The potential problem with InnoDB temporary tablesĪlthough using InnoDB is the best for performance, a new potential issue could arise. The variable can be set to innodb (default if not set) or myisam. The new default is the best option for the overall performance and should be used in the majority of the cases.Ī new configuration variable is available to set the storage engine for the temporary tables: internal_tmp_disk_storage_engine. Then you can rely on the advanced features. From MySQL 5.7, they are created as InnoDB by default. The temporary table is created in-memory or on-disk, depending on the configuration, and it’s dropped immediately at the end of the query. Until MySQL 5.6, all the on-disk temporary tables are created as MyISAM. Note: the temporary table is created on-disk if the tables involved have TEXT or BLOB columns, even if the size is less than the configured threshold. You cannot indefinitely increase the variables, as at some point you’ll need to let MySQL use on-disk temporary tables. When setting the threshold, take into consideration the available RAM and the number of concurrent connections you have during your peaks. If you run queries on a large amount of data, or if you have not optimized queries, you can increase the variables.
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The maximum size for in-memory temporary tables is defined by the tmp_table_size or max_heap_table_size value, whichever is smaller. MySQL creates an in-memory table, and if it becomes too large it is converted to an on-disk table. Needless to say that an in-memory temporary table is faster. If the temporary table is small it can be created into the memory, otherwise, it’s created on the disk. Such queries are executed in two stages: the first is to gather the data and put them into a temporary table, the second is to execute the sorting on the temporary table.Ī temporary table is also needed in case of some UNION statements evaluation, for VIEW that cannot use merge, for derived tables when using subqueries, for multiple-table UPDATE, and some other cases. For example, when you use GROUP BY, ORDER BY or DISTINCT. The queries that require a sorting stage most of the time need to rely on a temporary table.
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MySQL needs to create implicit temporary tables for solving some kinds of queries. What was happening? In this article, I’m going to explain it and propose solutions. This was a serious issue because of the continuous outages of the service, as the customer had to constantly restart the server and wait for the next outage. I was recently faced with a real issue about completely exhausting the disk space on MySQL.