CHECK Constraints¶

CHECK Constraints are named SQL expressions that are used to enforce data quality at table level (at write time).

CHECK constraints are registered (added) using the following high-level operator:

• ALTER TABLE ADD CONSTRAINT SQL statement

CHECK constraints are de-registered (dropped) using the following high-level operator:

• ALTER TABLE DROP CONSTRAINT SQL statement

DeltaCatalog is responsible to manage CHECK constraints of a delta table (using AddConstraint and DropConstraint).

Protocol¶

CHECK constraints require the Minimum Writer Version (of a delta table) to be at least 3.

delta.constraints¶

CHECK constraints are stored in the table configuration (of a table metadata) as delta.constraints.-keyed entries.

AlterTableCommands, DeltaCatalog and Delta Commands¶

At execution, AlterTableCommand (Spark SQL) is planned as a AlterTableExec (Spark SQL) that requests the active TableCatalog (Spark SQL) to alter a table.

That is when DeltaCatalog kicks in to alter a table as AlterTableAddConstraintDeltaCommand.