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Merged
ti-chi-bot merged 7 commits into from
Aug 14, 2025
Merged

system-variables: add optimizer cost factors #20748

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c053265
planner: add optimizer cost factors
terry1purcell Apr 13, 2025
b286347
Update system-variables.md
terry1purcell Apr 13, 2025
f0502d0
typo
terry1purcell Apr 25, 2025
689ea88
Merge branch 'costfactors' of github.com:terry1purcell/docs into cost…
terry1purcell Apr 25, 2025
27def08
system variables: add v8.5.3 where applicable and revise wording
Oreoxmt Jul 15, 2025
9c0d81e
Update wording and capitalization
lilin90 Aug 13, 2025
a9cb004
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 @@ -3932,7 +3932,7 @@
- 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 @@ -3942,7 +3942,7 @@
- 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 @@ -3976,7 +3976,7 @@
- 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 @@ -3995,7 +3995,7 @@
- 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 @@ -4005,7 +4005,7 @@
- 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 @@ -4210,7 +4210,7 @@
- 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 @@ -4229,7 +4229,7 @@
- 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 @@ -4633,7 +4633,7 @@
- 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 @@ -4643,7 +4643,7 @@
- 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 @@ -4698,6 +4698,210 @@
- 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>

> **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_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>

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[vale] reported by reviewdog 🐶
[PingCAP.Ambiguous] Consider using a clearer word than 'many' because it may cause confusion.

Raw Output:
{"message": "[PingCAP.Ambiguous] Consider using a clearer word than 'many' because it may cause confusion.", "location": {"path": "system-variables.md", "range": {"start": {"line": 4893, "column": 350}}}, "severity": "INFO"}

> **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