提交事务后，为什么MySQL`gtid_owned`会话变量总是为空？

Question

I'm testing some work with MySQL Global Transaction IDs (GTIDs) and I'm having a hard time getting the most-recent session GTID. I've enabled GTIDs (global gtid_mode is set to ON_PERMISSIVE ). According to the documentation for gtid_owned :

This read-only variable holds a list whose contents depend on its scope. When used with session scope, the list holds all GTIDs that are owned by this client; ...

So, I expect, after committing a transaction, that this session variable would contain GTIDs; but it is always empty, no matter what I do. However, the global gtid_executed changes every time I commit a transaction, so I know that GTIDs are working.

Here's a session that demonstrates this issue:

mysql> SELECT @@global.gtid_executed;
| a1c32161-89c4-11e8-856e-0242ac12001d:1-4 |

mysql> SELECT @@session.gtid_next;
| AUTOMATIC           |

mysql> SELECT @@session.gtid_owned;
|                      |

mysql> START TRANSACTION;

mysql> SELECT @@global.gtid_executed;
| a1c32161-89c4-11e8-856e-0242ac12001d:1-4 |

mysql> SELECT @@session.gtid_next;
| AUTOMATIC           |

mysql> SELECT @@session.gtid_owned;
|                      |

mysql> INSERT INTO ........

mysql> SELECT @@global.gtid_executed;
| a1c32161-89c4-11e8-856e-0242ac12001d:1-4 |

mysql> SELECT @@session.gtid_next;
| AUTOMATIC           |

mysql> SELECT @@session.gtid_owned;
|                      |

mysql> COMMIT;

mysql> SELECT @@global.gtid_executed;
| a1c32161-89c4-11e8-856e-0242ac12001d:1-5 |

mysql> SELECT @@session.gtid_next;
| AUTOMATIC           |

mysql> SELECT @@session.gtid_owned;
|                      |

mysql> INSERT INTO ........

mysql> SELECT @@global.gtid_executed;
| a1c32161-89c4-11e8-856e-0242ac12001d:1-6 |

mysql> SELECT @@session.gtid_next;
| AUTOMATIC           |

mysql> SELECT @@session.gtid_owned;
|                      |

Notice that each time a transaction is committed (explicitly or implicitly/autocommit), the global list of executed GTIDs is incremented, but the session gtid_owned is always empty.

What am I doing wrong/missing? Or have I found a bug?

Answer 1

A client only owns a transaction that is still open, so gtid_owned is cleared once the transaction commits. It also only shows a value if you have explicitly set one for gtid_next ; using gtid_next=automatic will not populate gtid_owned . The use of gtid_owned is therefore limited to some internal operations and to cases like replication where the GTID is explicitly set in the binary log.

To get the list of gtids set by the current session, you could enable session_track_gtids (see https://dev.mysql.com/doc/en/server-system-variables.html#sysvar_session_track_gtids ).

Note however that the 'mysql' command line client does not report the values returned by the session tracker; you'd need a client which lets you call the C API functions mysql_session_track_get_first() and mysql_session_track_get_next() (see https://dev.mysql.com/doc/en/mysql-session-track-get-first.html ).