What are sql-temptable files?

In MariaDB (and MySQL), temporary tables are often used for handling intermediate results during query execution. These temporary tables can sometimes be stored in memory, specifically in the /dev/shm directory on Linux systems. Here's a full explanation of how sql-temptable files in /dev/shm are created, used, and managed by MariaDB:

1. What is /dev/shm?

• /dev/shm is a shared memory directory in Linux, backed by RAM. It's a virtual file system (tmpfs) designed to store files in memory rather than on disk.
• Using /dev/shm allows for faster read/write operations compared to disk-based storage, which is especially useful for temporary data that doesn't need to persist.

2. Why are sql-temptable Files Created?

• Temporary Tables: When MariaDB executes complex queries (e.g., involving ORDER BY, GROUP BY, joins, or subqueries), it may create temporary tables to store intermediate results.
• In-Memory Storage: MariaDB tries to store these temporary tables in memory to optimize performance, avoiding the overhead of disk I/O.
• sql-temptable Files: These temporary tables are sometimes represented as files, and the files can be stored in memory (RAM) using a tmpfs filesystem like /dev/shm.

3. How does MariaDB use /dev/shm?

• Memory Storage for Temporary Tables: When MariaDB creates temporary tables, it first tries to allocate space for them in memory.
  • If enough memory is available, it uses /dev/shm (or similar locations based on the system configuration).
  • If the table size exceeds the available memory or configuration limits, MariaDB will automatically swap the temporary table to disk.
• Improved Performance: By placing temporary tables in memory, MariaDB reduces the performance overhead associated with disk-based temporary storage.

4. Configuration for Temp Table Location:

• tmpdir Setting: The directory where MariaDB creates temporary files and tables is controlled by the tmpdir configuration variable.
  • In a typical MariaDB configuration, you can set tmpdir to /dev/shm to encourage in-memory temporary table storage.
• Fallback to Disk: If the temporary table grows too large to fit in memory, MariaDB will write the table to disk, typically in the default temporary directory (e.g., /tmp or /var/tmp).

Example in my.cnf (MariaDB configuration file):

[mysqld]
tmpdir = /dev/shm

5. Types of Temporary Tables:

• Internal Temporary Tables: These are created automatically by MariaDB during query execution for handling intermediate results (e.g., sorting data, calculating aggregates).
  • MariaDB decides internally whether to store these tables in memory or on disk, depending on their size and the tmp_table_size and max_heap_table_size settings.
• Explicit Temporary Tables: Developers can create temporary tables explicitly using SQL (CREATE TEMPORARY TABLE). These tables are generally stored in memory, but if they exceed the memory limit, they are written to disk.

6. sql-temptable File Naming and Structure:

• The files in /dev/shm related to temporary tables typically have names like sql-temptable-M-N, where:
  • M: A unique identifier or number for the temporary table.
  • N: Another identifier for the specific session or query using the temporary table.
• These files are automatically managed by MariaDB and are removed when they are no longer needed (e.g., after the query finishes or the session ends).

7. Configurable Parameters Affecting Temporary Tables:

Several configuration parameters influence how MariaDB handles temporary tables and their storage in /dev/shm or elsewhere:

• tmp_table_size: The maximum size (in bytes) of in-memory temporary tables.
  • If a temporary table exceeds this size, it is converted to an on-disk table.
• max_heap_table_size: The maximum size of memory-allocated MEMORY tables.
  • This also affects how large in-memory temporary tables can be.
• innodb_temp_data_file_path: Defines the location and size of tablespace files for InnoDB on-disk temporary tables.
  • When memory is exhausted, or for large temporary tables, MariaDB may fall back to using InnoDB tables in the file system.

Example:

max_heap_table_size = 64M

Example:

tmp_table_size = 64M

8. Performance Considerations:

• Advantages of Using /dev/shm:
  • Faster performance due to memory-based storage, reducing the need for disk I/O.
  • Especially beneficial for workloads involving frequent use of temporary tables, large sorting operations, or complex joins.
• Potential Downsides:
  • Memory Usage: Since /dev/shm is backed by RAM, it reduces the available memory for other processes, including MariaDB itself. If the system runs out of memory, this could lead to out-of-memory (OOM) errors.
  • Limited Space: The size of /dev/shm is often limited (by default, half of the system's physical RAM). If your temporary tables exceed this space, they may overflow to disk.

9. Managing and Monitoring Temp Tables in /dev/shm:

• In case the usage of /dev/shm becomes problematic due to heavy query loads or large temporary tables, you might consider adjusting the tmp_table_size and max_heap_table_size to prevent MariaDB from over-utilizing shared memory.

You can monitor the use of /dev/shm with the df command to check available space.

df -h /dev/shm

10. Cleanup of Temp Tables:

• Temporary files in /dev/shm should be automatically cleaned up by MariaDB when they are no longer in use (e.g., after query execution or when a session closes).
• In some cases, leftover files may accumulate if queries are aborted unexpectedly or if the server crashes, requiring manual cleanup.

Summary:

• The sql-temptable files in /dev/shm are intermediate data stores used by MariaDB for temporary tables during query execution.
• These files exist in RAM (via the /dev/shm directory) to speed up performance by avoiding slower disk-based temporary storage.
• They are influenced by parameters like tmpdir, tmp_table_size, and max_heap_table_size, and may fall back to disk-based storage if the memory limits are exceeded.
• While /dev/shm offers performance benefits, it's important to monitor memory usage and configure limits to prevent overloading the system's RAM.