update "DB2" to "Db2" (#20411)

faq/migration-tidb-faq.md

@@ -81,9 +81,9 @@ You can use the following methods to export the data in TiDB:
 - Export data using mysqldump and the `WHERE` clause.
 - Use the MySQL client to export the results of `select` to a file.
 
-### How to migrate from DB2 or Oracle to TiDB?
+### How to migrate from Db2 or Oracle to TiDB?
 
-To migrate all the data or migrate incrementally from DB2 or Oracle to TiDB, see the following solution:
+To migrate all the data or migrate incrementally from Db2 or Oracle to TiDB, see the following solution:
 
 - Use the official migration tool of Oracle, such as OGG, Gateway, CDC (Change Data Capture).
 - Develop a program for importing and exporting data.

optimizer-fix-controls.md

@@ -88,7 +88,7 @@ SET SESSION tidb_opt_fix_control = '44262:ON,44389:ON';
 - Default value: `ON`
 - Possible values: `ON`, `OFF`
 - Due to challenges in accurately estimating the number of qualified rows for each plan step in a query plan, the optimizer might estimate a smaller value for `estRows`. This variable controls whether to limit the minimum value of `estRows`.
-- `ON`: limits the minimum value of `estRows` to 1, which is the new behavior introduced in v8.4.0 and is consistent with other databases, such as Oracle and DB2.
+- `ON`: limits the minimum value of `estRows` to 1, which is the new behavior introduced in v8.4.0 and is consistent with other databases, such as Oracle and Db2.
 - `OFF`: disables the minimum row estimate limit, which maintains the behavior consistent with versions before v8.4.0. In this case, `estRows` might be zero.
 
 ### [`52592`](https://github.com/pingcap/tidb/issues/52592) <span class="version-mark">New in v8.4.0</span>

releases/release-8.4.0.md

@@ -371,7 +371,7 @@ The following features are planned for deprecation in future versions:
     - Optimize the efficiency of constructing BatchCop tasks when scanning a large amount of data [#55915](https://github.com/pingcap/tidb/issues/55915) [#55413](https://github.com/pingcap/tidb/issues/55413) @[wshwsh12](https://github.com/wshwsh12)
     - Optimize the transaction's buffer to reduce write latency in transactions and TiDB CPU usage [#55287](https://github.com/pingcap/tidb/issues/55287) @[you06](https://github.com/you06)
     - Optimize the execution performance of DML statements when the system variable `tidb_dml_type` is set to `"bulk"` [#50215](https://github.com/pingcap/tidb/issues/50215) @[ekexium](https://github.com/ekexium)
-    - Support using [Optimizer Fix Control 47400](/optimizer-fix-controls.md#47400-new-in-v840) to control whether the optimizer limits the minimum value estimated for `estRows` to `1`, which is consistent with databases such as Oracle and DB2 [#47400](https://github.com/pingcap/tidb/issues/47400) @[terry1purcell](https://github.com/terry1purcell)
+    - Support using [Optimizer Fix Control 47400](/optimizer-fix-controls.md#47400-new-in-v840) to control whether the optimizer limits the minimum value estimated for `estRows` to `1`, which is consistent with databases such as Oracle and Db2 [#47400](https://github.com/pingcap/tidb/issues/47400) @[terry1purcell](https://github.com/terry1purcell)
     - Add write control to the [`mysql.tidb_runaway_queries`](/mysql-schema/mysql-schema.md#system-tables-related-to-runaway-queries) log table to reduce overhead caused by a large number of concurrent writes [#54434](https://github.com/pingcap/tidb/issues/54434) @[HuSharp](https://github.com/HuSharp)
     - Support Index Join by default when the inner table has `Selection`, `Projection`, or `Aggregation` operators on it [#47233](https://github.com/pingcap/tidb/issues/47233) @[winoros](https://github.com/winoros)
     - Reduce the number of column details fetched from TiKV for `DELETE` operations in certain scenarios, lowering the resource overhead of these operations [#38911](https://github.com/pingcap/tidb/issues/38911) @[winoros](https://github.com/winoros)