8000 Repair failure with SubPlans in multi-row VALUES lists. · koderP/postgres@d538f65 · GitHub
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Repair failure with SubPlans in multi-row VALUES lists.
When nodeValuesscan.c was written, it was impossible to have a SubPlan in VALUES --- any sub-SELECT there would have to be uncorrelated and thereby would produce an InitPlan instead. We therefore took a shortcut in the logic that throws away a ValuesScan's per-row expression evaluation data structures. This was broken by the introduction of LATERAL however; a sub-SELECT containing a lateral reference produces a correlated SubPlan. The cleanest fix for this would be to give up the optimization of discarding the expression eval state. But that still seems pretty unappetizing for long VALUES lists. It seems to work to just prevent the subexpressions from hooking into the ValuesScan node's subPlan list, so let's do that and see how well it works. (If this breaks, due to additional connections between the subexpressions and the outer query structures, we might consider compromises like throwing away data only for VALUES rows not containing SubPlans.) Per bug #14924 from Christian Duta. Back-patch to 9.3 where LATERAL was introduced. Discussion: https://postgr.es/m/20171124120836.1463.5310@wrigleys.postgresql.org
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‎src/backend/executor/nodeValuesscan.c

Lines changed: 16 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -92,6 +92,7 @@ ValuesNext(ValuesScanState *node)
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if (exprlist)
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{
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MemoryContext oldContext;
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List *oldsubplans;
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List *exprstatelist;
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Datum *values;
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bool *isnull;
@@ -114,12 +115,22 @@ ValuesNext(ValuesScanState *node)
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oldContext = MemoryContextSwitchTo(econtext->ecxt_per_tuple_memory);
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/*
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* Pass NULL, not my plan node, because we don't want anything in this
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* transient state linking into permanent state. The only possibility
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* is a SubPlan, and there shouldn't be any (any subselects in the
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* VALUES list should be InitPlans).
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* The expressions might contain SubPlans (this is currently only
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* possible if there's a sub-select containing a LATERAL reference,
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* otherwise sub-selects in a VALUES list should be InitPlans). Those
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* subplans will want to hook themselves into our subPlan list, which
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* would result in a corrupted list after we delete the eval state. We
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* can work around this by saving and restoring the subPlan list.
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* (There's no need for the functionality that would be enabled by
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* having the list entries, since the SubPlans aren't going to be
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* re-executed anyway.)
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*/
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exprstatelist = (List *) ExecInitExpr((Expr *) exprlist, NULL);
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oldsubplans = node->ss.ps.subPlan;
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node->ss.ps.subPlan = NIL;
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exprstatelist = (List *) ExecInitExpr((Expr *) exprlist, &node->ss.ps);
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node->ss.ps.subPlan = oldsubplans;
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/* parser should have checked all sublists are the same length */
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Assert(list_length(exprstatelist) == slot->tts_tupleDescriptor->natts);

‎src/test/regress/expected/subselect.out

Lines changed: 30 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -710,6 +710,36 @@ select exists(select * from nocolumns);
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f
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(1 row)
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--
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-- Check behavior with a SubPlan in VALUES (bug #14924)
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--
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select val.x
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from generate_series(1,10) as s(i),
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lateral (
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values ((select s.i + 1)), (s.i + 101)
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) as val(x)
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where s.i < 10 and (select val.x) < 110;
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x
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-----
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2
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102
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(17 rows)
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--
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-- Check sane behavior with nested IN SubLinks
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--

‎src/test/regress/sql/subselect.sql

Lines changed: 10 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -412,6 +412,16 @@ explain (verbose, costs off)
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create temp table nocolumns();
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select exists(select * from nocolumns);
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--
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-- Check behavior with a SubPlan in VALUES (bug #14924)
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--
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select val.x
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from generate_series(1,10) as s(i),
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lateral (
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values ((select s.i + 1)), (s.i + 101)
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) as val(x)
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where s.i < 10 and (select val.x) < 110;
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--
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-- Check sane behavior with nested IN SubLinks
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--

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