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GH-45847: Pre-grow SwissTable to avoid repeated grow_double in map_new_keys - #53816
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…map_new_keys When GrouperFastImpl processes a minibatch where every row is a new key (common after a hash join scan emits rows with one unique key per payload), SwissTable::map_new_keys previously called grow_double() inside its do-while loop once per capacity threshold. Starting from log_blocks_=0 (8 slots), handling 1024 new keys required ~9 individual grow_double calls, each re-processing all existing entries, causing quadratic-ish overhead. This change pre-computes the required log_blocks capacity (mirroring num_groups_for_resize load factors: 50% for small tables, 75% for large) and performs all needed grow_double calls upfront when more than one is required. init_slot_ids_for_new_keys is called after the batched grow so slot ids are consistent with the new table size. All existing Grouper and hash aggregate tests still pass (83 row tests, 97 hash aggregate tests).
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Background
Issue #45847: Acero HashAggregate performs very poorly on multi-batch build scenarios (32+ batches). Performance drops 10x+ with only 1 CPU core utilized.
Root Cause
SwissTable::map_new_keysinkey_map_internal.ccstarts withlog_blocks_=0(8 slots) and callsgrow_double()each time capacity is exceeded. For N unique keys this causes O(log N) separate resize operations, each requiring O(N) re-insertion of ALL existing entries. With 2M unique keys from 64x32K batches, cascading resize dominates execution time.Solution
Pre-compute target capacity before the do-while loop and batch all grows at once:
log_blocks_so capacity >= num_inserted_ + num_idsnum_groups_for_resize)Changes: +44 lines, 0 deleted in
cpp/src/arrow/compute/key_map_internal.ccTesting
arrow-compute-row-test(Grouper): 83/83 PASSarrow-acero-hash-aggregate-test: 97/97 PASSarrow_compute_shared,arrow_acero_sharedcompile cleanlyFixes: #45847