reference/performance: move optimizer hints to proper section

reference/performance/optimizer-hints.md

@@ -182,6 +182,34 @@ The `USE_INDEX_MERGE(t1_name, idx1_name [, idx2_name ...])` hint tells the optim
 select /*+ USE_INDEX_MERGE(t1, idx_a, idx_b, idx_c) */ * from t t1 where t1.a > 10 or t1.b > 10;
 ```
 
+### NO_INDEX_MERGE()
+
+The `NO_INDEX_MERGE()` hint disables the index merge feature of the optimizer.
+
+For example, the following query will not use index merge:
+
+{{< copyable "sql" >}}
+
+```sql
+select /*+ NO_INDEX_MERGE() */ * from t where t.a > 0 or t.b > 0;
+```
+
+In addition to this hint, setting the `tidb_enable_index_merge` environment variable also controls whether to enable this feature.
+
+### USE_TOJA(boolean_value)
+
+The `boolean_value` parameter can be `TRUE` or `FALSE`. The `USE_TOJA(TRUE)` hint enables the optimizer to convert an `in` condition (containing a sub-query) to join and aggregation operations. Comparatively, the `USE_TOJA(FALSE)` hint disables this feature.
+
+For example, the following query will convert `in (select t2.a from t2) subq` to corresponding join and aggregation operations:
+
+{{< copyable "sql" >}}
+
+```sql
+select /*+ USE_TOJA(TRUE) */ t1.a, t1.b from t1 where t1.a in (select t2.a from t2) subq;
+```
+
+In addition to this hint, setting the `tidb_opt_insubq_to_join_and_agg` environment variable also controls whether to enable this feature.
+
 ## Hints for setting operation parameters
 
 A hint for setting operation parameters follows behind **the first** `SELECT`, `UPDATE` or `DELETE` keyword in a SQL statement. A hint of this category modifies the operation parameter of the query to which this hint applies.
@@ -225,31 +253,3 @@ select /*+ READ_FROM_REPLICA() */ * from t;
 ```
 
 In addition to this hint, setting the `tidb_replica_read` environment variable to `'follower'` or `'leader'` also controls whether to enable this feature.
-
-### NO_INDEX_MERGE()
-
-The `NO_INDEX_MERGE()` hint disables the index merge feature of the optimizer.
-
-For example, the following query will not use index merge:
-
-{{< copyable "sql" >}}
-
-```sql
-select /*+ NO_INDEX_MERGE() */ * from t where t.a > 0 or t.b > 0;
-```
-
-In addition to this hint, setting the `tidb_enable_index_merge` environment variable also controls whether to enable this feature.
-
-### USE_TOJA(boolean_value)
-
-The `boolean_value` parameter can be `TRUE` or `FALSE`. The `USE_TOJA(TRUE)` hint enables the optimizer to convert an `in` condition (containing a sub-query) to join and aggregation operations. Comparatively, the `USE_TOJA(FALSE)` hint disables this feature.
-
-For example, the following query will convert `in (select t2.a from t2) subq` to corresponding join and aggregation operations:
-
-{{< copyable "sql" >}}
-
-```sql
-select /*+ USE_TOJA(TRUE) */ t1.a, t1.b from t1 where t1.a in (select t2.a from t2) subq;
-```
-
-In addition to this hint, setting the `tidb_opt_insubq_to_join_and_agg` environment variable also controls whether to enable this feature.