TiDB Memory Control

Currently, TiDB can track the memory quota of a single SQL query and take actions to prevent OOM (out of memory) or troubleshoot OOM when the memory usage exceeds a specific threshold value. In the TiDB configuration file, you can configure the options as below to control TiDB behaviors when the memory quota exceeds the threshold value:

# Valid options: ["log", "cancel"]
oom-action = "log"

If the configuration item above uses “log”, when the memory quota of a single SQL query exceeds the threshold value which is controlled by the tidb_mem_quota_query variable, TiDB prints an entry of log. Then the SQL query continues to be executed. If OOM occurs, you can find the corresponding SQL query in the log.
If the configuration item above uses “cancel”, when the memory quota of a single SQL query exceeds the threshold value, TiDB stops executing the SQL query immediately and returns an error to the client. The error information clearly shows the memory usage of each physical execution operator that consumes much memory in the SQL execution process.

Configure the memory quota of a query

You can control the memory quota of a query using the following session variables. Generally, you only need to configure tidb_mem_quota_query. Other variables are used for advanced configuration which most users do not need to care about.

Variable Name	Description	Unit	Default Value
tidb_mem_quota_query	Control the memory quota of a query	Byte	32 << 30
tidb_mem_quota_hashjoin	Control the memory quota of “HashJoinExec”	Byte	32 << 30
tidb_mem_quota_mergejoin	Control the memory quota of “MergeJoinExec”	Byte	32 << 30
tidb_mem_quota_sort	Control the memory quota of “SortExec”	Byte	32 << 30
tidb_mem_quota_topn	Control the memory quota of “TopNExec”	Byte	32 << 30
tidb_mem_quota_indexlookupreader	Control the memory quota of “IndexLookUpExecutor”	Byte	32 << 30
tidb_mem_quota_indexlookupjoin	Control the memory quota of “IndexLookUpJoin”	Byte	32 << 30
tidb_mem_quota_nestedloopapply	Control the memory quota of “NestedLoopApplyExec”	Byte	32 << 30

Some usage examples:

-- Set the threshold value of memory quota for a single SQL query to 8GB:
set @@tidb_mem_quota_query = 8 << 30;

-- Set the threshold value of memory quota for a single SQL query to 8MB:
set @@tidb_mem_quota_query = 8 << 20;

-- Set the threshold value of memory quota for a single SQL query to 8KB:
set @@tidb_mem_quota_query = 8 << 10;