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update several statements (#3309)
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Zejun Li authored May 25, 2020
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12 changes: 11 additions & 1 deletion sql-statements/sql-statement-do.md
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Expand Up @@ -7,7 +7,13 @@ aliases: ['/docs-cn/dev/reference/sql/statements/do/']

# DO

`DO` 语句用于执行表达式，而不返回结果。在 MySQL 中，`DO` 的一个常见用例是执行存储的程序，而无需处理结果。但是 TiDB 不提供存储例程，因此该功能的使用较为受限。
`DO` 语句用于执行表达式，但不返回任何结果。大部分情况下，`DO` 相当于不返回结果的 `SELECT expr, ...,`。

> **注意：**
>
> `DO` 只能执行表达式，所以不是所有能够用 `SELECT` 的地方都能用 `DO` 替换。例如 `DO id FROM t1` 就是不是合法的 SQL 语句，因为它引用了一张表。
`DO` 在 MySQL 中的一个主要应用场景是存储过程或者触发器。因为 TiDB 当前不支持存储过程和触发器，所以 `DO` 的实际使用场景较少。

## 语法图

Expand All @@ -25,6 +31,8 @@ aliases: ['/docs-cn/dev/reference/sql/statements/do/']

## 示例

这条 `SELECT` 语句会暂停执行，但同时也会返回一个结果集。

{{< copyable "sql" >}}

```sql
Expand All @@ -40,6 +48,8 @@ SELECT SLEEP(5);
1 row in set (5.00 sec)
```

如果使用 `DO` 的话，语句同样会暂停，但不会返回结果集。

{{< copyable "sql" >}}

```sql
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18 changes: 17 additions & 1 deletion sql-statements/sql-statement-set-variable.md
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Expand Up @@ -7,16 +7,26 @@ aliases: ['/docs-cn/dev/reference/sql/statements/set-variable/']

# SET [GLOBAL|SESSION] <variable>

`SET [GLOBAL|SESSION]` 语句用于在 `SESSION` 或 `GLOBAL` 的范围内，对某个 TiDB 的内置变量进行更改。需注意，对 `GLOBAL` 变量的更改不适用于已有连接或本地连接，这与 MySQL 类似。只有新会话才会反映值的变化。
`SET [GLOBAL|SESSION]` 语句用于在 `SESSION` 或 `GLOBAL` 的范围内，对某个 TiDB 的内置变量进行更改。

> **注意：**
>
> 与 MySQL 类似，对 `GLOBAL` 变量的更改不适用于已有连接或本地连接，只有新会话才会反映值的变化。
## 语法图

**SetStmt:**

![SetStmt](/media/sqlgram/SetStmt.png)

**VariableAssignment:**

![VariableAssignment](/media/sqlgram/VariableAssignment.png)

## 示例

获取 `sql_mode` 的值：

{{< copyable "sql" >}}

```sql
Expand Down Expand Up @@ -47,6 +57,8 @@ SHOW SESSION VARIABLES LIKE 'sql_mode';
1 row in set (0.00 sec)
```

更新全局的 `sql_mode`：

{{< copyable "sql" >}}

```sql
Expand All @@ -57,6 +69,8 @@ SET GLOBAL sql_mode = 'STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER';
Query OK, 0 rows affected (0.03 sec)
```

检查更新之后的 `sql_mode` 的取值，可以看到 SESSION 级别的值没有更新：

{{< copyable "sql" >}}

```sql
Expand Down Expand Up @@ -87,6 +101,8 @@ SHOW SESSION VARIABLES LIKE 'sql_mode';
1 row in set (0.00 sec)
```

`SET SESSION` 则可以立即生效：

{{< copyable "sql" >}}

```sql
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194 changes: 116 additions & 78 deletions sql-statements/sql-statement-show-variables.md
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Expand Up @@ -25,90 +25,128 @@ aliases: ['/docs-cn/dev/reference/sql/statements/show-variables/']

## 示例

查看 TiDB 定义的专用系统变量，关于这些变量的含义参见 [TiDB 专用系统变量和语法](/tidb-specific-system-variables.md)。

