document INSERT IGNORE ON DUP for pipelined dml (pingcap#17353)

system-variables.md

@@ -1822,7 +1822,7 @@ mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;
     - `"standard"` indicates the standard DML execution mode, where TiDB transactions are cached in memory before being committed. This mode is suitable for high-concurrency transaction scenarios with potential conflicts and is the default recommended execution mode.
     - `"bulk"` indicates the bulk DML execution mode, which is suitable for scenarios where a large amount of data is written, causing excessive memory usage in TiDB.
         - During the execution of TiDB transactions, the data is not fully cached in the TiDB memory, but is continuously written to TiKV to reduce memory usage and smooth the write pressure.
-        - Only `INSERT`, `UPDATE`, `REPLACE`, and `DELETE` statements are affected by the `"bulk"` mode.
+        - Only `INSERT`, `UPDATE`, `REPLACE`, and `DELETE` statements are affected by the `"bulk"` mode. Due to the pipelined execution in `"bulk"` mode, the usage of `INSERT IGNORE ... ON DUPLICATE UPDATE ...` might result in a `Duplicate entry` error when updates cause conflicts. In contrast, in `"standard"` mode, because the `IGNORE` keyword is set, this error would be ignored and not be returned to the user.
         - `"bulk"` mode is only suitable for scenarios where a large amount of **data is written without conflicts**. This mode is not efficient for handling write conflicts, as write-write conflicts might cause large transactions to fail and be rolled back.
         - `"bulk"` mode only takes effect on statements with auto-commit enabled, and requires the [`pessimistic-auto-commit`](https://docs.pingcap.com/tidb/stable/tidb-configuration-file#pessimistic-auto-commit-new-in-v600) configuration item to be set to `false`.
         - When using the `"bulk"` mode to execute statements, ensure that the [metadata lock](/metadata-lock.md) remains enabled during the execution process.