query stuck when trying to kill it because of OOM

[guo-shaoge]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. hack tidb code, delete following code, so we can set tidb_mem_quota_query a very small value to help reproduce this bug

   tidb/pkg/sessionctx/variable/sysvar.go

   Lines 1127 to 1130 in 199bf90

   	if intVal > 0 && intVal < 128 { // 128 Bytes
   	s.StmtCtx.AppendWarning(ErrTruncatedWrongValue.FastGenByArgs(TiDBServerMemoryLimitSessMinSize, originalValue))
   	intVal = 128
   	}

2. build tidb and start tiup playround
3. download 10-tidb-slow.log and set @@tidb_slow_query_file = '/home/guojiangtao/10-tidb-slow.log'; (remember to change the slow query file path)
4. set @@tidb_mem_quota_query = 10;
5. run select time,host host_ip,Query_time as exec_max_time,parse_time,compile_time,Query as sql_text,Digest as sql_id,is_internal,succ, Plan as plan_text,mem_max as mem_max,User as parse_user,DB as database_name,total_keys,request_count,process_time,process_keys from information_schema.SLOW_QUERY order by time desc;

2. What did you expect to see? (Required)

query is canceled

3. What did you see instead (Required)

query is stucked

4. What is your TiDB version? (Required)

master 560e92e

[yibin87]

/assign @yibin87

[yibin87]

Selecting plan from slow_query table without limit restriction is not recommanded behavior. And this is a long existing issue. Down to major.