只要做分组聚合字段数据的基数足够多，就能落盘。
如果内存给的太小有可能还没达到分组聚合这里就被其它地方占用内存过多导致oom-killer了。比如从tidb获取的数据缓存过来的占用内存多出发杀死会话。看我这里可以落盘，需要注意的是，落盘会导致性能很慢：
mysql> explain analyze select count(*) from orders group by o_custkey;
+---------------------------+-------------+----------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------------------------------------------------------------+----------+---------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+---------------------------+-------------+----------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------------------------------------------------------------+----------+---------+
| HashAgg_9 | 4008704.67 | 2999660 | root | | time:10m36.6s, loops:2931 | group by:tpch.orders.o_custkey, funcs:count(Column#11)->Column#10 | 395.5 MB | 10.7 GB |
| └─TableReader_10 | 4008704.67 | 37114427 | root | | time:77.6ms, loops:1215, cop_task: {num: 1214, max: 36.6ms, min: 554.7µs, avg: 12.4ms, p95: 23.1ms, max_proc_keys: 51200, p95_proc_keys: 51200, tot_proc: 13.1s, tot_wait: 42ms, rpc_num: 1214, rpc_time: 15s, copr_cache: disabled, build_task_duration: 58.1µs, max_distsql_concurrency: 15} | data:HashAgg_5 | 12.4 MB | N/A |
| └─HashAgg_5 | 4008704.67 | 37114427 | cop[tikv] | | tikv_task:{proc max:34ms, min:0s, avg: 11ms, p80:18ms, p95:21ms, iters:36586, tasks:1214}, scan_detail: {total_process_keys: 37438464, total_process_keys_size: 5686354813, total_keys: 37439678, get_snapshot_time: 66.8ms, rocksdb: {key_skipped_count: 37438464, block: {cache_hit_count: 199034}}} | group by:tpch.orders.o_custkey, funcs:count(1)->Column#11 | N/A | N/A |
| └─TableFullScan_8 | 37327872.00 | 37438464 | cop[tikv] | table:orders | tikv_task:{proc max:29ms, min:0s, avg: 8.75ms, p80:15ms, p95:17ms, iters:36586, tasks:1214} | keep order:false | N/A | N/A |
+---------------------------+-------------+----------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------------------------------------------------------------+----------+---------+
4 rows in set (10 min 38.10 sec)