{{< copyable "sql" >}}

```sql
SHOW GLOBAL VARIABLES LIKE 'tidb%';
```

```
+-------------------------------------+---------------+
| Variable_name | Value |
+-------------------------------------+---------------+
| tidb_ddl_error_count_limit | 512 |
| tidb_dml_batch_size | 20000 |
| tidb_force_priority | NO_PRIORITY |
| tidb_batch_insert | 0 |
| tidb_skip_utf8_check | 0 |
| tidb_backoff_lock_fast | 100 |
| tidb_opt_join_reorder_threshold | 0 |
| tidb_auto_analyze_end_time | 23:59 +0000 |
| tidb_slow_log_threshold | 300 |
| tidb_opt_correlation_exp_factor | 1 |
| tidb_mem_quota_sort | 1073741824 |
| tidb_current_ts | 0 |
| tidb_ddl_reorg_batch_size | 256 |
| tidb_checksum_table_concurrency | 4 |
| tidb_check_mb4_value_in_utf8 | 1 |
| tidb_distsql_scan_concurrency | 15 |
| tidb_optimizer_selectivity_level | 0 |
| tidb_opt_agg_push_down | 0 |
| tidb_max_chunk_size | 1024 |
| tidb_low_resolution_tso | 0 |
| tidb_index_lookup_size | 20000 |
| tidb_skip_isolation_level_check | 0 |
| tidb_opt_write_row_id | 0 |
| tidb_wait_split_region_finish | 1 |
| tidb_index_lookup_join_concurrency | 4 |
| tidb_mem_quota_indexlookupjoin | 34359738368 |
| tidb_replica_read | leader |
| tidb_ddl_reorg_priority | PRIORITY_LOW |
| tidb_batch_commit | 0 |
| tidb_mem_quota_mergejoin | 34359738368 |
| tidb_mem_quota_query | 34359738368 |
| tidb_batch_delete | 0 |
| tidb_build_stats_concurrency | 4 |
| tidb_enable_index_merge | 0 |
| tidb_enable_radix_join | 0 |
| tidb_retry_limit | 10 |
| tidb_query_log_max_len | 2048 |
| tidb_config | |
| tidb_ddl_reorg_worker_cnt | 4 |
| tidb_opt_insubq_to_join_and_agg | 1 |
| tidb_enable_vectorized_expression | 1 |
| tidb_mem_quota_hashjoin | 34359738368 |
| tidb_disable_txn_auto_retry | 1 |
| tidb_enable_window_function | 1 |
| tidb_init_chunk_size | 32 |
| tidb_constraint_check_in_place | 0 |
| tidb_wait_split_region_timeout | 300 |
| tidb_hash_join_concurrency | 5 |
| tidb_enable_fast_analyze | 0 |
| tidb_enable_cascades_planner | 0 |
| tidb_txn_mode | |
| tidb_index_serial_scan_concurrency | 1 |
| tidb_projection_concurrency | 4 |
| tidb_enable_noop_functions | 0 |
| tidb_index_lookup_concurrency | 4 |
| tidb_hashagg_partial_concurrency | 4 |
| tidb_general_log | 0 |
| tidb_enable_streaming | 0 |
| tidb_backoff_weight | 2 |
| tidb_mem_quota_topn | 34359738368 |
| tidb_scatter_region | 0 |
| tidb_index_join_batch_size | 25000 |
| tidb_snapshot | |
| tidb_expensive_query_time_threshold | 60 |
| tidb_slow_query_file | tidb-slow.log |
| tidb_auto_analyze_ratio | 0.5 |
| tidb_enable_table_partition | auto |
| tidb_auto_analyze_start_time | 00:00 +0000 |
| tidb_mem_quota_nestedloopapply | 34359738368 |
| tidb_mem_quota_indexlookupreader | 34359738368 |
| tidb_hashagg_final_concurrency | 4 |
| tidb_opt_correlation_threshold | 0.9 |
+-------------------------------------+---------------+
68 rows in set (0.01 sec)
```sql
+-------------------------------------+---------------------+
| Variable_name | Value |
+-------------------------------------+---------------------+
| tidb_allow_batch_cop | 0 |
| tidb_allow_remove_auto_inc | 0 |
| tidb_auto_analyze_end_time | 23:59 +0000 |
| tidb_auto_analyze_ratio | 0.5 |
| tidb_auto_analyze_start_time | 00:00 +0000 |
| tidb_backoff_lock_fast | 100 |
| tidb_backoff_weight | 2 |
| tidb_batch_commit | 0 |
| tidb_batch_delete | 0 |
| tidb_batch_insert | 0 |
| tidb_build_stats_concurrency | 4 |
| tidb_capture_plan_baselines | off |
| tidb_check_mb4_value_in_utf8 | 1 |
| tidb_checksum_table_concurrency | 4 |
| tidb_config | |
