planner: add variable tidb_enable_inl_join_inner_multi_pattern (#12944)…

… (#13296)

* planner: add variable tidb_enable_inl_join_inner_multi_pattern

Signed-off-by: Aolin <aolin.zhang@pingcap.com>

* Apply suggestions from code review

* Apply suggestions from code review

* Update system-variables.md

---------

Signed-off-by: Aolin <aolin.zhang@pingcap.com>
Co-authored-by: Aolin <aolin.zhang@pingcap.com>
Co-authored-by: Ran <huangran.alex@gmail.com>

system-variables.md

@@ -1683,6 +1683,14 @@ MPP is a distributed computing framework provided by the TiFlash engine, which a
     - If the TiDB version before the upgrade is earlier than v6.1.0, the default value of this variable after the upgrade is `ON`.
     - If the TiDB version before the upgrade is v6.1.0 or later, the default value of the variable after the upgrade follows the value before the upgrade.
 
+### `tidb_enable_inl_join_inner_multi_pattern` <span class="version-mark">New in v6.5.2</span>
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: Yes
+- Type: Boolean
+- Default value: `OFF`
+- This variable controls whether Index Join is supported when the inner table has `Selection` or `Projection` operators on it. The default value `OFF` means that Index Join is not supported in this scenario.
+
 ### tidb_enable_ordered_result_mode
 
 - Scope: SESSION | GLOBAL