Specifying WITHOUT SYSTEM VERSIONING might involve removing old history data for that column.

History is stored for rows, not columns. Thus it's not possible to remove history for some columns. And changing WITH/WITHOUT SYSTEM VERSIONING doens't need to read table data at all.

Apart from table rebuildint there is a related bug:

set @@system_versioning_alter_history=keep;

create table t (

  a int,

  b int,

  row_start bigint unsigned as row start invisible,

  row_end bigint unsigned as row end invisible,

  period for system_time(row_start, row_end)

) with system versioning engine=innodb;

insert into t values (1,1);

alter table t

  change a a int without system versioning;

update t set a=11;

select *,row_start,row_end from t for system_time all;

Here historical row is created on UPDATE.

Yes, history is stored for rows, not columns. I meant that it would be technically possible to remove historical data for a particular column by updating the value of the column in the history rows to the current value.

Come to think of it, if versioning is disabled for some columns, some historical values of non-versioned columns can be persisted in historical rows as ‘collateral damage’ if these historical rows are created due to updating versioned columns.

So, I would guess that adding the WITHOUT SYSTEM VERSIONING attribute to a column could be an instantaneous operation, only affecting subsequent versioned operations. (SELECT should only report the current value of non-versioned columns, and treat any values in historical rows as garbage.)

I posted my review comments, asking for some revision.

I improved PR and asking for another review round.

Some minor improvements are still needed.

PR updated.