update doc for IndexMerge GA (pingcap#7348)

explain-index-merge.md

@@ -36,10 +36,10 @@ For the scan operation that is performed on a specific range of data, such as `I
 
 > **Note:**
 >
-> At present, the `IndexMerge` feature is disabled by default in TiDB 4.0.0-rc.1. In addition, the currently supported scenarios of `IndexMerge` in TiDB 4.0 are limited to the disjunctive normal form (expressions connected by `or`). The conjunctive normal form (expressions connected by `and`) will be supported in later versions. Enable the `IndexMerge` in one of two ways:
+> - The Index Merge feature is enabled by default from v5.4.0. That is, [`tidb_enable_index_merge`](/system-variables.md#tidb_enable_index_merge-new-in-v40) is `ON`.
 >
-> - Set `tidb_enable_index_merge=1`;
+> - You can use the SQL hint [`USE_INDEX_MERGE`](/optimizer-hints.md#use_index_merget1_name-idx1_name--idx2_name-) to force the optimizer to apply Index Merge, regardless of the setting of `tidb_enable_index_merge`. To enable Index Merge when the filtering conditions contain expressions that cannot be pushed down, you must use the SQL hint [`USE_INDEX_MERGE`](/optimizer-hints.md#use_index_merget1_name-idx1_name--idx2_name-).
 >
-> - Use the SQL Hint [`USE_INDEX_MERGE`](/optimizer-hints.md#use_index_merget1_name-idx1_name--idx2_name-) in the query.
+> - Index Merge supports only disjunctive normal form (expressions connected by `or`) and does not support conjunctive normal form (expressions connected by `and`).
 >
-> SQL Hint has a higher priority than system variables.
+> - Index Merge is not supported in [tempoaray tables](/temporary-tables.md) for now.

optimizer-hints.md

@@ -236,7 +236,6 @@ When multiple `USE_INDEX_MERGE` hints are made to the same table, the optimizer
 This hint takes effect on strict conditions, including:
 
 - If the query can select a single index scan in addition to full table scan, the optimizer does not select index merge.
-- If the query is in an explicit transaction, and if the statements before this query has already written data, the optimizer does not select index merge.
 
 ## Hints that take effect in the whole query
 
@@ -262,7 +261,8 @@ In addition to this hint, setting the `tidb_enable_index_merge` system variable
 
 > **Note:**
 >
-> `NO_INDEX_MERGE` has a higher priority over `USE_INDEX_MERGE`. When both hints are used, `USE_INDEX_MERGE` does not take effect.
+> - `NO_INDEX_MERGE` has a higher priority over `USE_INDEX_MERGE`. When both hints are used, `USE_INDEX_MERGE` does not take effect. 
+> - For a subquery, `NO_INDEX_MERGE` only takes effect when it is placed at the outermost level of the subquery.
 
 ### USE_TOJA(boolean_value)
 

system-variables.md

@@ -758,9 +758,17 @@ Constraint checking is always performed in place for pessimistic transactions (d
 ### tidb_enable_index_merge <span class="version-mark">New in v4.0</span>
 
 - Scope: SESSION | GLOBAL
-- Default value: `OFF`
+- Default value: `ON`
 - This variable is used to control whether to enable the index merge feature.
 
+> **Note:**
+>
+> - After upgrading a TiDB cluster from versions earlier than v4.0.0 to v5.4.0 or later, this variable is disabled by default to prevent performance regression due to changes of execution plans.
+>
+> - After upgrading a TiDB cluster from v4.0.0 or later to v5.4.0 or later, this variable remains the setting before the upgrade.
+>
+> - Since v5.4.0, for a newly deployed TiDB cluster, this variable is enabled by default.
+
 ### tidb_enable_list_partition <span class="version-mark">New in v5.0</span>
 
 > **Warning:**