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MySQL 8.0 Reference Manual  /  ...  /  Internal Temporary Table Use in MySQL

8.4.4 Internal Temporary Table Use in MySQL

In some cases, the server creates internal temporary tables while processing statements. Users have no direct control over when this occurs.

The server creates temporary tables under conditions such as these:

To determine whether a statement requires a temporary table, use EXPLAIN and check the Extra column to see whether it says Using temporary (see Section 8.8.1, “Optimizing Queries with EXPLAIN”). EXPLAIN does not necessarily say Using temporary for derived or materialized temporary tables. For statements that use window functions, EXPLAIN with FORMAT=JSON always provides information about the windowing steps. If the windowing functions use temporary tables, it is indicated for each step.

Some query conditions prevent the use of an in-memory temporary table, in which case the server uses an on-disk table instead:

  • Presence of a BLOB or TEXT column in the table. However, the TempTable storage engine, which is the default storage engine for in-memory internal temporary tables in MySQL 8.0, supports binary large object types as of MySQL 8.0.13. See Internal Temporary Table Storage Engine.

  • Presence of any string column with a maximum length larger than 512 (bytes for binary strings, characters for nonbinary strings) in the SELECT list, if UNION or UNION ALL is used.

  • The SHOW COLUMNS and DESCRIBE statements use BLOB as the type for some columns, thus the temporary table used for the results is an on-disk table.

The server does not use a temporary table for UNION statements that meet certain qualifications. Instead, it retains from temporary table creation only the data structures necessary to perform result column typecasting. The table is not fully instantiated and no rows are written to or read from it; rows are sent directly to the client. The result is reduced memory and disk requirements, and smaller delay before the first row is sent to the client because the server need not wait until the last query block is executed. EXPLAIN and optimizer trace output reflects this execution strategy: The UNION RESULT query block is not present because that block corresponds to the part that reads from the temporary table.

These conditions qualify a UNION for evaluation without a temporary table:

  • The union is UNION ALL, not UNION or UNION DISTINCT.

  • There is no global ORDER BY clause.

  • The union is not the top-level query block of an {INSERT | REPLACE} ... SELECT ... statement.

Internal Temporary Table Storage Engine

An internal temporary table can be held in memory and processed by the TempTable or MEMORY storage engine, or stored on disk by the InnoDB storage engine.

Storage Engine for In-Memory Internal Temporary Tables

The internal_tmp_mem_storage_engine variable defines the storage engine used for in-memory internal temporary tables. Permitted values are TempTable (the default) and MEMORY.

Note

As of MySQL 8.0.27, configuring a session setting for internal_tmp_mem_storage_engine requires the SESSION_VARIABLES_ADMIN or SYSTEM_VARIABLES_ADMIN privilege.

The TempTable storage engine provides efficient storage for VARCHAR and VARBINARY columns, and other binary large object types as of MySQL 8.0.13.

The following variables control TempTable storage engine limits and behavior:

  • tmp_table_size: From MySQL 8.0.28, tmp_table_size defines the maximum size of any individual in-memory internal temporary table created by the TempTable storage engine. When the tmp_table_size limit is reached, MySQL automatically converts the in-memory internal temporary table to an InnoDB on-disk internal temporary table. The default tmp_table_size setting is 16777216 bytes (16 MiB).

    The tmp_table_size limit is intended to prevent individual queries from consuming an inordinate amount global TempTable resources, which can affect the performance of concurrent queries that require TempTable resources. Global TempTable resources are controlled by the temptable_max_ram and temptable_max_mmap settings.

    If the tmp_table_size limit is less than the temptable_max_ram limit, it is not possible for an in-memory temporary table to contain more data than permitted by the tmp_table_size limit. If the tmp_table_size limit is greater than the sum of the temptable_max_ram and temptable_max_mmap limits, it is not possible for an in-memory temporary table to contain more than the sum of the temptable_max_ram and temptable_max_mmap limits.

  • temptable_max_ram: Defines the maximum amount of RAM that can be used by the TempTable storage engine before it starts allocating space from memory-mapped files or before MySQL starts using InnoDB on-disk internal temporary tables, depending on your configuration. The default temptable_max_ram setting is 1073741824 bytes (1GiB).

    Note

    The temptable_max_ram setting does not account for the thread-local memory block allocated to each thread that uses the TempTable storage engine. The size of the thread-local memory block depends on the size of the thread's first memory allocation request. If the request is less than 1MB, which it is in most cases, the thread-local memory block size is 1MB. If the request is greater than 1MB, the thread-local memory block is approximately the same size as the initial memory request. The thread-local memory block is held in thread-local storage until thread exit.

  • temptable_use_mmap: Controls whether the TempTable storage engine allocates space from memory-mapped files or MySQL uses InnoDB on-disk internal temporary tables when the temptable_max_ram limit is exceeded. The default setting is temptable_use_mmap=ON.

    Note

    The temptable_use_mmap variable was introduced in MySQL 8.0.16 and deprecated in MySQL 8.0.26; expect support for it to be removed in a future version of MySQL. Setting temptable_max_mmap=0 is equivalent to setting temptable_use_mmap=OFF.

