Within the Performance Schema, the THREAD_ID  column in the error_log  table is most similar to the PROCESSLIST_ID  column of the threads  table: 

•For foreground threads, THREAD_ID  and PROCESSLIST_ID  represent a connection identifier. This is the same value displayed in the ID  column of the INFORMATION_SCHEMA PROCESSLIST  table, displayed in the Id  column of SHOW PROCESSLIST  output, and returned by the CONNECTION_ID() function within the thread. 

•For background threads, THREAD_ID  is 0 and PROCESSLIST_ID  is NULL . 

Many Performance Schema tables other than error_log  has a column named THREAD_ID , but in those tables, the THREAD_ID  column is a value assigned internally by the Performance Schema. 

• PRIO 

The event priority. Permitted values are System , Error , Warning , Note . The PRIO  column is based on the label  field of error events, which itself is based on the underlying numeric prio  field value. 

• ERROR_CODE 

The numeric event error code. ERROR_CODE  corresponds to the error_code  field of error events. 

• SUBSYSTEM 

The subsystem in which the event occurred. SUBSYSTEM  corresponds to the subsystem  field of error events. 

• DATA 

The text representation of the error event. The format of this value depends on the format produced by the log sink component that generates the error_log  row. For example, if the log sink is log_sink_internal  or log_sink_json , DATA  values represent error events in traditional or JSON format, respectively. (See Section 5.4.2.9, “Error Log Output Format” .) 

Because the error log can be reconfigured to change the log sink component that supplies rows to the error_log  table, and because different sinks produce different output formats, it is possible for rows written to the error_log  table at different times to have different DATA  formats. 

## Implementation and Configuration of the error_log Table 

The error_log  table has these indexes: 

•Primary key on ( LOGGED ) 

•Index on ( THREAD_ID ) 

•Index on ( PRIO ) 

•Index on ( ERROR_CODE ) 

•Index on ( SUBSYSTEM ) 

TRUNCATE TABLE  is not permitted for the error_log  table. 

The Performance Schema error_log  table is populated by error log sink components that write to the table in addition to writing formatted error events to the error log. Performance Schema support by log sinks has two parts: 

