Feature: internal columns

[sundy-li]

Summary

Description for this feature.

Fuse engine can have some virtual column, it's not in select * unless you specify it.

row_id
block_name
segment_name
snapshot_name

[xudong963]

It's useful to support delete with subquery: subquery outputs the rowid to be deleted -> all rowids and the rowid to be deleted can form a filter -> the final result.

[leiysky]

I prefer to call this a hidden column or internal column rather than virtual column since it usually refers to generated column.

[lichuang]

I prefer to call this a hidden column or internal column rather than virtual column since it usually refers to generated column.

agree

[sundy-li]

@lichuang internal column should have ColumnID, how about make it from u32::MAX

[lichuang]

@lichuang internal column should have ColumnID, how about make it from u32::MAX

OK

[sundy-li]

RowID could be represented by a UInt64 column where the high 32 bits as BlockID and the low 32 bits as row_in_block