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.Evaluation of some views, such those that use the
TEMPTABLE
algorithm,UNION
, or aggregation.Evaluation of derived tables (see Section 13.2.10.8, “Derived Tables”).
Tables created for subquery or semijoin materialization (see Section 8.2.2, “Optimizing Subqueries and Derived Tables”).
Evaluation of statements that contain an
ORDER BY
clause and a differentGROUP BY
clause, or for which theORDER BY
orGROUP 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 evaluate
INSERT ... 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. See Section 13.2.5.1, “INSERT ... SELECT Statement”.Evaluation of multiple-table
UPDATE
statements.Evaluation of
GROUP_CONCAT()
orCOUNT(DISTINCT)
expressions.
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.
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. This includes user-defined variables having a string value because they are treated asBLOB
orTEXT
columns, depending on whether their value is a binary or nonbinary string, respectively.Presence of any string column in a
GROUP BY
orDISTINCT
clause larger than 512 bytes for binary strings or 512 characters for nonbinary strings.Presence of any string column with a maximum length larger than 512 (bytes for binary strings, characters for nonbinary strings) in the
SELECT
list, ifUNION
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.
An internal temporary table can be held in memory and
processed by the MEMORY
storage engine, or
stored on disk and processed by the MyISAM
storage engine.
If an internal temporary table is created as an in-memory
table but becomes too large, MySQL automatically converts it
to an on-disk table. 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.
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 increasing the
tmp_table_size
and
max_heap_table_size
settings.
In-memory temporary tables are managed by the
MEMORY
storage engine, which uses
fixed-length row format. VARCHAR
and
VARBINARY
column values are padded to the
maximum column length, in effect storing them as
CHAR
and BINARY
columns.
On-disk temporary tables are managed by the
MyISAM
storage engine using dynamic-width
row format. Columns take only as much storage as needed, which
reduces disk I/O and space requirements, and processing time
compared to on-disk tables that use fixed-length rows.
For statements that initially create an internal temporary
table in memory, then convert it to an on-disk table, better
performance might be achieved by skipping the conversion step
and creating the table on disk to begin with. The
big_tables
variable can be
used to force disk storage of internal temporary tables.