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planner: Add new variable named "tidb_opt_selectivity_factor" #21655

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planner: Add new variable named "tidb_opt_skew_distinct_agg"
elsa0520 Aug 26, 2025
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Update system-variables.md
elsa0520 Aug 26, 2025
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10 changes: 10 additions & 0 deletions system-variables.md
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Expand Up @@ -1784,6 +1784,16 @@ mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;
- This variable is renamed from the variable [`tidb_ddl_version`](https://docs-archive.pingcap.com/tidb/v7.6/system-variables#tidb_ddl_version-new-in-v760) that is introduced in v7.6.0. Starting from v8.0.0, `tidb_ddl_version` no longer takes effect.
- Starting from TiDB v8.5.0, the accelerated table creation feature is enabled by default for newly created clusters, with `tidb_enable_fast_create_table` set to `ON`. For clusters upgraded from v8.4.0 or earlier versions, the default value of `tidb_enable_fast_create_table` remains unchanged.

### tidb_opt_selectivity_factor

- Scope: SESSION | GLOBAL
- Persists to cluster: Yes
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Float
- Range: `[0, 1]`
- Default value: `0.8`
- This variable specifies the pseudo selectivity for the TiDB optimizer. In some cases where the optimizer cannot deduce the selectivity of a predicate from statistical information, this pseudo selectivity value is used. It is **NOT** recommended that you modify this value.

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

- Scope: SESSION | GLOBAL
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