optimizer-fix-controls: Add optimizer fix control 47400 for controlling zero estRows (#18587)

Author: terry1purcell

optimizer-fix-controls.md

@@ -71,6 +71,14 @@ SET SESSION tidb_opt_fix_control = '44262:ON,44389:ON';
 - This variable sets the threshold for the optimizer's heuristic strategy to select access paths. If the estimated rows for an access path (such as `Index_A`) is much smaller than that of other access paths (default `1000` times), the optimizer skips the cost comparison and directly selects `Index_A`.
 - `0` means to disable this heuristic strategy.
 
+### [`47400`](https://github.com/pingcap/tidb/issues/47400) <span class="version-mark">New in v8.4.0</span>
+
+- 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.
+- `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>
 
 - Default value: `OFF`