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Commitbcddad2

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Ensure mark_dummy_rel doesn't create dangling pointers in RelOptInfos.
When we are doing GEQO join planning, the current memory context is ashort-lived context that will be reset at the end of geqo_eval(). However,the RelOptInfos for base relations are set up before that and then re-usedacross many GEQO cycles. Hence, any code that modifies a baserel duringjoin planning has to be careful not to put pointers to the short-livedcontext into the baserel struct. mark_dummy_rel got this wrong, leading toeasy-to-reproduce-once-you-know-how crashes in 8.4, as reported off-list byLeo Carson of SDSC. Some improvements made in 9.0 make it difficult todemonstrate the crash in 9.0 or HEAD; but there's no doubt that there'sstill a risk factor here, so patch all branches that have the function.(Note: 8.3 has a similar function, but it's only applied to joinrels andthus is not a hazard.)
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‎src/backend/optimizer/path/joinrels.c

Lines changed: 25 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -17,6 +17,7 @@
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#include"optimizer/joininfo.h"
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#include"optimizer/pathnode.h"
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#include"optimizer/paths.h"
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#include"utils/memutils.h"
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staticvoidmake_rels_by_clause_joins(PlannerInfo*root,
@@ -933,11 +934,32 @@ is_dummy_rel(RelOptInfo *rel)
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}
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/*
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* Mark a rel as proven empty.
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* Mark a relation as proven empty.
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*
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* During GEQO planning, this can get invoked more than once on the same
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* baserel struct, so it's worth checking to see if the rel is already marked
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* dummy.
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*
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* Also, when called during GEQO join planning, we are in a short-lived
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* memory context. We must make sure that the dummy path attached to a
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* baserel survives the GEQO cycle, else the baserel is trashed for future
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* GEQO cycles. On the other hand, when we are marking a joinrel during GEQO,
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* we don't want the dummy path to clutter the main planning context. Upshot
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* is that the best solution is to explicitly make the dummy path in the same
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* context the given RelOptInfo is in.
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*/
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staticvoid
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mark_dummy_rel(RelOptInfo*rel)
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{
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MemoryContextoldcontext;
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/* Already marked? */
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if (is_dummy_rel(rel))
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return;
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/* No, so choose correct context to make the dummy path in */
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oldcontext=MemoryContextSwitchTo(GetMemoryChunkContext(rel));
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/* Set dummy size estimate */
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rel->rows=0;
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@@ -949,6 +971,8 @@ mark_dummy_rel(RelOptInfo *rel)
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/* Set or update cheapest_total_path */
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set_cheapest(rel);
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MemoryContextSwitchTo(oldcontext);
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}
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