Function optimizeDupKeyCheckForUpdate

pkg/executor/update.go:691–722

optimizeDupKeyCheckForUpdate trys to optimize the DupKeyCheckMode for an update statement. If the DupKeyCheckMode of the current statement can be optimized, it will return `DupKeyCheckLazy` to avoid the redundant requests to TiKV, otherwise, `DupKeyCheckInPlace` will be returned. The second argument

(txn kv.Transaction, ignoreNeedsCheckInPlace bool)

// redundant requests to TiKV, otherwise, `DupKeyCheckInPlace` will be returned.
// The second argument `ignoreNeedsCheckInPlace` is true if `IGNORE` keyword is used in the update statement.
func optimizeDupKeyCheckForUpdate(txn kv.Transaction, ignoreNeedsCheckInPlace bool) table.DupKeyCheckMode {
	if txn.IsPipelined() {
		// It means `@@tidb_dml_type='bulk'` which indicates to insert rows in "bulk" mode.
		// At this time, `DupKeyCheckLazy` should be used to improve the performance.
		// If "bulk" mode and IGNORE keyword are used together, "bulk" is prior, see:
		// https://github.com/pingcap/tidb/issues/55187#issuecomment-2268356459
		return table.DupKeyCheckLazy
	}

	if ignoreNeedsCheckInPlace {
		// For `UPDATE IGNORE ...` and `INSERT IGNORE ... ON DUPLICATE KEY UPDATE ...` statements,
		// `DupKeyCheckInPlace` should be used to make sure the executor can get the error
		// immediately and ignore it then.
		return table.DupKeyCheckInPlace
	}

	if txn.IsPessimistic() {
		// We can just check duplicated key lazily without keys in storage for the below cases:
		// - `txn.Pipelined()` is true.
		//    It means the user is using `@@tidb_dml_type="bulk"` to insert rows in bulk mode.
		//    DupKeyCheckLazy should be used to improve the performance.
		// - The current transaction is pessimistic.
		//   The duplicate key check can be postponed to the lock stage.
		// Please notice that for optimistic transaction, it always returns `DupKeyCheckInPlace` even if
		// `tidb_constraint_check_in_place` is `OFF`.
		// That is because `tidb_constraint_check_in_place` is only designed for insert cases, see comments in issue:
		// https://github.com/pingcap/tidb/issues/54492#issuecomment-2229941881
		return table.DupKeyCheckLazy
	}

	return table.DupKeyCheckInPlace
}