| tidb_constraint_check_in_place | 0 |
| tidb_current_ts | 0 |
| tidb_ddl_error_count_limit | 512 |
| tidb_ddl_reorg_batch_size | 256 |
| tidb_ddl_reorg_priority | PRIORITY_LOW |
| tidb_ddl_reorg_worker_cnt | 4 |
| tidb_disable_txn_auto_retry | 1 |
| tidb_distsql_scan_concurrency | 15 |
| tidb_dml_batch_size | 20000 |
| tidb_enable_cascades_planner | 0 |
| tidb_enable_chunk_rpc | 1 |
| tidb_enable_collect_execution_info | 1 |
| tidb_enable_fast_analyze | 0 |
| tidb_enable_index_merge | 0 |
| tidb_enable_noop_functions | 0 |
| tidb_enable_radix_join | 0 |
| tidb_enable_slow_log | 1 |
| tidb_enable_stmt_summary | 1 |
| tidb_enable_streaming | 0 |
| tidb_enable_table_partition | on |
| tidb_enable_vectorized_expression | 1 |
| tidb_enable_window_function | 1 |
| tidb_evolve_plan_baselines | off |
| tidb_evolve_plan_task_end_time | 23:59 +0000 |
| tidb_evolve_plan_task_max_time | 600 |
| tidb_evolve_plan_task_start_time | 00:00 +0000 |
| tidb_expensive_query_time_threshold | 60 |
| tidb_force_priority | NO_PRIORITY |
| tidb_general_log | 0 |
| tidb_hash_join_concurrency | 5 |
| tidb_hashagg_final_concurrency | 4 |
| tidb_hashagg_partial_concurrency | 4 |
| tidb_index_join_batch_size | 25000 |
| tidb_index_lookup_concurrency | 4 |
| tidb_index_lookup_join_concurrency | 4 |
| tidb_index_lookup_size | 20000 |
| tidb_index_serial_scan_concurrency | 1 |
| tidb_init_chunk_size | 32 |
| tidb_isolation_read_engines | tikv, tiflash, tidb |
| tidb_low_resolution_tso | 0 |
| tidb_max_chunk_size | 1024 |
| tidb_max_delta_schema_count | 1024 |
| tidb_mem_quota_hashjoin | 34359738368 |
| tidb_mem_quota_indexlookupjoin | 34359738368 |
| tidb_mem_quota_indexlookupreader | 34359738368 |
| tidb_mem_quota_mergejoin | 34359738368 |
| tidb_mem_quota_nestedloopapply | 34359738368 |
| tidb_mem_quota_query | 1073741824 |
| tidb_mem_quota_sort | 34359738368 |
| tidb_mem_quota_topn | 34359738368 |
| tidb_metric_query_range_duration | 60 |
| tidb_metric_query_step | 60 |
| tidb_opt_agg_push_down | 0 |
| tidb_opt_concurrency_factor | 3 |
| tidb_opt_copcpu_factor | 3 |
| tidb_opt_correlation_exp_factor | 1 |
| tidb_opt_correlation_threshold | 0.9 |
| tidb_opt_cpu_factor | 3 |
| tidb_opt_desc_factor | 3 |
| tidb_opt_disk_factor | 1.5 |
| tidb_opt_distinct_agg_push_down | 0 |
| tidb_opt_insubq_to_join_and_agg | 1 |
| tidb_opt_join_reorder_threshold | 0 |
| tidb_opt_memory_factor | 0.001 |
| tidb_opt_network_factor | 1 |
| tidb_opt_scan_factor | 1.5 |
| tidb_opt_seek_factor | 20 |
| tidb_opt_write_row_id | 0 |
| tidb_optimizer_selectivity_level | 0 |
| tidb_pprof_sql_cpu | 0 |
| tidb_projection_concurrency | 4 |
| tidb_query_log_max_len | 4096 |
| tidb_record_plan_in_slow_log | 1 |
| tidb_replica_read | leader |
| tidb_retry_limit | 10 |
| tidb_row_format_version | 2 |
| tidb_scatter_region | 0 |
| tidb_skip_isolation_level_check | 0 |
| tidb_skip_utf8_check | 0 |
| tidb_slow_log_threshold | 300 |
| tidb_slow_query_file | tidb-slow.log |
| tidb_snapshot | |
| tidb_stmt_summary_history_size | 24 |
| tidb_stmt_summary_internal_query | 0 |
| tidb_stmt_summary_max_sql_length | 4096 |
| tidb_stmt_summary_max_stmt_count | 200 |
| tidb_stmt_summary_refresh_interval | 1800 |
| tidb_store_limit | 0 |
| tidb_txn_mode | |
| tidb_use_plan_baselines | on |
| tidb_wait_split_region_finish | 1 |
| tidb_wait_split_region_timeout | 300 |
| tidb_window_concurrency | 4 |
+-------------------------------------+---------------------+
108 rows in set (0.01 sec)
```

{{< copyable "sql" >}}
Expand Down

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