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update system-variables.md: add mpp_version and mpp_exchange_compres.. (
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TomShawn authored Feb 10, 2023
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30 changes: 30 additions & 0 deletions explain-mpp.md
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Expand Up @@ -170,3 +170,33 @@ EXPLAIN ANALYZE SELECT COUNT(*) FROM t1 GROUP BY id;
```

Compared to the output of `EXPLAIN`, the `operator info` column of the operator `ExchangeSender` also shows `tasks`, which records the id of the MPP task that the query fragment instantiates into. In addition, each MPP operator has a `threads` field in the `execution info` column, which records the concurrency of operations when TiDB executes this operator. If the cluster consists of multiple nodes, this concurrency is the result of adding up the concurrency of all nodes.

## MPP version and exchange data compression

Starting from v6.6.6, the new fields `MPPVersion` and `Compression` are added to the MPP execution plan.

- `MppVersion`: The version number of the MPP execution plan, which can be set through the system variable [`mpp_version`](/system-variables.md#mpp_version-new-in-v660).
- `Compression`: The data compression mode of the `Exchange` operator, which can be set through the system variable [`mpp_exchange_compression_mode`](/system-variables.md#mpp_exchange_compression_mode-new-in-v660). If data compression is not enabled, this field is not displayed in the execution plan.

See the following example:

```sql
mysql > EXPLAIN SELECT COUNT(*) AS count_order FROM lineitem GROUP BY l_returnflag, l_linestatus ORDER BY l_returnflag, l_linestatus;

+----------------------------------------+--------------+--------------+----------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+----------------------------------------+--------------+--------------+----------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Projection_6 | 3.00 | root | | Column#18 |
| └─Sort_8 | 3.00 | root | | tpch100.lineitem.l_returnflag, tpch100.lineitem.l_linestatus |
| └─TableReader_36 | 3.00 | root | | MppVersion: 1, data:ExchangeSender_35 |
| └─ExchangeSender_35 | 3.00 | mpp[tiflash] | | ExchangeType: PassThrough |
| └─Projection_31 | 3.00 | mpp[tiflash] | | Column#18, tpch100.lineitem.l_returnflag, tpch100.lineitem.l_linestatus |
| └─HashAgg_32 | 3.00 | mpp[tiflash] | | group by:tpch100.lineitem.l_linestatus, tpch100.lineitem.l_returnflag, funcs:sum(Column#23)->Column#18, funcs:firstrow(tpch100.lineitem.l_returnflag)->tpch100.lineitem.l_returnflag, funcs:firstrow(tpch100.lineitem.l_linestatus)->tpch100.lineitem.l_linestatus, stream_count: 20 |
| └─ExchangeReceiver_34 | 3.00 | mpp[tiflash] | | stream_count: 20 |
| └─ExchangeSender_33 | 3.00 | mpp[tiflash] | | ExchangeType: HashPartition, Compression: FAST, Hash Cols: [name: tpch100.lineitem.l_returnflag, collate: utf8mb4_bin], [name: tpch100.lineitem.l_linestatus, collate: utf8mb4_bin], stream_count: 20 |
| └─HashAgg_14 | 3.00 | mpp[tiflash] | | group by:tpch100.lineitem.l_linestatus, tpch100.lineitem.l_returnflag, funcs:count(1)->Column#23 |
| └─TableFullScan_30 | 600037902.00 | mpp[tiflash] | table:lineitem | keep order:false |
+----------------------------------------+--------------+--------------+----------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
```

In the preceding execution plan result, TiDB uses an MPP execution plan of version `1` to build `TableReader`. The `ExchangeSender` operator of the `HashPartition` type uses the `FAST` data compression mode. Data compression is not enabled for the `ExchangeSender` operator of the `PassThrough` type.
23 changes: 23 additions & 0 deletions system-variables.md
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Expand Up @@ -394,6 +394,29 @@ This variable is an alias for [`last_insert_id`](#last_insert_id).
- Range: `[0, 4294967295]`
- This variable is used to establish a password reuse policy that allows TiDB to limit password reuse based on the number of password changes. The default value `0` means disabling the password reuse policy based on the number of password changes. When this variable is set to a positive integer `N`, the reuse of the last `N` passwords is not allowed.

### mpp_exchange_compression_mode <span class="version-mark">New in v6.6.0</span>

- Scope: SESSION | GLOBAL
- Persists to cluster: Yes
- Default value: `UNSPECIFIED`
- Value options: `NONE`, `FAST`, `HIGH_COMPRESSION`, `UNSPECIFIED`
- This variable is used to specify the data compression mode of the MPP Exchange operator. This variable takes effect when TiDB selects the MPP execution plan with the version number `1`. The meanings of the variable values are as follows:
- `UNSPECIFIED`: means unspecified. TiDB will automatically select the compression mode. Currently, TiDB automatically selects the `FAST` mode.
- `NONE`: no data compression is used.
- `FAST`: fast mode. The overall performance is good and the compression ratio is less than `HIGH_COMPRESSION`.
- `HIGH_COMPRESSION`: the high compression ratio mode.

### mpp_version <span class="version-mark">New in v6.6.0</span>

- Scope: SESSION | GLOBAL
- Persists to cluster: Yes
- Default value: `UNSPECIFIED`
- Value options: `UNSPECIFIED`, `0`, `1`
- This variable is used to specify different versions of the MPP execution plan. After a version is specified, TiDB selects the specified version of the MPP execution plan. The meanings of the variable values are as follows:
- `UNSPECIFIED`: means unspecified. TiDB automatically selects the latest version `1`.
- `0`: compatible with all TiDB cluster versions. Features with the MPP version greater than `0` do not take effect in this mode.
- `1`: new in v6.6.0, used to enable data exchange with compression on TiFlash. For details, see [MPP version and exchange data compression](/explain-mpp.md#mpp-version-and-exchange-data-compression).

### password_reuse_interval <span class="version-mark">New in v6.5.0</span>

- Scope: GLOBAL
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