Extremely inefficient GetWarnings() makes the query slow

[tiancaiamao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Originally encountered by an user https://asktug.com/t/topic/994126
A minimal reproduce can be:

CREATE TABLE `t2` (`col` decimal(20,4) DEFAULT NULL);
INSERT INTO `t2` VALUES (round(rand() * 0.49 + 0.0123, 4));
....  (repeat several times)
INSERT INTO `t2` SELECT * FROM `t2`;
.... (repeat several times to construct enough data)

CREATE TABLE `t1` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `c1` decimal(20,2) DEFAULT NULL,
  `create_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  `update_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`) /*T![clustered_index] CLUSTERED */
)
INSERT INTO t1 (c1) SELECT * FROM t2;

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

The last query finish in maybe 5~10s

3. What did you see instead (Required)

It takes nearly 8min to run the query!

mysql> insert into t1 (c1) select * from t2;
Query OK, 491520 rows affected, 65535 warnings (7 min 53.87 sec)
Records: 491520  Duplicates: 0  Warnings: 65535

4. What is your TiDB version? (Required)

nightly

mysql> select tidb_version();
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version()                                                                                                                                                                                                                                                                                           |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v6.5.0-alpha
Edition: Community
Git Commit Hash: dc6d9a01112a54af1ac7d0200521ea20b6d09b30
Git Branch: heads/refs/tags/v6.5.0-alpha
UTC Build Time: 2022-12-06 14:32:10
GoVersion: go1.19.3
Race Enabled: false
TiKV Min Version: 6.2.0-alpha
Check Table Before Drop: false
Store: tikv |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

[tiancaiamao]

[tiancaiamao]

2 necessary conditions to trigger this:

• generate warning during execution
• check system variables (fill default timestamp column value checks the @@timestamp variable here)

The root cause is that GetWarnings() use a copy on read which is too inefficient!

tidb/sessionctx/stmtctx/stmtctx.go

Lines 730 to 735 in 26bbb71

	func (sc *StatementContext) GetWarnings() []SQLWarn {
	sc.mu.Lock()
	defer sc.mu.Unlock()
	warns := make([]SQLWarn, len(sc.mu.warnings))
	copy(warns, sc.mu.warnings)
	return warns