sql: block FKs to/from TTL tables

docs/RFCS/20220120_row_level_ttl.md

@@ -320,10 +320,16 @@ Rows that have expired their TTL can be optionally removed from all SQL query
 results. Work for this is required at the optimizer layer. However, this is not
 planned to be implemented for the first iteration of TTL.
 
-## Foreign Keys to a TTL Table
-To avoid additional complexity in the initial implementation, foreign keys to
-TTL tables will not be permitted. More thought has to be put on ON
-DELETE/ON UPDATE CASCADEs before we can look at allowing this functionality.
+## Foreign Keys to/from TTL Tables
+To avoid additional complexity in the initial implementation, foreign keys (FK)
+to and from TTL tables will not be permitted due to complexities with the
+implementation which are complex to handle, for example:
+* When having a non-TTL table with a FK dependent on a TTL table with an
+  `ON UPDATE/DELETE CASCADE`, the non-TTL table need to hide any rows which
+  are linked to an expired TTL row.
+* When having a TTL table with an FK dependent on a non-TTL table,
+  `ON DELETE RESTRICT` should only block a delete on the non-TTL table
+  if the row has expired.
 
 ## Introspection
 The TTL definition for the table will appear in `SHOW CREATE TABLE`. The options

pkg/sql/catalog/descriptor.go

@@ -638,6 +638,8 @@ type TableDescriptor interface {
 	GetRegionalByRowTableRegionColumnName() (tree.Name, error)
 	// GetRowLevelTTL returns the row-level TTL config for the table.
 	GetRowLevelTTL() *descpb.TableDescriptor_RowLevelTTL
+	// HasRowLevelTTL returns where there is a row-level TTL config for the table.
+	HasRowLevelTTL() bool
 	// GetExcludeDataFromBackup returns true if the table's row data is configured
 	// to be excluded during backup.
 	GetExcludeDataFromBackup() bool

pkg/sql/catalog/tabledesc/structured.go

@@ -2440,6 +2440,11 @@ func (desc *wrapper) GetRowLevelTTL() *descpb.TableDescriptor_RowLevelTTL {
 	return desc.RowLevelTTL
 }
 
+// HasRowLevelTTL implements the TableDescriptor interface.
+func (desc *wrapper) HasRowLevelTTL() bool {
+	return desc.RowLevelTTL != nil
+}
+
 // GetExcludeDataFromBackup implements the TableDescriptor interface.
 func (desc *wrapper) GetExcludeDataFromBackup() bool {
 	return desc.ExcludeDataFromBackup

pkg/sql/catalog/tabledesc/validate.go

@@ -176,6 +176,13 @@ func (desc *wrapper) ValidateCrossReferences(
 	}
 
 	// Check foreign keys.
+	if desc.HasRowLevelTTL() && (len(desc.OutboundFKs) > 0 || len(desc.InboundFKs) > 0) {
+		vea.Report(unimplemented.NewWithIssuef(
+			76407,
+			`foreign keys to/from table with TTL "%s" are not permitted`,
+			desc.Name,
+		))
+	}
 	for i := range desc.OutboundFKs {
 		vea.Report(desc.validateOutboundFK(&desc.OutboundFKs[i], vdg))
 	}

pkg/sql/logictest/testdata/logic_test/row_level_ttl

@@ -97,3 +97,28 @@ CREATE TABLE public.tbl (
                                                                                             CONSTRAINT tbl_pkey PRIMARY KEY (id ASC),
                                                                                             FAMILY fam_0_id_text_crdb_internal_expiration (id, text, crdb_internal_expiration)
 ) WITH (ttl = 'on', ttl_automatic_column = 'on', ttl_expire_after = '00:10:00':::INTERVAL)
+
+# Test we cannot add FKs to a TTL table.
+statement ok
+CREATE TABLE ref_table (id INT PRIMARY KEY, ref INT)
+
+statement error foreign keys to/from table with TTL "ttl_table" are not permitted
+CREATE TABLE ttl_table (id INT PRIMARY KEY, ref INT REFERENCES ref_table(id)) WITH (ttl_expire_after = '10 mins')
+
+statement ok
+CREATE TABLE ttl_table (id INT PRIMARY KEY, ref INT) WITH (ttl_expire_after = '10 mins')
+
+statement error foreign keys to/from table with TTL "ttl_table" are not permitted
+CREATE TABLE new_ref_table (id INT PRIMARY KEY, ref INT REFERENCES ttl_table(id))
+
+statement error foreign keys to/from table with TTL "ttl_table" are not permitted
+ALTER TABLE ref_table ADD CONSTRAINT fk FOREIGN KEY (ref) REFERENCES ttl_table (id)
+
+statement error foreign keys to/from table with TTL "ttl_table" are not permitted
+ALTER TABLE ttl_table ADD CONSTRAINT fk FOREIGN KEY (ref) REFERENCES ttl_table (id)
+
+statement ok
+CREATE TABLE ttl_become_table (id INT PRIMARY KEY, ref INT REFERENCES ref_table (id))
+
+statement error foreign keys to/from table with TTL "ttl_become_table" are not permitted
+ALTER TABLE ttl_become_table SET (ttl_expire_after = '10 minutes')