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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 will not necessarily say Using temporary for derived or materialized temporary tables.

When the server creates an internal temporary table (either in memory or on disk), it increments the Created_tmp_tables status variable. If the server creates the table on disk (either initially or by converting an in-memory table) it increments the Created_tmp_disk_tables status variable.

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 or MyISAM storage engine.

Storage Engine for In-Memory Internal Temporary Tables

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

The TempTable storage engine provides efficient storage for VARCHAR and VARBINARY columns. Storage of other binary large object types is supported as of MySQL 8.0.13. The temptable_max_ram configuration option defines the maximum amount of random access memory (RAM) that can be occupied by the TempTable storage engine before it starts allocating space from disk in the form of temporary files that are mapped into memory. The default temptable_max_ram setting is 1GiB. Use of temporary files by the TempTable storage engine as an overflow mechanism for in-memory temporary tables 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.

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. If the physical_disk instrument reports a value other than 0, the temptable_max_ram threshold 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 25.11.16.10, “Memory Summary Tables”.

When using the MEMORY storage engine for in-memory temporary tables, MySQL automatically converts an in-memory temporary table to an on-disk table if it becomes too large. The maximum size for in-memory temporary tables 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

The internal_tmp_disk_storage_engine variable defines the storage engine the server uses to manage on-disk internal temporary tables. Permitted values are INNODB (the default) and MYISAM.

For common table expressions (CTEs), the storage engine used for on-disk internal temporary tables cannot be MyISAM. If internal_tmp_disk_storage_engine=MYISAM, an error occurs for any attempt to materialize a CTE using an on-disk temporary table.

Note

When using internal_tmp_disk_storage_engine=INNODB, queries that generate on-disk internal temporary tables that exceed InnoDB row or column limits return Row size too large or Too many columns errors. The workaround is to set internal_tmp_disk_storage_engine to MYISAM.

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, or 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 exceeds the temptable_max_ram limit and starts allocating space from disk in the form of temporary files that are mapped into memory.

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.

On-disk internal temporary tables are 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.

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.


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