sysvar: add system variable tidb_opt_enable_hash_join (#14905)

optimizer-hints.md

@@ -938,3 +938,27 @@ EXPLAIN SELECT /*+ leading(t1, t3), inl_join(t3) */ * FROM t1, t2, t3 WHERE t1.i
 +---------------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------------------------------------------+
 9 rows in set (0.01 sec)
 ```
+
+### Using hints causes the `Can't find a proper physical plan for this query` error
+
+The `Can't find a proper physical plan for this query` error might occur in the following scenarios:
+
+- A query itself does not require reading indexes in order. That is, for this query, the optimizer does not generate a plan to read indexes in order in any case without using hints. In this case, if the `ORDER_INDEX` hint is specified, this error occurs. To resolve this issue, remove the corresponding `ORDER_INDEX` hint.
+- A query excludes all possible join methods by using the `NO_JOIN` related hints.
+
+```sql
+CREATE TABLE t1 (a INT);
+CREATE TABLE t2 (a INT);
+EXPLAIN SELECT /*+ NO_HASH_JOIN(t1), NO_MERGE_JOIN(t1) */ * FROM t1, t2 WHERE t1.a=t2.a;
+ERROR 1815 (HY000): Internal : Can't find a proper physical plan for this query
+```
+
+- The system variable [`tidb_opt_enable_hash_join`](/system-variables.md#tidb_opt_enable_hash_join-new-in-v712) is set to `OFF`, and all other join types are also excluded.
+
+```sql
+CREATE TABLE t1 (a INT);
+CREATE TABLE t2 (a INT);
+set tidb_opt_enable_hash_join=off;
+EXPLAIN SELECT /*+ NO_MERGE_JOIN(t1) */ * FROM t1, t2 WHERE t1.a=t2.a;
+ERROR 1815 (HY000): Internal : Can't find a proper physical plan for this query
+```

system-variables.md

@@ -3449,6 +3449,15 @@ mysql> desc select count(distinct a) from test.t;
 - Default value: `ON`
 - This variable is used to control whether the optimizer estimates the number of rows based on column order correlation
 
+### tidb_opt_enable_hash_join <span class="version-mark">New in v7.1.2</span>
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: Yes
+- Type: Boolean
+- Default value: `ON`
+- This variable is used to control whether the optimizer selects hash joins for tables. The value is `ON` by default. If it is set to `OFF`, the optimizer avoids selecting hash joins when generating execution plans, unless no other join algorithm is available.
+- If both the system variable `tidb_opt_enable_hash_join` and the `HASH_JOIN` hint are configured, the `HASH_JOIN` hint takes precedence. Even if `tidb_opt_enable_hash_join` is set to `OFF`, when you specify a `HASH_JOIN` hint in a query, the TiDB optimizer still enforces a hash join plan.
+
 ### tidb_opt_enable_late_materialization <span class="version-mark">New in v7.0.0</span>
 
 - Scope: SESSION | GLOBAL