Why the `Information schema is changed` error is reported?

FAQ
1

TiDB handles the SQL statement using the schema of the time and supports online asynchronous DDL change. A DML statement and a DDL statement might be executed at the same time and you must ensure that each statement is executed using the same schema. Therefore, when the DML operation meets the ongoing DDL operation, the Information schema is changed error might be reported. Some improvements have been made to prevent too many error reportings during the DML operation.

Now, there are still a few causes for this error reporting:

  • Cause 1: Some tables involved in the DML operation are the same tables involved in the ongoing DDL operation. To check the ongoing DDL operations, use the ADMIN SHOW DDL statement.
  • Cause 2: The DML operation goes on for a long time. During this period, many DDL statements have been executed, which causes more than 1024 schema version changes. You can modify this default value by modifying the tidb_max_delta_schema_count variable.
  • Cause 3: The TiDB server that accepts the DML request is not able to load schema information for a long time (possibly caused by the connection failure between TiDB and PD or TiKV). During this period, many DDL statements have been executed, which causes more than 100 schema version changes.
  • Cause 4: After TiDB restarts and before the first DDL operation is executed, the DML operation is executed and then encounters the first DDL operation (which means before the first DDL operation is executed, the transaction corresponding to the DML is started. And after the first schema version of the DDL is changed, the transaction corresponding to the DML is committed), this DML operation reports this error.

In the preceding causes, only Cause 1 is related to tables. Cause 1 and Cause 2 do not impact the application, as the related DML operations retry after failure. For cause 3, you need to check the network between TiDB and TiKV/PD.

Note:

  • Currently, TiDB does not cache all the schema version changes.
  • For each DDL operation, the number of schema version changes is the same with the number of corresponding schema state version changes.
  • Different DDL operations cause different number of schema version changes. For example, the CREATE TABLE statement causes one schema version change while the ADD COLUMN statement causes four.