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Merged
merged 5 commits into from
Aug 14, 2025
Merged

system-variables: add optimizer cost factors (#20748) #21602

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9fc5a02
planner: add optimizer cost factors
terry1purcell Apr 13, 2025
8c92cdf
Update system-variables.md
terry1purcell Apr 13, 2025
656a366
system variables: add v8.5.3 where applicable and revise wording
Oreoxmt Jul 15, 2025
f9d59f7
Update wording and capitalization
lilin90 Aug 13, 2025
3d5a26f
Update wording
lilin90 Aug 13, 2025
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222 changes: 213 additions & 9 deletions system-variables.md
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Original file line number Diff line number Diff line change
Expand Up @@ -4114,7 +4114,7 @@ As shown in this diagram, when [`tidb_enable_paging`](#tidb_enable_paging-new-in
- Type: Float
- Range: `[0, 18446744073709551615]`
- Default value: `3.0`
- Indicates the CPU cost of starting a Golang goroutine in TiDB. This variable is internally used in the [Cost Model](/cost-model.md), and it is **NOT** recommended to modify its value.
- Indicates the CPU cost of starting a Golang goroutine in TiDB. This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

### tidb_opt_copcpu_factor

Expand All @@ -4124,7 +4124,7 @@ As shown in this diagram, when [`tidb_enable_paging`](#tidb_enable_paging-new-in
- Type: Float
- Range: `[0, 18446744073709551615]`
- Default value: `3.0`
- Indicates the CPU cost for TiKV Coprocessor to process one row. This variable is internally used in the [Cost Model](/cost-model.md), and it is **NOT** recommended to modify its value.
- Indicates the CPU cost for TiKV Coprocessor to process one row. This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

### tidb_opt_correlation_exp_factor

Expand Down Expand Up @@ -4158,7 +4158,7 @@ As shown in this diagram, when [`tidb_enable_paging`](#tidb_enable_paging-new-in
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `3.0`
- Indicates the CPU cost for TiDB to process one row. This variable is internally used in the [Cost Model](/cost-model.md), and it is **NOT** recommended to modify its value.
- Indicates the CPU cost for TiDB to process one row. This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

### `tidb_opt_derive_topn` <span class="version-mark">New in v7.0.0</span>

Expand All @@ -4177,7 +4177,7 @@ As shown in this diagram, when [`tidb_enable_paging`](#tidb_enable_paging-new-in
- Type: Float
- Range: `[0, 18446744073709551615]`
- Default value: `3.0`
- Indicates the cost for TiKV to scan one row from the disk in descending order. This variable is internally used in the [Cost Model](/cost-model.md), and it is **NOT** recommended to modify its value.
- Indicates the cost for TiKV to scan one row from the disk in descending order. This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

### tidb_opt_disk_factor

Expand All @@ -4187,7 +4187,7 @@ As shown in this diagram, when [`tidb_enable_paging`](#tidb_enable_paging-new-in
- Type: Float
- Range: `[0, 18446744073709551615]`
- Default value: `1.5`
- Indicates the I/O cost for TiDB to read or write one byte of data from or to the temporary disk. This variable is internally used in the [Cost Model](/cost-model.md), and it is **NOT** recommended to modify its value.
- Indicates the I/O cost for TiDB to read or write one byte of data from or to the temporary disk. This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

### tidb_opt_distinct_agg_push_down

Expand Down Expand Up @@ -4392,7 +4392,7 @@ mysql> desc select count(distinct a) from test.t;
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `0.001`
- Indicates the memory cost for TiDB to store one row. This variable is internally used in the [Cost Model](/cost-model.md), and it is **NOT** recommended to modify its value.
- Indicates the memory cost for TiDB to store one row. This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

### tidb_opt_mpp_outer_join_fixed_build_side <span class="version-mark">New in v5.1.0</span>

Expand All @@ -4411,7 +4411,7 @@ mysql> desc select count(distinct a) from test.t;
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `1.0`
- Indicates the net cost of transferring 1 byte of data through the network. This variable is internally used in the [Cost Model](/cost-model.md), and it is **NOT** recommended to modify its value.
- Indicates the net cost of transferring 1 byte of data through the network. This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

### tidb_opt_objective <span class="version-mark">New in v7.4.0</span>

Expand Down Expand Up @@ -4825,7 +4825,7 @@ SHOW WARNINGS;
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `1.5`
- Indicates the cost for TiKV to scan one row of data from the disk in ascending order. This variable is internally used in the [Cost Model](/cost-model.md), and it is **NOT** recommended to modify its value.
- Indicates the cost for TiKV to scan one row of data from the disk in ascending order. This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

