Disallow deferrable unique constraints

Prior to this change, if Psycopack was used on a table with a deferrable
unique constraint, then after the "swap" stage, any updates to the original
table would cause the trigger that updates the psycopack table to fail with
this error:
    ON CONFLICT does not support deferrable unique constraints/exclusion
     constraints as arbiters

That error results from the use of "ON CONFLICT DO NOTHING" in the copy
function used by the trigger.

This change removes test coverage of deferrable unique constraints and adds
a check to the pre_validate stage to reject a table that has a deferrable
unique constraint.

Author: timb07

src/psycopack/__init__.py

@@ -8,6 +8,7 @@
 from ._repack import (
     BasePsycopackError,
     CompositePrimaryKey,
+    DeferrableUniqueConstraint,
     InheritedTable,
     InvalidIndexes,
     InvalidPrimaryKeyTypeForConversion,
@@ -32,6 +33,7 @@
     "BackfillBatch",
     "BasePsycopackError",
     "CompositePrimaryKey",
+    "DeferrableUniqueConstraint",
     "FailureDueToLockTimeout",
     "InheritedTable",
     "InvalidIndexes",

src/psycopack/_commands.py

@@ -295,23 +295,12 @@ def create_unique_constraint_using_idx(
         table: str,
         constraint: str,
         index: str,
-        is_deferrable: bool,
-        is_deferred: bool,
     ) -> None:
         add_constraint_sql = dedent("""
             ALTER TABLE {schema}.{table}
             ADD CONSTRAINT {constraint}
-            UNIQUE USING INDEX {index}
+            UNIQUE USING INDEX {index} NOT DEFERRABLE
         """)
-        if is_deferrable:
-            add_constraint_sql += " DEFERRABLE"
-        else:
-            add_constraint_sql += " NOT DEFERRABLE"
-
-        if is_deferred:
-            add_constraint_sql += " INITIALLY DEFERRED"
-        else:
-            add_constraint_sql += " INITIALLY IMMEDIATE"
 
         self.cur.execute(
             psycopg.sql.SQL(add_constraint_sql)

src/psycopack/_repack.py

@@ -59,6 +59,10 @@ class ReferringForeignKeyInDifferentSchema(BasePsycopackError):
     pass
 
 
+class DeferrableUniqueConstraint(BasePsycopackError):
+    pass
+
+
 class NoCreateAndUsagePrivilegeOnSchema(BasePsycopackError):
     pass
 
@@ -333,6 +337,21 @@ def pre_validate(self) -> None:
                     f"schema: {fks_in_different_schema}."
                 )
 
+            deferrable_unique_constraints = [
+                c.name
+                for c in self.introspector.get_constraints(
+                    table=self.table, types=["u"]
+                )
+                if c.is_deferrable
+            ]
+            if deferrable_unique_constraints:
+                raise DeferrableUniqueConstraint(
+                    f"Psycopack does not currently support tables with "
+                    f"deferrable unique constraints. The table {self.table} "
+                    f"has the following deferrable unique constraints: "
+                    f"{deferrable_unique_constraints}."
+                )
+
     def setup_repacking(self) -> None:
         with (
             self.tracker.track(_tracker.Stage.SETUP),
@@ -758,8 +777,6 @@ def _create_unique_constraints(self) -> None:
                 # From previous steps, the index name is the same as the
                 # constraint, not a typo!
                 index=constraint_name,
-                is_deferrable=cons.is_deferrable,
-                is_deferred=cons.is_deferred,
             )
 
     def _create_check_and_fk_constraints(self) -> None:

tests/test_repack.py

@@ -7,6 +7,7 @@
 from psycopack import (
     BackfillBatch,
     CompositePrimaryKey,
+    DeferrableUniqueConstraint,
     FailureDueToLockTimeout,
     InheritedTable,
     InvalidIndexes,
@@ -1752,6 +1753,37 @@ def test_with_fks_from_another_schema(connection: _psycopg.Connection) -> None:
             repack.full()
 
 
+def test_with_deferrable_unique_constraint(connection: _psycopg.Connection) -> None:
+    with _cur.get_cursor(connection, logged=True) as cur:
+        factories.create_table_for_repacking(
+            connection=connection,
+            cur=cur,
+            table_name="to_repack",
+            rows=100,
+        )
+        repack = Psycopack(
+            table="to_repack",
+            batch_size=1,
+            conn=connection,
+            cur=cur,
+        )
+        cur.execute(
+            "ALTER TABLE to_repack DROP CONSTRAINT to_repack_var_with_unique_const_key;"
+        )
+        cur.execute(
+            dedent("""
+                ALTER TABLE to_repack ADD CONSTRAINT to_repack_var_with_unique_const_key
+                UNIQUE (var_with_unique_const)
+                DEFERRABLE;
+            """)
+        )
+
+        with pytest.raises(
+            DeferrableUniqueConstraint, match="to_repack_var_with_unique_const_key"
+        ):
+            repack.full()
+
+
 def test_without_schema_privileges(connection: _psycopg.Connection) -> None:
     schema = "sweet_schema"
     with _cur.get_cursor(connection, logged=True) as cur: