add tidb_opt_ordering_index_selectivity_ratio in release notes 8.0.0

releases/release-8.0.0.md

@@ -333,6 +333,7 @@ Quick access: [Quick start](https://docs.pingcap.com/tidb/v8.0/quick-start-with-
 | [`tidb_enable_fast_create_table`](/system-variables.md#tidb_enable_fast_create_table-new-in-v800) | Newly added | Controls whether to enable [TiDB Accerates Table Creation](/accelerated-table-creation.md). Set the value to `ON` to enable it and `OFF` to disable it. The default value is `ON`. When this variable is enabled, TiDB accelerates table creation by using [`CREATE TABLE`](/sql-statements/sql-statement-create-table.md). |
 | [`tidb_load_binding_timeout`](/system-variables.md#tidb_load_binding_timeout-new-in-v800) | Newly added | Controls the timeout of loading bindings. If the execution time of loading bindings exceeds this value, the loading will stop. |
 | [`tidb_low_resolution_tso_update_interval`](/system-variables.md#tidb_low_resolution_tso_update_interval-new-in-v800) | Newly added | Controls the interval for updating TiDB [cache timestamp](/system-variables.md#tidb_low_resolution_tso). |
+| [`tidb_opt_ordering_index_selectivity_ratio`](/system-variables.md#tidb_opt_ordering_index_selectivity_ratio-new-in-v800) | Newly added | Controls the estimated number of rows for an index that matches a SQL statements `ORDER BY` when there are `ORDER BY` and `LIMIT` clauses with filter conditions that are not covered by the index. The default value is `-1`, which means to disable this system variable. |
 | [`tidb_opt_use_invisible_indexes`](/system-variables.md#tidb_opt_use_invisible_indexes-new-in-v800) | Newly added | Controls whether the optimizer can select [invisible indexes](/sql-statements/sql-statement-create-index.md#invisible-index) for query optimization in the current session. When the variable is set to `ON`, the optimizer can select invisible indexes for query optimization in the session. |
 | [`tidb_schema_cache_size`](/system-variables.md#tidb_schema_cache_size-new-in-v800)  | Newly added | Controls the upper limit of memory that can be used for caching the schema information to avoid occupying too much memory. When this feature is enabled, the LRU algorithm is used to cache the required tables, effectively reducing the memory occupied by the schema information.    |