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Correctly re-use hash tables in buildSubPlanHash().

Browse Source 
Commit 356687bd8 omitted to remove leftover code for destroying
a hashed subplan's hash tables, with the result that the tables
were always rebuilt not reused; this leads to severe memory
leakage if a hashed subplan is re-executed enough times.
Moreover, the code for reusing the hashnulls table had a typo
that would have made it do the wrong thing if it were reached.

Looking at the code coverage report shows severe under-coverage
of the potential callers of ResetTupleHashTable, so add some test
cases that exercise them.

Andreas Karlsson and Tom Lane, per reports from Ranier Vilela
and Justin Pryzby.

Backpatch to v11, as the faulty commit was.

Discussion: https://postgr.es/m/edb62547-c453-c35b-3ed6-a069e4d6b937@proxel.se
Discussion: https://postgr.es/m/CAEudQAo=DCebm1RXtig9OH+QivpS97sMkikt0A9qHmMUs+g6ZA@mail.gmail.com
Discussion: https://postgr.es/m/20200210032547.GA1412@telsasoft.com
This commit is contained in:
Tom Lane
2020-02-29 13:48:09 -05:00
parent 6afc8aefd3
commit 58c47ccfff
3 changed files with 191 additions and 3 deletions
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6
src/backend/executor/nodeSubplan.c
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@ -495,8 +495,6 @@ buildSubPlanHash(SubPlanState *node, ExprContext *econtext)
* need to store subplan output rows that contain NULL.
*/
MemoryContextReset(node->hashtablecxt);
node->hashtable = NULL;
node->hashnulls = NULL;
node->havehashrows = false;
node->havenullrows = false;
@ -533,7 +531,7 @@ buildSubPlanHash(SubPlanState *node, ExprContext *econtext)
}
if (node->hashnulls)
ResetTupleHashTable(node->hashtable);
ResetTupleHashTable(node->hashnulls);
else
node->hashnulls = BuildTupleHashTableExt(node->parent,
node->descRight,
@ -549,6 +547,8 @@ buildSubPlanHash(SubPlanState *node, ExprContext *econtext)
node->hashtempcxt,
false);
}
else
node->hashnulls = NULL;
/*
* We are probably in a short-lived expression-evaluation context. Switch