Truncation/Reset support

[wangandi]

A number of sources or potential sources emit an event that calls for the deletion of all existing rows in previously in the source.

Examples:

• server sent sources can have reset events https://github.com/MaterializeInc/materialize/issues/2237#issuecomment-729259735
• Debezium has a truncate event. https://github.com/MaterializeInc/materialize/issues/6596
• Postgresql also has a truncate event. cc: @petrosagg

@frankmcsherry mentioned that differential is capable of supporting truncation.

[frankmcsherry]

I did?

Truncation in DD is usually "turn off dataflow and turn it on again". DD doesn't have an easy concept of that, unless you've retained an arrangement to explicitly retract.

[wangandi]

I definitely thought you mentioned you had some idea for truncation/reset support that we could hash out in greater detail this week. I could've been wrong on DD being the source of that support.