ified semijoin with LATERAL
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explain (verbose, costs off)
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select * from

‎src/test/regress/sql/join.sql

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@@ -1666,6 +1666,16 @@ select * from
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select * from (select 3 as z offset 0) z where z.z = x.x
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) zz on zz.z = y.y;
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-- check handling of nested appendrels inside LATERAL
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select * from
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((select 2 as v) union all (select 3 as v)) as q1
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cross join lateral
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((select * from
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((select 4 as v) union all (select 5 as v)) as q3)
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union all
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(select q1.v)
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) as q2;
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-- check we don't try to do a unique-ified semijoin with LATERAL
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explain (verbose, costs off)
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select * from

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