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Commit69995c3

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Fix cost_rescan() to account for multi-batch hashing correctly.
cost_rescan assumed that we don't need to rebuild the hash table whenrescanning a hash join. However, that's currently only true forsingle-batch joins; for a multi-batch join we must charge full freight.This probably has escaped notice because we'd be unlikely to put a hashjoin on the inside of a nestloop anyway. Nonetheless, it's wrong.Fix in HEAD, but don't backpatch for fear of destabilizing plans instable releases.
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‎src/backend/optimizer/path/costsize.c

Lines changed: 14 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -3114,11 +3114,21 @@ cost_rescan(PlannerInfo *root, Path *path,
31143114
caseT_HashJoin:
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/*
3117-
*Assume that all of the startup cost represents hash table
3118-
*building, which we won't have to do over.
3117+
*If it's a single-batch join, we don't need to rebuild the hash
3118+
*table during a rescan.
31193119
*/
3120-
*rescan_startup_cost=0;
3121-
*rescan_total_cost=path->total_cost-path->startup_cost;
3120+
if (((HashPath*)path)->num_batches==1)
3121+
{
3122+
/* Startup cost is exactly the cost of hash table building */
3123+
*rescan_startup_cost=0;
3124+
*rescan_total_cost=path->total_cost-path->startup_cost;
3125+
}
3126+
else
3127+
{
3128+
/* Otherwise, no special treatment */
3129+
*rescan_startup_cost=path->startup_cost;
3130+
*rescan_total_cost=path->total_cost;
3131+
}
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break;
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caseT_CteScan:
31243134
caseT_WorkTableScan:

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