# System Variables


System variables control the behavior of a session or the entire instance engine. Use `SET` to change them, subject to privileges and whether a variable can be changed dynamically, and use `SHOW VARIABLES` to inspect them.

## Global and session scopes

| Scope | Meaning |
| --- | --- |
| **Global (GLOBAL)** | Affects the default behavior of new sessions. Some values persist for the instance lifecycle. |
| **Session (SESSION)** | Applies only to the current connection and expires when it disconnects. |
| **Both** | Has both a global default and a session override. |

```sql
SHOW VARIABLES;
SHOW VARIABLES LIKE 'sql_mode';
SHOW GLOBAL VARIABLES;
SHOW SESSION VARIABLES LIKE '%timeout%';

SET SESSION autocommit = 0;
SET GLOBAL max_allowed_packet = 33554432;  -- Requires sufficient privileges, and the variable must support dynamic modification
```

Custom user variables such as `@var` differ from system variables. They temporarily store scalar values within a session. Refer to client usage and statements such as `SET @a = ...`.

## Common variables

The following table lists variables commonly encountered in the documentation and product. Refer to `SHOW VARIABLES` for the complete list.

| Variable | Typical purpose | Notes |
| --- | --- | --- |
| `autocommit` | Enables automatic commits | Transaction scripts commonly disable it temporarily. |
| `sql_mode` | SQL mode set | Affects strictness, GROUP BY behavior, and more. |
| `character_set_*` / `collation_*` | Character sets and collations | A common default is `utf8mb4`. |
| `time_zone` | Session time zone | Affects `TIMESTAMP` display and conversion. |
| `lower_case_table_names` | Table-name case behavior | Commonly 1 (case-insensitive) and usually should not be changed arbitrarily. |
| `max_allowed_packet` | Maximum packet size in bytes | Increase it for large results or statements. |
| `interactive_timeout` / `net_write_timeout` | Idle and write timeouts | Relevant to long-lived connections and slow networks. |
| `auto_increment_increment` / `auto_increment_offset` | Auto-increment step and offset | |
| `foreign_key_checks` | Enables foreign-key checks | Can be disabled temporarily during bulk loading after consistency risks are assessed. |
| `save_query_result` / `query_result_*` | Query-result persistence | MatrixOne extension capabilities |
| `sql_safe_updates` | Safe updates | Restricts updates and deletes without a key or WHERE clause. |
| Password / login policy variables | Complexity, failure limits, allowlists, and more | Related to instance security policies |

## Query-result extensions

If query-result persistence is enabled for the instance:

- `save_query_result`: Whether to save results.
- `query_result_maxsize` / `query_result_timeout`: Size and retention-time limits.

Whether these capabilities are available to your tenant depends on the console and `SHOW VARIABLES`.