  • temptable_max_mmap: Introduced in MySQL 8.0.23. Defines the maximum amount of memory the TempTable storage engine is permitted to allocate from memory-mapped files before MySQL starts using InnoDB on-disk internal temporary tables. The default setting is 1073741824 bytes (1GiB). The limit is intended to address the risk of memory mapped files using too much space in the temporary directory (tmpdir). A temptable_max_mmap=0 setting disables allocation from memory-mapped files, effectively disabling their use, regardless of the temptable_use_mmap setting.

Use of memory-mapped files by the TempTable storage engine is governed by these rules:

  • Temporary files are created in the directory defined by the tmpdir variable.

  • Temporary files are deleted immediately after they are created and opened, and therefore do not remain visible in the tmpdir directory. The space occupied by temporary files is held by the operating system while temporary files are open. The space is reclaimed when temporary files are closed by the TempTable storage engine, or when the mysqld process is shut down.

  • Data is never moved between RAM and temporary files, within RAM, or between temporary files.

  • New data is stored in RAM if space becomes available within the limit defined by temptable_max_ram. Otherwise, new data is stored in temporary files.

  • If space becomes available in RAM after some of the data for a table is written to temporary files, it is possible for the remaining table data to be stored in RAM.

When using the MEMORY storage engine for in-memory temporary tables (internal_tmp_mem_storage_engine=MEMORY), MySQL automatically converts an in-memory temporary table to an on-disk table if it becomes too large. The maximum size of an in-memory temporary table is defined by the tmp_table_size or max_heap_table_size value, whichever is smaller. This differs from MEMORY tables explicitly created with CREATE TABLE. For such tables, only the max_heap_table_size variable determines how large a table can grow, and there is no conversion to on-disk format.

Storage Engine for On-Disk Internal Temporary Tables

In MySQL 8.0.15 and earlier, the internal_tmp_disk_storage_engine variable defined the storage engine used for on-disk internal temporary tables. Supported storage engines were InnoDB and MyISAM.

From MySQL 8.0.16, MySQL uses only the InnoDB storage engine for on-disk internal temporary tables. The MYISAM storage engine is no longer supported for this purpose.

InnoDB on-disk internal temporary tables are created in session temporary tablespaces that reside in the data directory by default. For more information, see Section 15.6.3.5, “Temporary Tablespaces”.

In MySQL 8.0.15 and earlier:

Internal Temporary Table Storage Format

When in-memory internal temporary tables are managed by the TempTable storage engine, rows that include VARCHAR columns, VARBINARY columns, and other binary large object type columns (supported as of MySQL 8.0.13) are represented in memory by an array of cells, with each cell containing a NULL flag, the data length, and a data pointer. Column values are placed in consecutive order after the array, in a single region of memory, without padding. Each cell in the array uses 16 bytes of storage. The same storage format applies when the TempTable storage engine allocates space from memory-mapped files.

When in-memory internal temporary tables are managed by the MEMORY storage engine, fixed-length row format is used. VARCHAR and VARBINARY column values are padded to the maximum column length, in effect storing them as CHAR and BINARY columns.

Prior to MySQL 8.0.16, on-disk internal temporary tables were managed by the InnoDB or MyISAM storage engine (depending on the internal_tmp_disk_storage_engine setting). Both engines store internal temporary tables using dynamic-width row format. Columns take only as much storage as needed, which reduces disk I/O, space requirements, and processing time compared to on-disk tables that use fixed-length rows. Beginning with MySQL 8.0.16, internal_tmp_disk_storage_engine is not supported, and internal temporary tables on disk are always managed by InnoDB.

When using the MEMORY storage engine, statements can initially create an in-memory internal temporary table and then convert it to an on-disk table if the table becomes too large. In such cases, better performance might be achieved by skipping the conversion and creating the internal temporary table on disk to begin with. The big_tables variable can be used to force disk storage of internal temporary tables.

Monitoring Internal Temporary Table Creation

When an internal temporary table is created in memory or on disk, the server increments the Created_tmp_tables value. When an internal temporary table is created on disk, the server increments the Created_tmp_disk_tables value. If too many internal temporary tables are created on disk, consider adjusting the engine-specific limits described in Internal Temporary Table Storage Engine.

Note

Due to a known limitation, Created_tmp_disk_tables does not count on-disk temporary tables created in memory-mapped files. By default, the TempTable storage engine overflow mechanism creates internal temporary tables in memory-mapped files. See Internal Temporary Table Storage Engine.

The memory/temptable/physical_ram and memory/temptable/physical_disk Performance Schema instruments can be used to monitor TempTable space allocation from memory and disk. memory/temptable/physical_ram reports the amount of allocated RAM. memory/temptable/physical_disk reports the amount of space allocated from disk when memory-mapped files are used as the TempTable overflow mechanism. If the physical_disk instrument reports a value other than 0 and memory-mapped files are used as the TempTable overflow mechanism, a TempTable memory limit was reached at some point. Data can be queried in Performance Schema memory summary tables such as memory_summary_global_by_event_name. See Section 27.12.20.10, “Memory Summary Tables”.