问题
I'm seeing a large number of internal temporary disk tables being written. I can see the count with SHOW GLOBAL STATUS where Variable_name like 'Created_tmp_disk_tables'.
I know I can update max_heap_table_size and tmp_table_size to help prevent this, but without knowing the size of the tables getting written to disk, it's difficult to know what values to use.
Does anyone know how to go about finding this value?
回答1:
This is not easy to get. In Percona Server, there are options to add additional information in the slow query log that shows the size of temp tables (see https://www.percona.com/doc/percona-server/5.7/diagnostics/slow_extended.html)
# User@Host: mailboxer[mailboxer] @ [192.168.10.165]
# Thread_id: 11167745 Schema: board
# Query_time: 1.009400 Lock_time: 0.000190 Rows_sent: 4 Rows_examined: 1543719 Rows_affected: 0 Rows_read: 4
# Bytes_sent: 278 Tmp_tables: 0 Tmp_disk_tables: 0 Tmp_table_sizes: 0
# QC_Hit: No Full_scan: Yes Full_join: No Tmp_table: No Tmp_table_on_disk: No
# Filesort: No Filesort_on_disk: No Merge_passes: 0
(The example above, taken from the Percona documentation, shows the extended fields, although the example is for a query that did not create temp tables, so the size is shown as 0.)
In Oracle MySQL, some of the same extended information is available in query events in the PERFORMANCE_SCHEMA—but not the temp table sizes.
In 2014, I logged a feature request to supply this information: https://bugs.mysql.com/bug.php?id=74484 and this bug has been acknowledged, but this has not been implemented as far as I know.
It's a little bit unclear how this would be implemented, since it's possible for any given query to create multiple temp tables of different sizes. I believe the Percona feature shows the sum total of the temp table sizes in such cases.
All I can offer as a suggestion is to increase the max_heap_table_size and tmp_table_size in increments, and monitor the rate of increase of the Created_tmp_disk_tables reported by SHOW GLOBAL STATUS, compared to Created_tmp_tables (temp tables that did not use disk). As the allowed tmp table size is able to hold a greater percentage of temp tables created, you should start to see the ratio of on-disk temp tables to in-memory temp tables decrease.
It's typically not necessary to increase tmp_table_size to hold every possible temp table, no matter how large. You want the largest outliers to use the disk. But as long as the temp tables use memory 98% of the time, you should be fine. That would mean that the ratio of Created_tmp_disk_tables to Created_tmp_tables should be 1:50 or more.
回答2:
(Addenda to Bill's Answer.)
Some significant changes have been made in the handling of tmp tables in 5.7 and 8.0. I am not sure if any of them apply here, but aware.
Exceeding tmp_table_size is not the only reason for using a disk-based temp table -- TEXT, BLOB, @variables, UNION, etc. (More details in https://dev.mysql.com/doc/refman/5.7/en/internal-temporary-tables.html ) For that reason, I would question Bill's 1:50 advice.
Tmp tables (on-disk or in-memory) can be created by lots of queries; multiple tmp tables may be needed for a complex query. So, the number of simultaneous tmp tables is potentially more than max_connections. For this reason, I recommend keeping tmp_table_size less than 1% of available RAM. If you did have a high setting and need lots of simultaneous tmp tables, you could cause swapping, which is terribly bad for performance.
There are many "fixes" to having high values for Created_tmp*tables; too many to itemize here. If you would like to present a query (and the relevant SHOW CREATE TABLEs); we can discuss that particular case.
When analyzing a customer's system, I look for things such as
Created_tmp_disk_tables > 1/second
Created_tmp_disk_tables > 4% of queries
Created_tmp_tables > 20/second
Any of those, in my opinion, indicate the need for scrutiny of slow queries. But if tmp_table_size is greater than 1% of RAM, I advise lowering it, even if it hurts some queries.
回答3:
Just looking through the documentation, and assuming that just knowing the number of tables is not sufficient (looking at Created_tmp_tables), the variable tmpdir would give you the location of the directory storing these tables. Lowering tmp_table_size should ensure temp tables are created on disk, so you're able to look at the size.
来源：https://stackoverflow.com/questions/47913129/how-to-see-size-of-mysql-internal-innodb-temporary-tables