Ignore lossy DDL change for TiCDC

[Rustin170506]

Enhancement

What is lossy DDL and why do we need to special handle it?

Lossy DDL means we execute a DDL which changes the data type. For example:
Create table:

CREATE TABLE movies(title VARCHAR(50) NOT NULL,
genre VARCHAR(30) NOT NULL,director VARCHAR(60) NOT NULL,
release_year INT NOT NULL,PRIMARY KEY(title));

Notice the current length of the genre is VARCHAR(30). If you do an alter:

ALTER TABLE movies MODIFY COLUMN genre VARCHAR(20)

You changed the length from 30 to 20. This is a lossy DDL, it would cause the full amount of data to change.

Also, it causes the TiCDC to spend a lot of resources on handling these data changes from TiCDC. But maybe the users don't care about these changes. They only want to get the right DDL to notice that the data type of this column was changed.

See more at: Needs Analysis For Optimizing Lossy DDL Synchronization Process and Solution For Optimizing Lossy DDL in TiCDC

[nongfushanquan]

/label affects-7.1

[nongfushanquan]

/area ticdc

[nongfushanquan]

/label affects-6.5