Metadata columns and deletes

Targets can add _SDC_ columns that describe ingestion time and source-delete events. Configure the behaviour per tap because pipelines sharing a target can have different delete requirements.

Columns

Column	Meaning
`_SDC_EXTRACTED_AT`	Time the tap extracted the record.
`_SDC_BATCHED_AT`	Time the record entered a target load batch.
`_SDC_DELETED_AT`	Time a source delete event was received; `NULL` for active rows.

Configuration outcomes

Settings	Metadata	Delete behaviour
`hard_delete: true`	Enabled automatically	Physically removes a target row after a source delete event.
`hard_delete: false` and `add_metadata_columns: true`	Enabled	Retains the row and sets `_SDC_DELETED_AT`.
Both `false`	Disabled	Retains the target row without marking the source delete.

add_metadata_columns: true
hard_delete: true

hard_delete defaults to true. Only LOG_BASED replication emits individual source-delete events; incremental replication cannot detect deletes. Full-table and PartialSync publication semantics can remove rows as part of replacing or reconciling a selected range.

Deprecated since version 0.79.0: Soft delete (hard_delete: false) is scheduled for removal. New pipelines should use hard delete and model retention in a controlled downstream layer.

Changing delete mode does not rewrite historical target rows. Plan a resync or explicit target migration when existing data must adopt the new behaviour.