### tidb_opt_seek_factor

Expand All @@ -4835,7 +4835,7 @@ SHOW WARNINGS;
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `20`
- Indicates the start-up cost for TiDB to request data from TiKV. This variable is internally used in the [Cost Model](/cost-model.md), and it is **NOT** recommended to modify its value.
- Indicates the start-up cost for TiDB to request data from TiKV. This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

### tidb_opt_skew_distinct_agg <span class="version-mark">New in v6.2.0</span>

Expand Down Expand Up @@ -4890,6 +4890,210 @@ SHOW WARNINGS;
- Default value: `OFF`
- This variable is used to control whether to allow `INSERT`, `REPLACE`, and `UPDATE` statements to operate on the `_tidb_rowid` column. This variable can be used only when you import data using TiDB tools.

### tidb_opt_hash_agg_cost_factor <span class="version-mark">New in v8.5.3 and v9.0.0</span>
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low

For consistency with other variable headings in this document and to adhere to the style guide, please wrap the system variable name in backticks.1

This suggestion applies to all new tidb_opt_*_cost_factor variables added in this pull request.

Suggested change
### tidb_opt_hash_agg_cost_factor <span class="version-mark">New in v8.5.3 and v9.0.0</span>
### `tidb_opt_hash_agg_cost_factor` <span class="version-mark">New in v8.5.3 and v9.0.0</span>

Style Guide References

Footnotes

  1. The style guide specifies that code snippets, command names, options, and paths should be enclosed in backticks. (link) ↩


> **Warning:**
>
> This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `1`
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medium

To improve clarity and completeness, please add a brief description of what this variable controls. This comment applies to all the new tidb_opt_*_cost_factor variables added in this pull request.1

For example, for tidb_opt_hash_agg_cost_factor, you could add the following bullet point to its property list:

- This variable is used to adjust the cost of the HashAgg operator.

Style Guide References

Footnotes

  1. Documentation should be clear, simple, and complete. (link) ↩


### tidb_opt_hash_join_cost_factor <span class="version-mark">New in v8.5.3 and v9.0.0</span>

> **Warning:**
>
> This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `1`

### tidb_opt_index_join_cost_factor <span class="version-mark">New in v8.5.3 and v9.0.0</span>

> **Warning:**
>
> This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `1`

### tidb_opt_index_lookup_cost_factor <span class="version-mark">New in v8.5.3 and v9.0.0</span>

> **Warning:**
>
> This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `1`

### tidb_opt_index_merge_cost_factor <span class="version-mark">New in v8.5.3 and v9.0.0</span>

> **Warning:**
>
> This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `1`

### tidb_opt_index_reader_cost_factor <span class="version-mark">New in v8.5.3 and v9.0.0</span>

> **Warning:**
>
> This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `1`

### tidb_opt_index_scan_cost_factor <span class="version-mark">New in v8.5.3 and v9.0.0</span>

> **Warning:**
>
> This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `1`

### tidb_opt_limit_cost_factor <span class="version-mark">New in v8.5.3 and v9.0.0</span>

> **Warning:**
>
> This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `1`

### tidb_opt_merge_join_cost_factor <span class="version-mark">New in v8.5.3 and v9.0.0</span>

> **Warning:**
>
> This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `1`

### tidb_opt_sort_cost_factor <span class="version-mark">New in v8.5.3 and v9.0.0</span>

> **Warning:**
>
> This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `1`

### tidb_opt_stream_agg_cost_factor <span class="version-mark">New in v8.5.3 and v9.0.0</span>

> **Warning:**
>
> This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `1`

### tidb_opt_table_full_scan_cost_factor <span class="version-mark">New in v8.5.3 and v9.0.0</span>

> **Warning:**
>
> This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `1`

### tidb_opt_table_range_scan_cost_factor <span class="version-mark">New in v8.5.3 and v9.0.0</span>

> **Warning:**
>
> This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `1`

### tidb_opt_table_reader_cost_factor <span class="version-mark">New in v8.5.3 and v9.0.0</span>

> **Warning:**
>
> This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `1`

### tidb_opt_table_rowid_cost_factor <span class="version-mark">New in v8.5.3 and v9.0.0</span>

> **Warning:**
>
> This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `1`

### tidb_opt_table_tiflash_scan_cost_factor <span class="version-mark">New in v8.5.3 and v9.0.0</span>

> **Warning:**
>
> This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `1`

### tidb_opt_topn_cost_factor <span class="version-mark">New in v8.5.3 and v9.0.0</span>

> **Warning:**
>
> This variable is used internally by the [cost model](/cost-model.md), and it is **NOT** recommended to modify its value.

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Float
- Range: `[0, 2147483647]`
- Default value: `1`

### tidb_optimizer_selectivity_level

- Scope: SESSION
Expand Down
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