In some cases, the server creates internal temporary tables
while processing queries. Such a table can be held in memory and
processed by the
MEMORY storage engine, or
stored on disk and processed by the
storage engine. The server may create a temporary table
initially as an in-memory table, then convert it to an on-disk
table if it becomes too large. Users have no direct control over
when the server creates an internal temporary table or which
storage engine the server uses to manage it.
Temporary tables can be created under conditions such as these:
UNIONqueries use temporary tables.
Some views require temporary tables, such those evaluated using the
TEMPTABLEalgorithm, or that use
If there is an
ORDER BYclause and a different
GROUP BYclause, or if the
GROUP BYcontains columns from tables other than the first table in the join queue, a temporary table is created.
ORDER BYmay require a temporary table.
If you use the
SQL_SMALL_RESULToption, MySQL uses an in-memory temporary table, unless the query also contains elements (described later) that require on-disk storage.
If an internal temporary table is created initially as an
in-memory table but becomes too large, MySQL automatically
converts it to an on-disk table. The maximum size for in-memory
temporary tables is the minimum of the
This differs from
MEMORY tables explicitly
CREATE TABLE: For
such tables, only the
variable determines how large the table is permitted to grow and
there is no conversion to on-disk format.
When the server creates an internal temporary table (either in
memory or on disk), it increments the
variable. If the server creates the table on disk (either
initially or by converting an in-memory table) it increments the
Some conditions prevent the use of an in-memory temporary table, in which case the server uses an on-disk table instead:
Presence of any string column in a
DISTINCTclause larger than 512 bytes