10.4.4 Internal Temporary Table Use in MySQL (original) (raw)

10.4.4 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:

• Evaluation of UNION statements, with some exceptions described later.
• Evaluation of some views, such those that use theTEMPTABLE algorithm,UNION, or aggregation.
• Evaluation of derived tables (seeSection 15.2.15.8, “Derived Tables”).
• Evaluation of common table expressions (seeSection 15.2.20, “WITH (Common Table Expressions)”).
• Tables created for subquery or semijoin materialization (seeSection 10.2.2, “Optimizing Subqueries, Derived Tables, View References, and Common Table Expressions”).
• Evaluation of statements that contain an ORDER BY clause and a different GROUP BY clause, or for which the ORDER BY or GROUP BY contains columns from tables other than the first table in the join queue.
• Evaluation of DISTINCT combined withORDER BY may require a temporary table.
• For queries that use the SQL_SMALL_RESULT modifier, MySQL uses an in-memory temporary table, unless the query also contains elements (described later) that require on-disk storage.
• To evaluateINSERT ... SELECT statements that select from and insert into the same table, MySQL creates an internal temporary table to hold the rows from theSELECT, then inserts those rows into the target table. SeeSection 15.2.7.1, “INSERT ... SELECT Statement”.
• Evaluation of multiple-tableUPDATE statements.
• Evaluation of GROUP_CONCAT() or COUNT(DISTINCT) expressions.
• Evaluation of window functions (seeSection 14.20, “Window Functions”) uses temporary tables as necessary.

To determine whether a statement requires a temporary table, useEXPLAIN and check theExtra column to see whether it saysUsing temporary (seeSection 10.8.1, “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 orTEXT column in the table. TheTempTable storage engine, which is the default storage engine for in-memory internal temporary tables in MySQL 8.4, supports binary large object types. SeeInternal 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 orUNION ALL is used.
• The SHOW COLUMNS andDESCRIBE statements useBLOB 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 forUNION 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: TheUNION 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, notUNION 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 orMEMORY storage engine, or stored on disk by the InnoDB storage engine.

Storage Engine for In-Memory Internal Temporary Tables

Theinternal_tmp_mem_storage_engine variable defines the storage engine used for in-memory internal temporary tables. Permitted values areTempTable (the default) andMEMORY.

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

The following variables control TempTable storage engine limits and behavior:

• tmp_table_size: Defines the maximum size of any individual in-memory internal temporary table created using theTempTable storage engine. When the limit determined bytmp_table_size is reached, MySQL automatically converts the in-memory internal temporary table to an InnoDB on-disk internal temporary table. The default value is 16777216 bytes (16 MiB).
  The tmp_table_size limit is intended to prevent individual queries from consuming an inordinate amount of globalTempTable resources, which can affect the performance of concurrent queries that require such resources. Global TempTable resources are controlled bytemptable_max_ram andtemptable_max_mmap.
  If tmp_table_size is less than temptable_max_ram, it is not possible for an in-memory temporary table to use more than tmp_table_size. If tmp_table_size is greater than the sum oftemptable_max_ram andtemptable_max_mmap, an in-memory temporary table cannot use more than the sum of the temptable_max_ram andtemptable_max_mmap limits.
• temptable_max_ram: Defines the maximum amount of RAM that can be used by theTempTable 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. If not set explicitly, the value oftemptable_max_ram is 3% of the total memory available on the server, with a minimum of 1 GB and a maximum of 4 GB.
  Note
  temptable_max_ram does not account for the thread-local memory block allocated to each thread that uses theTempTable 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 limit determined bytemptable_max_ram is exceeded. The default value is OFF.
• temptable_max_mmap: Sets the maximum amount of memory theTempTable storage engine is permitted to allocate from memory-mapped files before MySQL starts using InnoDB on-disk internal temporary tables. The default value is 0 (disabled). The limit is intended to address the risk of memory mapped files using too much space in the temporary directory (tmpdir).temptable_max_mmap = 0 disables allocation from memory-mapped files, effectively disabling their use, regardless of the value oftemptable_use_mmap.

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 theTempTable storage engine, or when themysqld 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 bytemptable_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 thetmp_table_size ormax_heap_table_size value, whichever is smaller. This differs fromMEMORY tables explicitly created withCREATE 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

MySQL 8.4 uses only theInnoDB storage engine for on-disk internal temporary tables. (TheMYISAM 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, seeSection 17.6.3.5, “Temporary Tablespaces”.

Internal Temporary Table Storage Format

When in-memory internal temporary tables are managed by theTempTable storage engine, rows that includeVARCHAR columns,VARBINARY columns, and other binary large object type columns 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 theTempTable storage engine allocates space from memory-mapped files.

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

Internal temporary tables on disk are always managed byInnoDB.

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. Thebig_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 theCreated_tmp_tables value. When an internal temporary table is created on disk, the server increments theCreated_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.

The memory/temptable/physical_ram andmemory/temptable/physical_disk Performance Schema instruments can be used to monitorTempTable 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 thephysical_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 asmemory_summary_global_by_event_name. SeeSection 29.12.20.10, “Memory Summary Tables”.