Auto-sync: Update English docs from Chinese PR qiancai/docs-cn#15

  Synced from: qiancai/docs-cn#15
  Target PR: #50
  AI Provider: gemini

  Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

system-variables.md

@@ -1791,6 +1791,16 @@ Assume that you have a cluster with 4 TiDB nodes and multiple TiKV nodes. In thi
 - 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` <span class="version-mark">Introduced in v9.0.0</span>
+
+- Scope: SESSION | GLOBAL
+- Is persisted to the cluster: Yes
+- Is controlled by the Hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
+- Type: Floating-point number
+- Value range: `[0, 1]`
+- Default value: `0.8`
+- This variable is used to specify the default selectivity of the TiDB optimizer. In some cases, when the optimizer cannot derive the predicate selectivity based on statistics, the optimizer uses this default selectivity as an alternative value. **It is not recommended** to modify this value.
+
 ### tidb_default_string_match_selectivity <span class="version-mark">New in v6.2.0</span>
 
 - Scope: SESSION | GLOBAL