Fix oversized memory allocation in Parallel Hash Join

During the calculations of the maximum for the number of buckets, take into
account that later we round that to the next power of 2.

Reported-by: Karen Talarico
Bug: #16925
Discussion: https://postgr.es/m/16925-ec96d83529d0d629%40postgresql.org
Author: Thomas Munro, Andrei Lepikhov, Alexander Korotkov
Reviewed-by: Alena Rybakina
Backpatch-through: 12

diff --git a/src/backend/executor/nodeHash.c b/src/backend/executor/nodeHash.c
index 42099b6d7515d7e653f45ac7dc1ec48bdb3c07a6..6e5297a159b92e89b13c6a0a4233e6d4a9ad3149 100644 (file)
--- a/src/backend/executor/nodeHash.c
+++ b/src/backend/executor/nodeHash.c
@@ -1155,6 +1155,7 @@ ExecParallelHashIncreaseNumBatches(HashJoinTable hashtable)
                    double      dtuples;
                    double      dbuckets;
                    int         new_nbuckets;
+                   uint32      max_buckets;
 
                    /*
                     * We probably also need a smaller bucket array.  How many
@@ -1167,9 +1168,16 @@ ExecParallelHashIncreaseNumBatches(HashJoinTable hashtable)
                     * array.
                     */
                    dtuples = (old_batch0->ntuples * 2.0) / new_nbatch;
+                   /*
+                    * We need to calculate the maximum number of buckets to
+                    * stay within the MaxAllocSize boundary.  Round the
+                    * maximum number to the previous power of 2 given that
+                    * later we round the number to the next power of 2.
+                    */
+                   max_buckets = pg_prevpower2_32((uint32)
+                                                  (MaxAllocSize / sizeof(dsa_pointer_atomic)));
                    dbuckets = ceil(dtuples / NTUP_PER_BUCKET);
-                   dbuckets = Min(dbuckets,
-                                  MaxAllocSize / sizeof(dsa_pointer_atomic));
+                   dbuckets = Min(dbuckets, max_buckets);
                    new_nbuckets = (int) dbuckets;
                    new_nbuckets = Max(new_nbuckets, 1024);
                    new_nbuckets = pg_nextpower2_32(new_nbuckets);