TiDB OOM when prepare/execute long insert statements

[jackysp]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. enable prepared plan cache
2. prepare some long insert statements, e.g. insert into t values (...),...(...);
3. execute prepared statements in concurrency.

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

No OOM

3. What did you see instead (Required)

TiDB OOM

4. What is your TiDB version? (Required)

v5.4.0

[jackysp]

When there're many placeholders in prepared statements, the plan cache for them will use much memory when executing them, and keep the memory usage in the TiDB server without releasing them.

[qw4990]

Fixed by #40080. Now the plan cache ignores simple insert-values-stmt like insert into t values (...).