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Update the description of tidb_distsql_scan_concurrency (#4401) #4424

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Update the description of tidb_distsql_scan_concurrency (#4401) #4424

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58471e4
cherry pick #4401 to release-4.0
CharLotteiu Dec 17, 2020
ad265b8
Update system-variables.md
TomShawn Dec 17, 2020
23fe102
Update system-variables.md
TomShawn Dec 17, 2020
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19 changes: 19 additions & 0 deletions system-variables.md
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Expand Up @@ -254,6 +254,25 @@ Constraint checking is always performed in place for pessimistic transactions (d

For more details, see [limits of retry](/optimistic-transaction.md#limits-of-retry).

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### tidb_distsql_scan_concurrency

- Scope: SESSION | GLOBAL
- Default value: 15
- This variable is used to set the concurrency of the `scan` operation.
- Use a bigger value in OLAP scenarios, and a smaller value in OLTP scenarios.
- For OLAP scenarios, the maximum value cannot exceed the number of CPU cores of all the TiKV nodes.
- If a table has a lot of partitions, you can reduce the variable value appropriately to avoid TiKV becoming out of memory (OOM).

### tidb_dml_batch_size

- Scope: SESSION | GLOBAL
- Default value: 0
- When this value is greater than `0`, TiDB will batch commit statements such as `INSERT` or `LOAD DATA` into smaller transactions. This reduces memory usage and helps ensure that the `txn-total-size-limit` is not reached by bulk modifications.
- Only the value `0` provides ACID compliance. Setting this to any other value will break the atomicity and isolation guarantees of TiDB.

>>>>>>> e5197085... Update the description of `tidb_distsql_scan_concurrency` (#4401)
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### `tidb_enable_amend_pessimistic_txn` <span class="version-mark">New in v4.0.7</span>

- Scope: SESSION | GLOBAL
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